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A Meta-analysis of the Uncanny Valley's Independent and Dependent Variables

  • Indiana University Luddy School of Informatics Computing and Engineering
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Abstract and Figures

The uncanny valley (UV) effect is a negative affective reaction to human-looking artificial entities. It hinders comfortable, trust-based interactions with android robots and virtual characters. Despite extensive research, a consensus has not formed on its theoretical basis or methodologies. We conducted a meta-analysis to assess operationalizations of human likeness (independent variable) and the UV effect (dependent variable). Of 468 studies, 72 met the inclusion criteria. The studies employed 10 different stimulus creation techniques, 39 affect measures, and 14 indirect measures. Based on 247 effect sizes, a three-level meta-analysis model revealed the UV effect had a large effect size, Hedges' g = 1.01 [0.80, 1.22]. A mixed-effects meta-regression model with creation technique as the moderator variable revealed face distortion produced the largest effect size, g = 1.46 [0.69, 2.24], followed by distinct entities, g = 1.20 [1.02, 1.38], realism render, g = 0.99 [0.62, 1.36], and morphing, g = 0.94 [0.64, 1.24]. Affective indices producing the largest effects were threatening, likable, aesthetics, familiarity, and eeriness, and indirect measures were dislike frequency, categorization reaction time, like frequency, avoidance, and viewing duration. This meta-analysis-the first on the UV effect-provides a methodological foundation and design principles for future research.
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A meta-analysis of the uncanny valley’s
independent and dependent variables
Alexander Diel
School of Psychology, Cardiff University, 70 Park Place, Cardiff CF10 3AT, United
Sarah Weigelt
Department of Vision, Visual Impairments & Blindness, Faculty of Rehabilitation
Sciences, Technical University of Dortmund, Emil-Figge-Straße 50, 44227 Dortmund,
Karl F. MacDorman
School of Informatics and Computing, Indiana University, 535 West Michigan St.,
Indianapolis, IN 46202, USA,
The uncanny valley (UV) effect is a negative affective reaction to human-looking artificial entities.
It hinders comfortable, trust-based interactions with android robots and virtual characters. Despite
extensive research, a consensus has not formed on its theoretical basis or methodologies. We
conducted a meta-analysis to assess operationalizations of human likeness (independent variable)
and the UV effect (dependent variable). Of 468 studies, 72 met the inclusion criteria. The studies
employed 10 different stimulus creation techniques, 39 affect measures, and 14 indirect measures.
Based on 247 effect sizes, a three-level meta-analysis model revealed the UV effect had a large
effect size, Hedges’ g = 1.01 [0.80, 1.22]. A mixed-effects meta-regression model with creation
technique as the moderator variable revealed face distortion produced the largest effect size, g =
1.46 [0.69, 2.24], followed by distinct entities, g = 1.20 [1.02, 1.38], realism render, g = 0.99 [0.62,
1.36], and morphing, g = 0.94 [0.64, 1.24]. Affective indices producing the largest effects were
threatening, likable, aesthetics, familiarity, and eeriness, and indirect measures were dislike
frequency, categorization reaction time, like frequency, avoidance, and viewing duration. This
meta-analysisthe first on the UV effectprovides a methodological foundation and design
principles for future research.
CCS Concepts
• Human-centered computing → HCI design and evaluation methods; • Computer systems
organization → External interfaces for robotics; • Computing methodologies → Animation
Anthropomorphism, computer animation, face perception, robotics, uncanny valley
1 Introduction
Royle (2003) gives an evocative and succinct description of the uncanny experience:
The uncanny is ghostly. It is concerned with the strange, weird, and mysterious, with
a flickering sense (but not conviction) of something supernatural. The uncanny
involves feelings of uncertainty, in particular regarding the reality of who one is and
what is being experienced. (p. 1)
Figure 1. The uncanny valley as proposed by Mori in 1970. The affective reaction towards an
entity (y-axis) is a function of its degree of human likeness (x-axis) and whether it is still or
moving (solid or dashed line). Bunraku puppets play character roles in ningyō jōruri, a
traditional form of musical puppet theater in Japan. Actors in theater wear masks: The yase
otoko mask (literally, thin man) signifies a ghost from hell, and the okina mask signifies an old
Objects, situations, and events that do not fit our everyday understanding of the world are often
described as eerie, creepy, or uncanny. These ascriptions can be made regarding new technologies
(Langer & König, 2018), unusual human behavior (McAndrew & Koehnke, 2016), or peculiar
coincidences (Freud, 1919/2003). Negative evaluations can hinder the adoption of supportive
products like healthcare robots (Olaronke, Ojerinde, & Ikono, 2017) or service chatbots
(Ciechanowski, Przegalińska, Magnuski, & Gloor, 2019). As the robotics pioneer Mori proposed
in 1970, human-looking androids and other objects could elicit a reaction unlike the one typically
elicited by people or stylish technology. Mori (2012) illustrated this phenomenon with a graph
(Figure 1). The y-axis depicts affinity, the dependent variable (DV), as a function of human
likeness, the independent variable (IV), on the x-axis (Bartneck, Kulić, Croft, & Zoghbi, 2009b;
Ho & MacDorman, 2010, 2017; MacDorman & Ishiguro, 2006). The stimulus sets in Figure 2
show how different creation techniques have been used to operationalize the independent
According to Mori (2012), affinity for an entity increases with its human likeness but only up to a
point. Beyond this point, affinity falls and becomes negative, and the entity elicits a cold, eerie,
repellant feeling. Then, affinity rises again, becoming positive, as human likeness increases
toward indistinguishability. When graphed, the fall and rise in affinity resemble a valleyhence,
the term uncanny valley (UV).
Since Mori’s proposal, a substantial body of research has replicated a valley-shaped curve and
found a significant effect (Burleigh, Schoenherr, & Lacroix, 2013; Ferrey, Burleigh, & Fenske,
2015; Jung & Cho, 2018; MacDorman, Green, Ho, & Koch, 2009; Mäkäräinen, Kätsyri, &
Takala, 2014; Mathur & Reichling, 2016; Mathur et al., 2020; McDonnell, Breidt, & Bülthoff,
2012; Palomäki et al., 2018; Sasaki, Ihaya, & Yamada, 2017; Strait et al., 2017; Strait, Vujovic,
Floerke, Scheutz, & Urry, 2015; Tinwell, Grimshaw, & Nabi, 2015; Tinwell, Grimshaw, Nabi, &
Williams, 2011; Tinwell & Sloan, 2014; Yamada, Kawabe, & Ihaya, 2013). However, some
studies have plotted functions other than a valley-shaped curve: For example, Kätsyri, de Gelder,
and Takala (2019) found affinity increased with human likeness, an “uncanny slope”; Cheetham,
Suter, and Jäncke (2014) interpreted increasing familiarity ratings with the transition from avatar
to ambiguous morph to human as a “happy valley”; and Bartneck, Kanda, Ishiguro, and Hagita
(2009a) and Cheetham, Wu, Pauli, and Jäncke (2015) found no difference in affective responses
toward androids and humans. Although the UV effect is seldom disputed, its theoretical basis and
methodologies have eluded consensus. This motivated us to examine how the independent and
dependent variables in Mori’s graph have been operationalized in the literature.
Although several reviews have examined the UV effect (Kätsyri, Förger, Mäkäräinen, & Takala,
2015; Lay, Brace, Pike, & Pollick, 2016; Wang, Lilienfeld, & Rochat, 2015; Zhang et al., 2020),
this is the first meta-analysis to do so. It confirmed the effect’s significance and determined its
effect size. This is also, of course, the first meta-analysis to evaluate the uncanny valley’s
stimulus creation methods and affect and indirect measures. The evaluation was accomplished
using meta-regression models. From the results, we distill design principles for future
The UV effect has been conceptualized in different ways. These conceptualizations often stem
from different theories and their assumptions about elicitors of the effect (Diel & MacDorman,
2021). They include
1. a function like Mori’s graph that maps a given degree of human likeness to a level of affect
(Bartneck et al., 2009a; Burleigh, Schoenherr, & Lacroix, 2013; Chen, Russel, Nakayama, &
Livingstone, 2010; Gray & Wegner, 2012; Kätsyri, de Gelder, & Takala, 2019; Lin et al, 2021;
Ramey, 2005; Sasaki, Ihaya, & Yamada, 2017; Schneider, Wang, & Yang, 2009; Schwind et
al., 2018; Seyama & Nagayama, 2007);
2. deviations from norms of human appearance and movement (Chaminade, Hodgins, & Kawato,
2007; MacDorman & Ishiguro, 2006; Mathur & Reichling, 2016; Palomäki et al., 2018;
Schoenherr & Burleigh, 2015; Seyama & Nagayama, 2007; Tinwell, 2009; Tinwell &
Grimshaw, 2009; Tinwell, Grimshaw, & Nabi, 2014);
3. violations of expectations of human appearance and behavior (Bartneck et al., 2009a;
MacDorman & Ishiguro, 2006);
4. sensitivity to nonhuman features that increases with an entity’s human likeness (Chattopadhyay
& MacDorman, 2016; Green, MacDorman, Ho, & Vasudevan, 2008; MacDorman, Srinivas, &
Patel, 2013);
5. a mismatch between human and nonhuman features (Ho & MacDorman, 2010; MacDorman,
Green, Koch, & Ho, 2009; Mitchell et al., 2011b; Moore, 2012; Takahashi, Fukuda, Samejima,
Watanabe, & Ueda, 2015; Tinwell & Sloan, 2014);
6. entities that elicit the concept human but have nonhuman traits (Steckenfinger & Ghazanfar,
2009); and
7. difficulty distinguishing between categories, such as human and robot, or a conflict between
categories (Cheetham, Pavlović, Jordan, Suter, & Jäncke, 2013; Cheetham, Suter, & Jäncke,
2011, 2014; Cheetham, Wu, Pauli, & Jäncke, 2015; Matsuda, Okamoto, Ida, Okanoya, &
Myowa-Yamakoshi, 2012).
Figure 2. Different operationalizations of the independent variable human likeness (Feng et al.,
2018; Ferrey, Burleigh, and Fenske, 2015; MacDorman et al., 2009; Mäkäräinen, Kätsyri, &
Talaka, 2014, derived from Langner et al., 2010; Mathur & Reichling, 2016; Schindler et al.,
Ferrey, Burleigh, and Fenske, 2015
MacDorman et al., 2009
Märäinen, tsyri, and Takala, 2014
Mathur and Reichling, 2016
Schindler et al., 2017
Feng et al., 2018
1.1 The independent variable
1.1.1 Construct
In experiments on the UV effect, the independent variable is typically human likeness or a similar
term. However, it is unclear precisely how human likeness relates to the UV curve. Human
likeness can be characterized along many dimensions, which interact to create an overall
impression of humanness (Bartneck et al., 2009b; von Zitzewitz, Boesch, Wolf, & Riener, 2013).
Mori (2012) examines both the outward appearance and the behavior of androids, corpses, and
industrial and toy robots. In discussing mannequins, prostheses, and bunraku puppets, he draws in
other dimensions, such as the setting, lighting, story, time of day, and the perceiver’s gender and
distance. Research corroborates the multidimensionality of human likeness in exploring the
relation between the UV effect and an entity’s physical (MacDorman & Ishiguro, 2006; Seyama
& Nagayama, 2007), behavioral (MacDorman et al., 2005; Złotowski et al., 2015), and perceived
mental similarity to humans (Gray & Wegner, 2012; Stein & Ohler, 2017). The perception of
nonhuman animals can also elicit the UV effect (Chattopadhyay & MacDorman, 2016; Löffler,
Dörenbächer, & Hassenzahl, 2020; Schwind et al., 2018; Takahashi et al., 2015; Yamada,
Kawabe, & Ihaya, 2013). This result casts doubt on whether the independent variable solely
concerns human likeness. Realism or zoomorphism have served as alternative concepts.
Furthermore, Mori (2012) uses human likeness to denote interchangeably both an entity’s
physical properties and how it is perceived. In research, however, the distinction is necessary.
Physical properties, for example, can be directly manipulated as an independent variable.
1.1.2 Stimulus range
We compiled a list of categories to summarize stimulus creation techniques. The list derives from
the stimuli appearing in publications of empirical research and descriptions of how they were
created (e.g., Mitchell et al., 2011b; Seyama & Nagayama, 2007). We started with six a priori
categories and added categories during the literature search when a paper’s stimuli did not fit in
any existing category. Saturation was reached at 10 categories. The categories encompass the
research reviewed, enabling its techniques to be easily classified, and reflect its theoretical and
methodological breadth. The 10 categories of techniques are listed below:
Distinct entities: Selecting images or videos of existing robots, androids, computer-animated
characters, humans, or other entities (e.g., Mathur et al., 2020). This technique is theory-
independent and can be used with both still and moving entities, such as characters from films,
video games, and virtual worlds.
Emotion manipulation: Distorting affective expressions (e.g., Qiao & Roger, 2011; Qiao, Eglin,
& Beck, 2011; Tinwell et al., 2014). This technique visually manipulates the emotional
expression of the face. It has been used mainly to test empathy-related theories.
Face distortion: Distorting facial features and proportions (e.g., Mäkäräinen et al., 2014). This
technique visually manipulates facial features or the relations among them until the face no longer
appears real. The emotional expression is not intentionally manipulated. This technique has been
used to test theories related to configural processing (e.g., MacDorman et al., 2009).
Mismatch: Swapping facial features with those of another face that differs along one or more
dimensionstypically animacy, human likeness, and realism (e.g., Seyama & Nagayama, 2007).
This technique has been used to test theories related to perceptual mismatch (MacDorman &
Chattopadhyay, 2016).
Morphing: Varying the stimulus in a stepwise transition between a pair of images to create a
range of stimuli (e.g., MacDorman & Ishiguro, 2006). This technique has been used to transform
the stimulus gradually from one kind of entity to another, thus making it suitable for testing
category-related theories (e.g., Cheetham et al., 2015; Sasaki, Ihaya, & Yamada, 2017).
Motion manipulation: Distorting an animation’s biological motion (e.g., gait, Destephe et al.,
2014; Handzic & Reed, 2015; motion quality, Piwek, McKay, & Pollick, 2014; Thompson,
Trafton, & McKnight, 2011). This technique has been used to test whether the UV effect occurs
in motion perception.
Realism render: Varying how real the stimuli appear by representing them as cartoons or as
computer models with a reduced polygon count or simplified textures (e.g., McDonnell et al.,
2012; Muniady & Ali, 2020). This technique is theory-independent and relevant to practical
applications of visual design.
Real-life encounter: Presenting different embodied entities like robots, androids, and humans for
observation or interaction (e.g., Złotowski et al., 2015). This technique encompasses multiple
modalities and, thus, can be used to measure a holistic UV effect. It is also useful because a
physical object could be perceived and evaluated differently from its two-dimensional depiction
(Snow, Skiba, Coleman, & Berryhill, 2014). Moreover, this technique is ecologically valid.
Visuo-auditory mismatch: Replacing a human voice with a synthesized voice or vice versa in an
animation (e.g., Mitchell et al., 2011b; Stein & Ohler, 2018). Although typically motivated by
perceptual mismatch theories, this technique differs from the mismatch category because the
mismatch is crossmodal.
Voice distortion: Distorting natural human voices as auditory stimuli (e.g., Baird et al., 2018;
Kühne et al., 2020). This technique has been used to test whether the UV effect can occur solely
within audition.
1.1.3 Measurement
To assess the degree of human likeness (or related concepts), either single-scale measures or
indices consisting of multiple scales have been used (e.g., Burleigh, Schoenherr, & Lacroix,
2013; Ho & MacDorman, 2010, 2017). Experiments typically vary the stimulus systematically in
its degree of human similarity. Manipulations include distorting it (Mäkäräinen, Kätsyri, &
Takala, 2014) or controlling its morphing proportion between two images (Cheetham & Jäncke,
2013). Experiments may include a manipulation check, such as rating the stimulus on human
likeness. For computer-modeled stimuli only, Burleigh, Schoenherr, and Lacroix (2013) proposed
two objective properties, which they define as follows: texture resolution, the number of pixels
per unit of surface area, and polygon count, the number of polygons constituting a three-
dimensional model. However, human likeness and realism are two different constructs. Thus, the
results of a study measuring human likeness may not be comparable to the results of a study
measuring realism. Research has not compared how changes in these independent variables or
others may influence affect measures differently.
1.2 The dependent variable
1.2.1 Construct
Mori (2012) represents the y-axis with the term shinwakan, a neologism he translates as affinity.
The y-axis had initially been translated as familiarity (Reichardt, 1978). Other proposed
constructs include interpersonal warmth (or likability) and reverse-scaled eeriness (Bartneck et
al., 2009b; Ho & MacDorman, 2010, 2017; Redstone, 2013). Eeriness and its synonym
creepiness correlate with aversive experiences like disgust, fear, and anxiety (Ho, MacDorman, &
Pramono, 2008).
1.2.2 Measurement
In experiments on the UV effect, the dependent variable is typically measured with single-scale
measures or indices composed of self-reported affective items. Semantic differential scales are
common. Semantically, some items like eerie, creepy, and uncanny are specific and, on face
value, capture the distinctive experiential quality of the UV effect (Ho & MacDorman, 2010;
Mangan, 2015; Palomäki et al., 2018; Redstone, 2013; Tinwell, Nabi, & Charlton, 2013). Other
items like pleasantness or likability are nonspecific. An entity could rate low on them without
being uncanny at all (e.g., items in Bartneck et al., 2009b; Ferrey, Burleigh, & Fenske, 2015;
Rosenthalvon der Pütten & Krämer, 2014; Yamada, Kawabe, & Ihaya, 2013).
Questionnaires that have been developed to evaluate robots in general have been repurposed to
measure the UV effect. Examples include the Godspeed indices (Bartneck et al., 2009b) and the
Robotic Social Attribution Scale (Carpinella, Wyman, Perez, & Stroessner, 2017). Ho and
MacDorman’s (2010, 2017) set of indices includes humanness, interpersonal warmth,
attractiveness, and eeriness. They developed the set to decorrelate these dimensions so they could
be plotted against each other on orthogonal axes.
Indirect measures may indicate a construct by measuring a different construct. For example, the
UV effect may correlate with trust behavior (Mathur & Reichling, 2016). For simplicity, we
categorize implicit measures as indirect measures. Implicit measures center on processes that are
automatic, effortless, fast, goal-independent, stimulus-driven, uncontrolled, or unintentional. For
example, response time and other performance measures of the UV effect typically are implicit
measures. Implicit measures counter self-presentational bias, that is, respondents attempts to
influence how others perceive them. Implicit measures may indicate the UV effect in otherwise
inaccessible populations, such as infants or nonhuman animals.
Apart from trust behavior, the UV effect has been measured by such indirect measures as
avoidance behavior (Matsuda et al., 2012), perceived responsiveness (Tinwell et al., 2013), and
cognitive conflict and categorization reaction time (RT, Cheetham & Jäncke, 2013).
1.2.3 Other constructs
Other constructs and their associated measures and theories include the following:
Aesthetics: Items measuring aesthetic appeal (Sansoni, Wodehouse, McFayden, & Buis, 2015;
Schwind et al., 2018). These items conceptualize the UV effect as a lack of physical
attractiveness. Thus, they can serve as a practical tool for design (Hanson et al., 2005; Ho &
MacDorman, 2010, 2017). Research has used nonhuman (e.g., Schwind et al., 2018) as well as
human stimuli with the latter leveraging on theories of evolutionary aesthetics. These theories
frame the UV effect as resulting from a mechanism for avoiding mates with low fitness as
determined by the absence of physical markers of fertility, health, and youthfulness (MacDorman
et al., 2009; MacDorman & Ishiguro, 2006).
Animacy and experience: Items measuring perceived animacy (Looser & Wheatley, 2010),
responsiveness (Tinwell et al., 2014), and mind (Appel et al., 2016). These items relate to theories
about how the perceived presence or absence of these qualities elicits the UV effect. For example,
Gray and Wegner (2012) proposed that a machine having conscious experiencesor a human
being lacking themwould be perceived as uncanny; the authors’ creation techniques are broad:
android robot videos, text about a supercomputer, and a photo of a man.
Anomaly: Items measuring an entity’s perceived deviation from the norm. Anomaly items, such
as strange or weird, are associated with atypicality theories. These theories predict that the UV
effect is elicited by an entity whose features cause it to deviate strongly from its prototype
(Kätsyri et al., 2015; Strait et al., 2017). Anomalies are easily created in images, where features
can be moved, reflected, rotated, and scaled (e.g., Diel & MacDorman, 2021).
Disgust: Items measuring disgust, a predictor of the UV effect (Ho, MacDorman, & Pramono,
2008). These items relate to the theory that the UV effect results from an evolved mechanism for
pathogen avoidance (MacDorman & Entezari, 2015).
Distinctive experience: Items measuring the UV effect as the subjective experience of
uncanniness or eeriness, which may be correlated with fear, anxiety, and disgust (Bartneck et al.,
2009a; Ho, MacDorman, & Pramono, 2008). This research conceives of the UV effect as an
experience distinct from general psychological discomfort or anxiety. Gahrn-Andersen (2020)
and Mangan (2015) have related the phenomenological study of the uncanny to the theories of
Martin Heidegger and William James.
Familiarity: Items measuring the UV effect as feelings of unfamiliarity, based on Reichardt’s
(1978) translation of shinwakan as familiarity. Typically, in cognitive psychology, familiarity is
contrasted with novelty: 0% familiarity is 100% novelty. However, when inspecting the y-axis of
Mori’s (2012) graph, the familiar–novel contrast leads to contradiction. On this interpretation, the
bottom of the valley lies in negative familiarity, beyond 100% novelty, which cannot exist. One
finds a different interpretation in Freud’s (1919/2003) theory of the uncanny. To Freud, the
uncanny is not the perception of something novel or unfamiliar. Rather, it is the recollection of
something intimately familiar, perhaps from early childhood, that has long been estranged
through repression (MacDorman & Entezari, 2015; MacDorman & Ishiguro, 2006). Freud asserts
that repression transforms every emotional affectincluding uncanninessinto anxiety (Angst).
General anxiety: Items measuring a state of anxiety or stress without relating it specifically to the
subjective experience of the uncanny. The items are associated with theories based on category
inhibition, cognitive conflict (Ferrey et al., 2015), and perceptual tension (Moore, 2012). Their
use may reflect the assumption that the experiential quality of the UV effect is no more specific
than the psychological discomfort caused by cognitive dissonance or cognitive load.
Interpersonal warmth: Items measuring the primary dimension of social perception, interpersonal
warmth, which accounts for 53% of the variance in perceptions of social behaviors (Fiske,
Cuddy, & Glick, 2007; Fiske, Cuddy, Glick, & Xu, 2002). This dimension is measured with
positive affect items, like likable, pleasant, and friendly, which load on the same factor in factor
analyses (Bartneck et al., 2009a; Ho & MacDorman, 2010). The construct is intended to measure
how feelings about an entity change with its degree of human likeness. The dimension is roughly
synonymous with affinity, the y-axis of Mori’s (2012) graph, though as a construct warmth has
been more thoroughly investigated. The use of warmth items to measure the UV effect is
grounded in the assumption that warmth and uncanniness are inversely related. However, feelings
of coldnessthe low end of the scalediffer from feelings of uncanniness. For example, we
might have warm feelings for the conductor (Tom Hanks) in The Polar Express (2004) while also
having uncanny feelings because of the way he is computer animated. Furthermore, the generality
of warmth items makes them susceptible to confounds. Stimulus evaluation could be influenced
by, for example, background, clothing, color, narrative and framing, verbal and nonverbal
behavior, interactivity, personality, relationships, and culture (Brink et al., 2019; Kennedy, 2014;
Łupkowski, Rybka, Dziedzic, & Włodarczyk, 2018; MacDorman, 2019; Shin, Kim, & Biocca,
2019). Thus, warmth items do not indicate the UV effect but a related construct.
Threat: Items measuring a negative emotional response to dead animals, ranked by the species’
similarity to living humans, motivated by theories that conceive of the UV effect as an evolved
threat-avoidance mechanism (Moosa & Ud-Dean, 2010; Palomäki et al.,2018; Rosenthal et al.,
2014). The entities could also appear threatening because of their ambiguity (McAndrew &
Koehnke, 2016).
Trust: Numerical indicators of trust, such as the amount of money invested while playing a game,
with a smaller investment indicating less trust. A decrease in trust could result from the UV effect
in perceiving android robots or avatars. Mathur and Reichling (2016) relate this measure of trust
to Hardin’s (2002) theory of encapsulated interest: We trust those whose interest encapsulates our
own. In their game, they raise the question of whether human players were really taking an
intentional stance toward the robot or merely acting as if they were.
2 Methods
The lack of consensus in the UV literature, both theoretical and methodological, should now be
evident. It motivates our meta-analysis, the first of its kind. We evaluate the effectiveness of
stimulus creation techniques as well as affect and indirect measures. Based on the results, we
propose empirically derived design principles for future research.
2.1 Inclusion criteria
The meta-analysis only included a study if it met the criteria below based on the information
Empirical study: The study contains the results of at least one data analysis conducted by its
Representative participants: The study uses healthy adults, children, or infants. Excluded were
studies restricted to a specific subgroup, such as people with autism spectrum disorder.
Relevant stimuli: The stimuli belong to at least one of the 10 creation techniques.
Adequate stimuli: The stimuli lack obvious confounds like noise created by editing images.
Affect or indirect measures: Affect measures include single-scale items or indices used to self-
report an affective appraisal of the stimulus. Indirect measures include everything else. Studies
with either or both were included.
Testing a UV hypothesis for statistical significance: The study has one or more hypotheses
designed to test the UV effect. For each hypothesis, a test statistic is applied to the collected data.
Studies with both significant and nonsignificant effects were included.
Appropriate variables: Testing for a change in an affect or indirect measure resulting from a
change in human likeness or a related variable (e.g., realism, zoomorphism). Thus, all studies
were experiments.
Effect size determinable: The study must give enough information to calculate an effect size and
its variance.
Figure 3. The flowchart depicts the process of study selection.
2.2 Study search and selection
In March 2021, we searched on PubMed, Science.Gov, and the Web of Science for papers with
uncanny valley in their title, abstract, or keywords. After removing 33 duplicates, 488 studies
remained of which 155 included UV significance testing (see Data Availability). Although 98 met
other review criteria, only 72 had determinable effect sizes. These studies appeared in 56 papers
published from 2008 to 2021. Figure 3 summarizes the article selection process.
From its description, we placed each IV operationalization under the best-fitting stimulus creation
For DV operationalizations, single items were generally grouped separately. Nouns formed from
adjectives were grouped with those adjectives (e.g., eeriness with eerie). The item creepy and
semantic differential scales like creepyfriendly and creepypleasant were group as creepy*.
Affect measures were grouped separately from indirect measures. For example, the item
trustworthy was counted as an affect measure, separate from trust behavior, an indirect measure.
If a study used a negative variant of an often-used positive item, the item was grouped with the
positive variant (e.g., unpleasant with pleasant). Indices used in multiple studies were counted as
separate index items and marked with the suffix -i (e.g., those developed by Bartneck et al.,
2009b; Ho and MacDorman, 2010, 2017; Schwind et al., 2018).
We then recorded or calculated effect sizes and effect size variances, labeling each with its
corresponding IV and DV. If a study used more than one IV or DV operationalization, each effect
size was recorded or calculated.
2.3 Data analysis
A random-effects model was selected for the meta-analysis because study populations and
designs differed and affect and indirect measures were used in combination with different
stimulus creation techniques. A three-level model was used with effect nested by study. The
meta-regression for moderation analysis was performed using a mixed-effects model. The model
was fitted by restricted maximum-likelihood estimation.
Effect size is reported here as Hedges’ g. The effect size, its 95% confidence interval, and the
number of measures from which it was derived, k, are all reported. Effect size is interpreted with
small = 0.20, medium = 0.50, and large = 0.80 thresholds.
If three or more conditions were compared, such as robot, android, and human, two separate g’s
were calculated: one for the posited descent from the first peak in Mori’s graph to the base of the
valley and the second for the posited ascent from the base of the valley to the second peak. For
convenience, the descent is denoted as the UV’s nonhuman side and the ascent as the UV’s
human side.
The definition of an influential effect was adopted from Viechtbauer and Cheung (2010), as
explained in the results section.
Moderator variables for the independent variable were the creation technique. Moderator
variables for the dependent variable were (separately) the side of the valley, side × valence
(positive or negative) × measure type (affect or indirect), affect measure, indirect measure, and
other construct. Finally, paper was used as a moderator variable.
2.3.1 Effect size calculation
The meta-analysis used the standardized mean difference and its variance. Hedges’ g was used to
correct for the positive bias of Cohen’s d in smaller studies,
  13
4df1, (1)
4df12, (2)
where df indicates the degrees of freedom (Borenstein et al., 2011). If a study did not report g, it
was calculated from the means and standard deviations or by converting another reported
measure of effect size. For within-group studies, which were the majority, dav and vav were used,
av 12
212, (3)
av 1
2, (4)
where n is the number of participants (Lakens, 2013). This approach leads to slightly wider
confidence intervals than d for repeated measures. However, the calculation of drm requires the
correlation between means, which no study reported. For ANOVAs, η2 was first calculated:
df1df2 (5)
Next, to calculate g, η2 was converted to d (Cohen, 1988):
  2η2
1η2 (6)
R2, Pearson’s r, and Cramér’s V were plugged into the same formula. For the t statistic, d was
calculated for between-groups studies by imputing r = 0.5 in the formula
  21
3 Results
The 72 studies in the meta-analysis employed 10 different stimulus creation techniques and 53
different measures, 39 of which were affect measures and 14 of which were indirect measures.
In total, 61 studies included affect measures, and 23 included indirect measures. The studies
ranged in size from 10 to 1,311 participants with a median size of 64.5 and an interquartile range
of 34 to 203.5. Of the 249 measured effects, 85 involve the nonhuman side of the UV, 71 involve
the human side, and 93 involve both sides simultaneously.
The three-level meta-analysis model, including two outliers, revealed that the UV effect had a
large effect size, g = 0.95 [0.76, 1.14], p < .001, k = 249, Akaike information criterion (AIC) =
724.92, QE(248) = 10241.38, p < .001, QM(1) = 93.30, p < .001. Excluding the two outliers,
discussed below, increased the effect size, g = 1.01 [0.80, 1.22], p < .001, k = 247.
3.1 Three-level model
The meta-analysis often draws multiple effect sizes from the same paper and even from the same
study. Thus, the effect sizes are not statistically independent (Cheung, 2019). To address this, we
investigated different three-level models.
The model with the lowest estimated prediction error, excluding outliers, has paper as its higher-
order grouping variable and effect as its nested lower-order grouping variable, QE(246) =
9725.21, p < .001, QM(1) = 88.53, p < .001. The model has lower estimated prediction error
(paper/effect: AIC = 675.17) than the other three-level models (study/effect: AIC = 683.05,
technique/effect: AIC = 714.85, measure/effect: AIC = 715.20). Its prediction error is significantly
lower than two-level models (effect: AIC = 717.57, p < .001, paper: AIC = 4915.67, p < .001). Of
the total variance, 38.53% is between-paper heterogeneity, 60.34% is within-paper heterogeneity
(total I² = 98.87), and 1.13% is sampling error.
3.2 Bias
Figure 4(a) shows a funnel plot of effect sizes against their standard errors for meta-analysis.
Since standard error is inversely proportional to sample size, larger studies appear at the top and
smaller studies at the bottom. In the absence of bias, sampling error should distribute effect sizes
randomly but symmetrically about their weighted mean. In the funnel plot, however, the effect
sizes tend to increase with their standard errors. A regression test with standard error as the
predictor variable and Hedges’ g as the outcome variable indicated significant funnel plot
asymmetry (z = 6.72, p < .001, k = 249).
Funnel plot asymmetry could result from publication bias because the meta-analysis relied on
published data only. In general, studies reporting a significant effect are more likely to be
published. If a true effect exists, a smaller study will require a larger effect size to reach
significance. Moreover, given that large studies constitute a major commitment of resources, they
are more likely to be published even if their effects are nonsignificant.
One approach to addressing bias is to limit the meta-analysis to larger studies and then to check
whether bias is still present and whether the effect size is still large enough to be of substantive
importance (Borenstein et al., 2009). We tried a version of this approach by excluding the effects
with the largest standard errors and retesting for funnel plot asymmetry. After excluding 66
effectsthat is, 27% of the total, as shown in Figure 4(b)funnel plot asymmetry for the
remaining effects became nonsignificant (z = 1.95, p = .051, k = 183). The effect size, however,
was reduced 28%, g = 0.68 [0.51, 0.85], k = 183. Though smaller, it remains of substantive
Figure 4. The funnel plot graphs effect sizes from the meta-analysis against their standard
errors: (a) all standard errors; (b) the lowest 73% of standard errors. Influential effects are
indicated in red.
Figure 5. The p-curve for the meta-analysis’s 249 effects.
Bias was next assessed by p-curve analysis. A plot of p values against percentage of effects
should be flat if there is no effect and right skewed if there is one. A left skew indicates bias, a
publication environment in which obtaining significance at the .05 level is incentivized, but lower
p values are unnecessary. This could result from publication bias or from p-hacking, mining the
data for patterns and then failing to control for multiplicity in reporting significance. Of 249
effects, p ≤ .05 for 213 (86%), and p ≤ .025 for 207 (83%). The right-skewness test, pbinomial <
.001, zfull = 73.80, pfull < .001, zhalf = 72.50, phalf < .001, was significant, which indicates a true
effect (Figure 5). The flatness test was nonsignificant, pbinomial > .999, zfull = 65.35, pfull > .999, zhalf
= 69.70, phalf > .999; thus, the test did not indicate insufficient power or the absence of a true
effect. The power estimate is 0.99 [0.99, 1.00]. The tests were repeated, with similar results, for
only the 66 effects with the largest standard errors. Thus, p-curve analysis supports the conclusion
that the effect is true. It is not simply the result of publication bias or p-hacking.
3.3 Influential effects
Viechtbauer and Cheung (2010) proposed that an effect is influential if it meets one of the
following four criteria:
, (8)
where p is the number of model coefficients and k the number of effects, the Cook’s distance,
2, (9)
where p is the model’s degrees of freedom, indicating the deletion if the i’th effect decreases the
Mahalanobis distance between effects,
 3
, and any (10)
DFBETA 1. (11)
Figure 6. DFFITS and Cook’s D for the effects in the meta-analysis, sorted from lowest to
highest standard error. Influential effects are indicated in red.
Figure 7. Creation technique is the moderator variable in the meta-regression model. For each
of its values, Hedges’ g, the 95% confidence interval, and number of effects (k) are listed. The
position of the blue square depicts the effect size, and its relative size depicts the precision. The
width of the diamond depicts the confidence interval of the summary effect size.
Two effects were identified as influential by the first two criteria (Figure 6), and both pertained to
the UV’s nonhuman side: Rosenthal et al.’s (2014) unfamiliar-i, g = 2.95, DFFITS = 0.224, D
= 0.047, hat = 0.004, DFBETA = –0.224, and Wang et al.’s (2020) alive, g = 2.77, DFFITS =
0.205, D = 0.040, hat =0.004, DFBETA = 0.205. They were treated as outliers for reasons
discussed below and included in analyses selectively.
3.4 Independent variable operationalizations
3.4.1 Moderator: Creation techniques
Moderation analysis was performed, excluding outliers, using a mixed-effects meta-regression
model with effect as the random variable and creation technique as the moderator variable, AIC =
701.33, QE(237) = 8984.08, p < .001, τ² = 0.91, I² = 98.62, QM(10) = 272.53, p < .001. Face
distortion produced the largest effect size, followed by distinct entities, realism render, and
morphing (Figure 7).
Distinct entities studies typically used stimuli that could have confounding effects (e.g., body
language, facial expressions, lighting, viewing perspective). To reduce their risk, a few studies
applied standards for stimulus selectionfor example, full face shown in frontal or three-fourths
aspect, resolution sufficient to generate a final image three inches in height at 100 dpi, and no
other body parts visible (Brink, Gray, & Wellman, 2017; Mathur & Reichling, 2016). When only
distinct entities studies with standardized stimuli were considered, three in total, g fell to 0.82 [
0.12, 1.77], k = 4, and the effect became nonsignificant, p = .089.
Four studies used nonhuman animal stimuli, AIC = 32.95, QE(17) = 373.46, p < .001, QM(1) =
32.95, p < .001 (MacDorman & Chattopadhyay, 2017; Schwind et al., 2018; Yamada et al.,
2013). Their 18 effects were all significant, g = 1.94 [1.28, 2.60], k = 18. Stimulus
operationalization techniques for animal stimuli were comparable with those for human stimuli,
including distinct entities (Rativa et al., 2020; Takahashi et al., 2015), emotion manipulation, face
distortion, realism render (Chattopadhyay & MacDorman, 2016; Schwind et al., 2018), and
morphing (Yamada et al., 2013).
Figure 8. Side of the uncanny valley is the moderator variable in the meta-regression model.
3.5 Dependent variable operationalizations
3.5.1 Moderator: Side of the uncanny valley, valence, and type of measure
Moderation analysis was performed, including outliers, with effect as the random variable and
side of the valley as the moderator variable, AIC = 731.92, QE(246) = 9942.04, p < .001, τ² =
1.00, I² = 98.80, QM(3) = 239.92, p < .001. If possible, an effect size was calculated for each side
of the uncanny valley. However, this was not possible for 37% of effect sizes, usually because the
means and standard deviations were not reported. In these cases, a combined effect size for both
sides of the valley was calculated (e.g., based on an F statistic). For the human side, g = 1.34
[1.10, 1.57], p < .001, and k = 71, for the nonhuman side, g = 0.64 [0.43, 0.86], p < .001, and k =
85, and for both sides, g = 0.98 [0.77, 1.19], p <.001, and k = 93. Thus, the effect size for the
human side was more than double that of the nonhuman side.
To investigate this disparity, we repeated the analysis with side × valence (positive or negative) ×
measure type (affect or indirect) as the moderator variable (Figure 8). The combined value human
positive affect had the largest affect size, g = 1.69 [1.34, 2.03], p < .001, k = 32 and nonhuman
positive affect had the smallest. Thus, among all measures, positive affect measures were the most
effective at measuring the human side of the valley and the least effective at measuring the
nonhuman side. A Wald-type test revealed this difference in effectiveness was significant,
QM(12) = 276.73, p < .001. For the human side, affect measures were more effective than
indirect measures. For the nonhuman side, indirect measures were more effective than affect
measures, and negative measures were more effective than positive ones.
3.5.2 Moderator: Affect measures
Moderation analysis was performed, excluding outliers, with effect as the random variable and
affect measure as the moderator variable, AIC = 537.05, QE(159) = 4544.64, p < .001, τ² = 0.92,
I² = 98.51, QM(38) = 247.70, p < .001 (Figure 9). Indices producing effects that were larger than
average include threatening-i (threatening, eerie, uncanny, dominant, harmless), likable-i
(pleasant, likable, attractive, familiar, natural, intelligent), aesthetics-i (uglybeautiful,
unaestheticaesthetic), familiarity-i (uncannyfamiliar, freakynumbing), and eeriness-i (dull
freaky, predictableeerie, plainweird, ordinarysupernatural, boringshocking, uninspiring
spine-tingling, predictablethrilling, blanduncanny, unemotionalhair-raising). Individual items
include reassuring, threatening, believable, appealing, acceptable, alive, and eerie. However,
when the two outliers are included, alive falls from the 12th highest effect size, g = 1.19 [0.33,
2.06], p = .007, k = 5, to the 29th, g = 0.55 [0.27, 1.37], p = .191, k = 6, and is no longer
significant. The other outlier, unfamiliar-i (strange, unfamiliar) appears last, g = 2.95 [4.94,
0.95], p = .004, k = 1.
3.5.3 Indices and multiple scale analyses
A variety of terms have been used to measure different constructs underlying the UV effect. The
relations among the terms can give insight into the UV effect’s experiential quality. In studies
with several terms, we investigated their intercorrelations to determine whether they reflect the
UV effect or instead a related construct. Table A1 in the Appendix lists the interscale correlations
observed in the reviewed research.
Figure 9. Affect measure is the moderator variable in the meta-regression model. Creepy*
combines the item creepy with scales including the term, such as creepypleasant and creepy
As a measure of reliability, 15 studies in the meta-analysis reported the Cronbach’s α of the
indices used. Ho and MacDorman’s (2010, 2017) eeriness and warmth indices and their
derivations were generally reliable. Distinctive experience terms (e.g., creepy, eerie, and
uncanny) tended to load on the same factor (e.g., Destephe et al., 2015; Lischetzke et al. 2017). In
a principal component analysis (PCA), the items uncanny and eerie loaded on the same
component as threat-related items, and the items strange and unfamiliar as anxiety-related items
(Rosenthalvon der Pütten & Krämer, 2014; Ho, MacDorman, & Pramono, 2008, found fear and
disgust to be stronger predictors of eerie and creepy than anxiety). In a similar vein, removing
strange from an index consisting of eerie, unsettling, and strange improved its reliability (Kätsyri,
Mäkäräinen, & Takala, 2017). This indicates uncanniness and strangeness may be different
Finally, likable, friendly, pleasant, and other warmth items typically comprise reliable indices
(e.g., Kätsyri, Mäkäräinen, & Takala, 2017; Rosenthalvon der Pütten & Krämer, 2014; Tung,
2016), which indicates an interpersonal warmth construct for the tested stimuli (e.g., Bartneck et
al., 2009a).
Figure 10. Indirect measure is the moderator variable in the meta-regression model.
3.5.4 Moderator: Indirect measures
Moderation analysis was performed, excluding outliers, with effect as the random variable and
indirect measure as the moderator variable (Figure 10). Dislike frequency, which indicates the
number of times disliked, had the largest effect size (Strait et al., 2019), followed by
categorization reaction time (Carr et al., 2017; Cheetham & Jäncke, 2013; MacDorman &
Chattopadhyay, 2017; Wang & Rochat, 2017; Yamada et al., 2013), like frequency (Strait et al.,
2019), avoidance behavior attributions to uncanniness (Perez et al., 2020), viewing duration
(Strait et al., 2015, 2019), preference choice in a two-alternative forced-choice categorization task
(Feng et al., 2018; Prakash & Rogers, 2015), and preferential looking, that is, preferring to view
one stimulus more than another (Matsuda et al., 2015; Nitta & Hashiya, 2021).
Nonsignificant effect sizes include lie detection, that is, frequency of rating a statement as a lie
(McDonnell & Breidt, 2010), cognitive conflict, operationalized as number of reversals of
direction when moving a stimulus with a mouse pointer towards one of two categories (Weis &
Wiese, 2017), trust behavior, specifically the amount of money entrusted with an entity in an
investment game (Mathur & Reichling, 2016), encounter duration, that is, viewing duration until
the participant terminates the encounter (Perez et al., 2020), termination frequency, measured by
the number of times terminated (Perez et al., 2020; Strait et al., 2015, 2017, 2019), information
processing about an entity, as indicated by the number of personality judgments made (Shin,
Kim, & Biocca, 2019), and ABX task, which entails visual samedifferent discriminations
(Cheetham et al., 2014).
Figure 11. Construct is the moderator variable in the meta-regression model.
3.6 Other constructs
After grouping measures by other UV construct, moderation analysis was performed, excluding
outliers, with effect as the random variable and other construct as the moderator variable, AIC =
386.28, QE(122) = 2999.63, p < .001, τ² = 1.02, I² = 98.29, QM(10) = 119.67, p < .001 (Figure
11). Animacy and experience had the largest effect size, g = 1.26 [0.44, 2.09], p = .003, k = 6.
However, if outliers are included, this construct falls from first to eighth and becomes
nonsignificant, g = 0.70 [0.10, 1.51], p = .088, k = 7. Other constructs with significant effects, in
decreasing order of effect size, were aesthetics, interpersonal warmth, distinctive experience,
threat, trust, anomaly, and disgust. General anxiety and familiarity had nonsignificant effects.
3.7 Papers
For reference, a moderation analysis was performed, excluding outliers, with effect as the random
variable and paper as the moderator variable, AIC = 585.95, QE(191) = 5058.35, p < .001, τ² =
0.61, I² = 98.05, QM(56) = 552.95, p < .001 (Figure 12).
3.8 Data availability
The meta-analysis was performed in the R statistical computing environment with the metafor
package. The p-curve analysis and variance distribution analysis of the three-level model were
performed with the dmetar package. The remaining R packages were devtools, forestplot,
ggplot2, and readxl. The dataset, R script, and other supplementary materials are available at
Figure 12. Paper is the moderator variable in the meta-regression model.
4 Discussion
4.1 Independent variable operationalizations
Among all the stimulus creation techniques, face distortion produced the largest effect size,
followed by distinct entities, realism render, morphing, voice distortion, and motion
manipulation. Techniques producing a nonsignificant effect include mismatch, visuo-auditory
mismatch, emotion manipulation, and real-life encounter, though real-life encounter was based
on only one paper. Nonhuman animal stimuli performed well. Our evaluation of stimulus creation
techniques is summarized in Table A2 of the Appendix.
Face distortion was only tested in four of the papers reviewed (Feng et al., 2018; MacDorman et
al., 2009; Mäkäräinen et al., 2014; Schwind et al., 2018). Nevertheless, it is a promising
technique to explore configural processing theories (Diel & MacDorman, 2021).
Distinct entities were used in 46% of significance tests (114 out of 249), more than any other
technique. This creation technique has greater ecological validity than all techniques exceptat
least for robotsreal-life encounter. However, stimuli in these studies typically varied in body
language, facial expression, familiarity, gaze direction, lighting, perspective, and other aspects.
These potential confounding variables indicate a lack of experimental control, which could limit
the generalizability of the results (Kätsyri, Förger, Mäkäräinen, & Takala, 2015; Kätsyri, de
Gelder, & Takala, 2019). This interpretation aligns with our results. When the moderation
analysis was limited to studies using standardized stimuli, distinct entities produced a
nonsignificant effect.
Although morphing produced a large effect size in the meta-analysis, it was nonsignificant for 8
out of 44 effects. Nonsignificance may stem from the choice of endpoint stimuli. Studies that did
not find a UV effect used endpoint stimuli with the same shape, such as a human face and a
matching avatar face (Cheetham et al., 2015; Kätsyri, de Gelder, & Takala, 2019; the same issue
arises for realism render, MacDorman & Chattopadhyay, 2016). By contrast, studies that did find
a UV effect used morphologically different endpoint stimuli to produce a robot-to-human,
animal-to-human, or cartoon-to-real transition (Ferrey et al., 2015; Lischetzke et al., 2017;
Palomäki et al., 2018; Sasaki, Ihaya, & Yamada, 2017).
Creating stimuli from insufficiently distinct endpoint images may result in a morphing sequence
with too narrow a range in human likeness to include the uncanny valley part of the graph. For
example, although animals and robots have facial proportions that are atypical for humans, they
are not judged by human standards. Morphing them with human faces may elicit human-specific
processing, heightening sensitivity to those features that still deviate from human proportions,
thus eliciting the UV effect. This effect could not occur if the facial proportions of the low human
likeness endpoint stimuli were already human (e.g., human avatars). Thus, it is possible that, for
morphing stimuli to elicit a UV effect reliably, they must distort an entity’s configural pattern,
which would support theories predicting the UV effect results from configural processing
(Chattopadhyay & MacDorman, 2016; Diel & MacDorman, 2021; Kätsyri, 2018).
Alternatively, the large effect sizes for endpoint stimuli that differ greatly in their morphology
may be an unintended consequence of the creation technique. Endpoint stimuli like robots and
dolls tend to be attractive because they are the product of design. Human beings, though not
designed, tend to find each other attractive because their faces and bodies co-evolved with their
perceptual systems. In this context, attractiveness serves a purpose: It supports mate bonds and
parental bonds (see Kozak, Head, Lackey, & Boughman, 2013; Wyman, Charlton, Locatelli, &
Reby, 2011). However, intermediate stimuli in a morphing sequence neither evolved nor were
designed to be perceived as anything. This arbitrariness could heighten their uncanniness.
We advise researchers to avoid using similar endpoint images when creating stimuli through
morphing, or to use such techniques as morphing different regions of the face in different
morphing steps (Seyama & Nagayama, 2007). However, it is also important to avoid creating
strange or ghostly artifacts that could appear eerie for reasons other than their being intermediate
in human likeness (discussed in MacDorman & Chattopadhyay, 2016). The effect of endpoint
stimulus choice on the UV effect is a topic for investigation.
In their review, Wang, Lilienfeld, and Rochat (2015) found evidence against the UV effect comes
from studies using distinct entities, while evidence for the UV effect comes from studies using
morphing. The reason is perhaps that Wang and colleagues cited studies our analysis excluded for
not using a test statistic (Hanson et al., 2005) or for having image noise (e.g., one face with two
sets of hair, Seyama & Nagayama, 2007). In addition, several distinct entities studies with
supportive results were published after their review (Brink, Gray, & Wellman, 2017; Jung & Cho,
2018; Kätsyri, de Gelder, & Takala, 2019; Mathur & Reichling, 2016; Mathur et al., 2020;
Palomäki et al., 2018; Strait et al., 2017).
Finally, Wang, Lilienfeld, and Rochat (2015) criticizes using face distortion as an independent
variable because face distortion differs from human likeness. However, our review found face
distortion can elicit UV-specific subjective experiences (e.g., Mäkäräinen et al., 2014). Moreover,
our meta-analysis found a significant UV effect in perceiving animal stimuli (e.g., Löffler et al.,
2020; Schwind et al., 2017, 2018). Thus, human likeness alone cannot predict the range of
observed UV effects. A more encompassing DV conceptualization, like norm deviation, would
predict a broader range of UV effects. However, norm deviation is not necessarily uncanny.
Sometimes it does harm aesthetics but rather improves it (e.g., supernormal stimuli, Diel &
MacDorman, 2021).
4.2 Dependent variable operationalizations
The effect size of the uncanny valley’s human side was more than double that of its nonhuman
side. This difference may seem to reflect Mori’s graph because the second peak is higher than the
first. However, we also noted that, among all measures, positive affect produced the largest effect
sizes for the human side and the smallest for the nonhuman side. Thus, a more plausible
explanation is that positive affect is a poor measure of the UV effect.
Setting aside the miraculous and the extraterrestrial, people tend to perceive human beings as
superior to nonhuman entities. This applies to stimuli appearing in UV experiments to date, such
as robots, animals, and dolls. Perceived limitations in present-day human artifacts or other species
reinforce our ingroup bias, rooted in our common identity, to privilege the human (MacDorman
& Entezari, 2015; Mitchell et al., 2011a). Humans are often seen as more appealing, attractive,
friendly, likable, pleasant, reassuring, and warm than nonhuman alternatives, not to mention more
cultured, intelligent, and sociable. We can immediately see why positive affect measures are poor
for measuring the UV effect because, despite how uncanny an android may appear, it will still
appear more lifelike and less unfamiliar than a mechanical-looking robot of a novel design. Thus,
it is important to focus on effective measures for the uncanny valley’s nonhuman side: negative
affect measures and positive indirect measures.
The effectiveness of negative affect measures like eerie, creepy, threatening, and disgusting align
with the view that the UV effect is characterized by a distinctive experience of uncanniness rather
than an overall decrease in positive affect (e.g., Ho, MacDorman, & Pramono, 2008; Mangan,
2015; Redstone, 2013). This negative experience may still reduce positive affect, though
indirectly (Patrick & Lavoro, 1997).
The most frequently used item was eerie (e.g., Ho & MacDorman, 2010, 2017; Kätsyri, de
Gelder, & Takala, 2019). Other negative items included creepy, disgusting, repulsive, strange,
threatening, and weird. Concordantly, positive items with the largest effect sizes were
nonspecific, such as interpersonal warmth items (likable, pleasant) or familiar (e.g., MacDorman
& Ishiguro, 2006). Despite a correlation between the UV effect and feelings of disgust (e.g., Ho,
MacDorman, & Pramono, 2008; MacDorman & Entezari, 2015), the item repulsive was
Among indirect measures, dislike frequency produced the largest effect size, followed by
categorization RT, like frequency, avoidance, viewing duration, preference choice, and
preferential looking. Indirect measures, such as performance measures, are not without their
limitations. Although some research uses performance measures to quantify a construct related to,
but distinct from, the UV effect, other research claims they measure the UV effect itself (e.g.,
Lewkowicz & Ghazanfar, 2012; Matsuda et al., 2015). Measures like preferential looking and
preference choice reflect general avoidance behavior, which could be elicited by the UV effect or
by extraneous factors that must be controlled for, such as an ugly appearance or inhospitable
disposition. Furthermore, most studies measuring performance omitted affect. Those that
measured it tended to find a UV effect for affect but not for performance (Strait et al., 2015;
Strait, Urry, & Muentener, 2019; for the opposite case, see Wang & Rochat, 2017).
These findings point to broader issues with measurement in UV research: First, many studies do
not measure affect, but they should endeavor to do so insofar as it is possible. It is better to avoid
relying solely on task performance measures (e.g., categorization RT, Cheetham, Suter, & Jäncke,
2011; Cheetham et al., 2013; Cheetham, Suter, & Jäncke, 2014; Chen, Russell, & Nakayama,
2010; Saygin, Chaminade, Ishiguro, Driver, & Frith, 2012; avoidance or preference, Lewkowicz
& Ghazanfar, 2012; Matsuda et al., 2012; Steckenfinger & Ghazanfar, 2009). The reason is that
we cannot infer affect and its influence on motivation solely from nonaffective behavior, though
we can code it from displays of emotion. For example, in a study that used termination frequency
to measure the UV effect, “the stimulus was boring” had a larger effect size than “the stimulus
was unnerving” (Strait et al., 2015; Strait, Urry, & Muentener, 2019). However, boring has never
been considered the dependent variable in Mori’s graph. In addition, task performance measures
can diverge from affect measures (MacDorman & Chattopadhyay, 2016, 2017; Mathur et al.,
2020). Research should aim to validate performance measures by testing their specificity for the
UV effect.
Second, although likability, pleasantness, and other nonspecific items used to measure overall
affect tend to correlate with UV-specific items, they do not capture the experiential quality of the
UV effect. Thus, unrelated factors could cause them to increase or decrease. This makes
nonspecific items more susceptible to confounding variables. Perceptual variables that can
influence stimulus evaluation include attractiveness (Ho & MacDorman, 2010, 2017; Principe &
Langlois, 2011), atypical (Kätsyri et al., 2015; Strait et al., 2017), disgusting (Curtis, de Barra, &
Aunger, 2011), or misaligned features (MacDorman & Chattopadhyay, 2016), background
(Łupkowski et al., 2019), color (Kennedy, 2014; Valdez & Mehrabian, 1994), morphing artifacts
(MacDorman & Chattopadhyay, 2016), realism (McDonnell et al., 2012), and size (Cesarei &
Codispoti, 2006). These variables tend to be automatic and stimulus-driven. Perceptual-cognitive
variables include categorization difficulty (Cheetham et al., 2013; Yamada et al., 2013),
expectation violation (Saygin et al., 2012), frequency (Burleigh & Schoenherr, 2015; Moreland &
Zajonc, 1982), inhibitory devaluation (Ferrey, Burleigh, & Fenske, 2015; Weis & Wiese, 2017),
and multimodal mismatch (Mitchell et al., 2011b; Tinwell et al., 2015). Social variables include
animacy (Koldewyn, Hanus, & Balas, 2014; Mäkäräinen et al., 2014), context (Jung & Cho,
2018), facial expressions (Paulus & Wentura, 2015; Tinwell et al., 2011), mind (Gray & Wegner,
2012), narrative structure (MacDorman, 2019), outgroup membership (Hugenberg, 2005), and
perceived warmth or competence (MacDorman, 2019). Thus, studies should include UV-specific
measures to mitigate potential confounds.
Third, even when UV-specific measures are used, they can be influenced by the flow of the
interaction and its narrative structure (Dai & MacDorman, 2018). Thus, it may be necessary to
test for the UV effect before the interaction begins.
Fourth, the UV effect is correlated with fear, anxiety, and disgust (Ho, MacDorman, & Pramono,
2008). Thus, a UV measure should be able to discriminate UV stimuli from non-UV stimuli that
elicit similar emotions. However, discriminant validity has not yet been demonstrated for a UV
Fifth, regardless of the strength of a change in affect, at least three stimulus conditions are
necessary to produce measurements that could fit a U-shaped curve—the valley part of Mori’s
graph. Even if those measurements fit, a dip in a measure like interpersonal warmth could occur
for a myriad of reasons other than the UV effect. Thus, experimental control is vital.
Sixth, what eeriness is and which situations elicit it has not been specified precisely. Redstone
(2013) proposed that eeriness is elicited when the ontological nature of a stimulus is unclear.
Langer and König (2018) differentiate between eeriness (which they assert is a fear-related
response to humanoid entities) and creepiness (an anxiety-related response to novel or
unpredictable people or situations). However, these claims are untested. In general, UV research
lacks a common definition and conceptualization of the UV effect.
4.3 Limitations
4.3.1 Study exclusion
This meta-analysis excluded a wide range of impactful UV studies that were never intended to
replicate a UV curve. For example, Gray and Wegner (2012) found the UV effect was elicited by
a conscious machine or the philosophers’ zombie (a person lacking conscious experience). Their
findings were replicated by Appel and colleagues (2020). Schein and Gray (2015) found that,
among facial features, the UV effect was especially sensitive to the manipulation of the eyes. The
review also excluded specific subgroups and nonhuman primates. For example, Steckenfinger
and Ghazanfar (2009) found a UV effect in macaque monkeys. The meta-analysis also excluded
studies on the neurophysiological correlates of the perception of humanlike appearance or
behavior, which shed light on the neural mechanisms underlying the UV effect (e.g., Saygin et
al., 2011; Urgen et al., 2018).
The meta-analysis excluded interaction effects for simplicity. However, these effects have
elucidated the UV effect. For example, Green and colleagues (2008) found an interaction between
the degree of face distortion and realism render by showing that sensitivity to acceptable facial
proportions increased as the stimulus appeared more human. Similarly, Mäkäräinen and
colleagues (2014) showed that the strangeness of faces with exaggerated expressions increased as
faces were rendered more realistically. Both studies indicate realism increases the perceiver’s
sensitivity to human features. Thus, deviations from norms are more likely to be noticed and
perceived as uncanny in realistic representations. Sensitivity increases with realism logistically
(S-shaped curve), not linearly, indicating a perceptual magnet effect (Chattopadhyay &
MacDorman, 2016) like the one found for animacy (Looser and Wheatley, 2010). In a similar
vein, Deska and colleagues (2017) found that the perception of a mind occurs when a face
appears nearly human and is processed configurally (cf. Gray & Wegner, 2012; Tinwell et al.,
Smaller studies, which require a larger effect size to obtain significance, tended to have larger
effect sizes in our meta-analysis. Specifically, the average effect size of smaller studies, those in
the quartile with the largest standard errors, was more than double that of the other three quartiles.
Typically, inflated effect sizes in smaller studies are explained by publication bias or p-hacking.
Publication bias results from unpublished or unreported nonsignificant effects in a meta-analysis,
and p-hacking is the failure to control for multiplicity in significance testing. However, p-curve
analysis found no signs of publication bias or p-hacking.
Twenty-six of 98 studies that met selection criteria, including significance testing, were excluded
from the meta-analysis because they provided insufficient information to calculate effect sizes.
This issue arose mainly for nonsignificant effect sizes. Nevertheless, the field has shown interest
in nonsignificant and contrary effects, and papers reporting them have been well-cited (e.g.,
Cheetham, Suter, & Jäncke, 2014; Thompson, Trafton, & McKnight, 2011). Because this paper
focuses on comparing methodologies, bias affecting relative comparisons between effect sizes is
more worrisome than bias affecting their absolute magnitude.
4.3.2 Diverse methodologies
The diversity of UV methodologies impeded the meta-analysis. The volume of IVDV
combinations complicated the interpretation of effect sizes for creation techniques and for
measures, especially for IVDV combinations used in only a few studies. Precision in meta-
regression requires having enough combinations in each cell. At least five is one rule of thumb
(Borenstein et al., 2009). However, three of 10 techniques, 23 of 39 affect measures, and 12 of 14
indirect measures were used in fewer than five studies. The variety of experimental designs and
other study-specific variables also complicates interpretation of the results. To draw conclusions
about techniques and methods simultaneously requires enough significance tests or effect sizes to
make comparisons (Lay, Brace, Pike, & Pollick, 2016). Future research could give priority to the
validation of rarely used methods.
5 Conclusion
This is the first meta-analysis on the UV effect. We used meta-regression to evaluate the methods
used to operationalize the axes of Mori’s graph. Our findings provide a methodological
foundation for UV research. After discussing the conceptual foundations of the uncanny valley,
we have presented successful research methodologies and raised methodological concerns.
5.1 Recommendations
We end by proposing the following design principles for stimulus creation techniques and
measures in UV research:
Items that measure the UV experience as a distinct experience of uncanniness, such as uncanny
and eerie, or of strangeness, such as weird or strange, are preferred to nonspecific items. They
also have face validity. In this vein, negative items are preferred to positive ones. Negative items
can always be reverse scaled to plot the valley.
Affect or preference measures are necessary to assess the UV effect. Although indirect measures
may complement them, a study should not rely solely on indirect measures, if possible. The
validity of performance measures warrants further investigation.
The stimulus creation techniques producing the largest effect sizes were face distortion, distinct
entities, realism render, and morphing.
A drawback of morphing is that, if the endpoint images are too similar, the x-axis may not include
the uncanny valley. Morphing that disrupts the configural pattern may produce a larger effect;
however, it should avoid creating visual artifacts from the morphing process. How best to
approach morphing is a topic for future research.
Useful stimulus creation techniques include distorting facial features, rendering at different
realism levels, and using different emotional expressions. Their choice depends on theoretical
considerations and the research question. Further investigation is needed on realism rendering
and how it influences UV-specific negative measures compared with nonspecific positive
When using distinct entities, researchers should apply standards for stimulus selection (e.g.,
similar size, perspective, facial expression, and lighting). The effect of stimulus standardization
on the UV effect also warrants investigation.
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Table A1. Indices and Cronbach’s α’s of UV studies.
Authors (year)
[study no.]
Indices: separate scales
UV effect
Cronbach’s α
per condition
Bartneck et al.
Likability: awfulnice,
unfriendlyfriendly, unkind
kind, and unpleasant
.92, .88, .84
Destephe et al.
Eeriness: eeriereassuring,
supernaturalordinary, spine-
thrillingboring, mortal
predictable, uncannybland,
and hair-raisingunemotional
Ho &
Eeriness: dullfreaky,
predictableeerie, plain
weird, ordinarysupernatural,
predictablethrilling, bland
uncanny, and unemotional
Ho &
Eeriness: reassuringeerie,
numbingfreaky, ordinary
supernatural, and
Warmth: cold-hearted
warm-hearted, hostile
friendly, spitefulwell-
intentioned, ill-tempered
good-natured, and grumpy
Mäkäräinen, &
Takala (2017)
Likable: likable, aesthetic,
and pleasant
Eerie: eerie and unsettling
Eerie: eerie, unsettling, and
Lischetzke et
al. (2017)
Index: creepy, eerie, and
MacDorman &
Eeriness: ordinarycreepy,
plainweird, and
Realism render
Warmth: cold-hearted
warm-hearted, hostile
friendly, and grumpy
Mitchell et al.
Eeriness (see Ho &
MacDorman, 2010)
Warmth (see Ho &
MacDorman, 2010)
der Pütten &
Krämer (2014)
Threatening: threatening,
eerie, uncanny, dominant,
and harmless
Likable: pleasant, likable,
attractive, familiar, natural,
and intelligent
Submissive: incompetent,
weak, and submissive
Unfamiliar: strange and
Schwind et al.
Familiarity: uncanny
familiar and freakynumbing
entities (cats)
Aesthetics: uglybeautiful
and unaestheticaesthetic
Shin, Kim, &
Biocca (2019)
Eeriness: reassuringeerie,
numbingfreaky, and
Realism render
Stein & Ohler
Eeriness (n.a.)
face distortion,
realism render,
Tinwell et al.
Uncanniness: eerie,
nonhumanlike, repulsive,
unattractive, unlikable, and
.74, .80, .80
Tung (2016)
Social attraction: friendly,
likable, and pleasant
Yes [1]
No [2]
≥ .70
Zlotowski et
al. (2015)
Eeriness (n.a.)
.62 (lowest of
Note. Eeriness and Warmth denote the indices developed by Ho and MacDorman (2010,
2017) and their derivations. We did not find studies with information on correlations
between individual scale items.
Table A2. Summary and evaluation of stimulus creation
Distinct entities
Mathur et al.,
der Pütten &
Krämer, 2014
Relatively high
control, easy
variables, no
gradual range
Additional control
when selecting
stimuli can
Tinwell et al.,
stimulus noise
Face distortion
Mäkäräinen et
al., 2014
MacDorman et
al., (2009)
gradual range
Stimulus noise
Strength of
distortion should
have a sufficient
Lischetzke et
al., 2017
Sasaki, Ihaya, &
gradual range
Results depend on
endpoint stimuli
choice, stimulus
Endpoint stimuli
should be
Seyama &
Stimulus noise, no
gradual range
Selection of
features (e.g.,
Lack of research
Handzic &
Reed, 2015
Lack of research
Realism render
McDonnell et
al., 2012
MacDorman &
Stimulus noise
Zlotowski et al.,
Ishiguro, &
Hagita, 2009
High ecological
validity for
android science
Low internal
validity, difficult
setup and stimulus
and human
counterpart stimuli
should match
Lack of research
Mitchell et al.,
Lack of research
Voice distortion
Baird et al.,2018
Lack of research
... The UV is one of the most discussed ideas in the literature, having received attention from scholars across various fields, not just in humancomputer interaction, but also psychology, philosophy, culture studies, and design. Many experimental studies tested the hypothesis of anthropomorphic realism having a counterproductive effect on observers (see Kätsyri et al., 2015 andDiel et al., 2021 for reviews). However, the empirical basis of the UV has mostly relied on subjective measures. ...
... Not all experimental work found evidence supporting the valley-shaped curve predicted by the UV theory in the results. In fact, results have been mixed, with some studies reporting the UV shape (Diel et al., 2021), whereas others reporting a different type of relationship, including a linear one (Kätsyri et al., 2019) .Consequently, Kätsyri et al. (2019) proposed three different shapes for the relationship between the perception of human-likeness and affinity. A distinction was made between a 'strong uncanny valley' and a 'weak uncanny valley', as well as an 'uncanny slope' (i.e., a positive linear relationship between human-likeness and affinity, whereby increasing human-likeness entails increasing affinity) (Fig. 2a). ...
... The shapes for the human-likeness and affinity relationships presented in Fig. 2 are beneficial for making sense of the observed results across different studies. It is possible that depending on the stimuli used and the method to create the stimuli (e.g., facial feature distortion and morphing), the UV shape varies (Diel et al., 2021). For example, Kätsyri et al. (2019) showed that computer-generated faces elicited only a weak UV. ...
Full-text available
The Uncanny Valley (UV) theory predicts that imperfectly human-like artificial agents elicit negative reactions in perceivers. While to date most studies investigating the UV have been behavioral, there is a growing number of neuroscientific studies that hold the potential of shedding light on the automatic processes related to the UV. The current paper provides a scoping review of studies using brain imaging techniques that addressed the UV. Of the total of 74 studies found in the database search, 13 met the inclusion criteria and compared the neural processing of human vs. artificial agent stimuli. Neural differences were found when processing the faces of humans and artificial agents, with reduced responses for the latter in a face-selective brain region, the fusiform face area. At the temporal level, specific event-related potential (ERP) components were susceptible to facial appearance, such as the Late Positive Potential. The studies that employed mentalizing, i.e., reasoning about other agents’ behavior, showed that different brain regions of the mentalizing network were engaged, with the temporo-parietal junction being more responsive to humans, while the ventromedial prefrontal cortex and the precuneus were more responsive when reasoning about artificial agents. Some commonalities were also observed: the processing of human and artificial agent actions activated comparable brain areas in the sensorimotor cortex. Not only does this scoping review shed light on the neural processes that may underlie the UV, but it also allows for generating predictions with respect to processing differences regarding human and artificial agents.
... The affective or perceptual component of the uncanny valley has been described as a specific sensational response related to eeriness, creepiness, strangeness, and coldness [2,[30][31][32][33]. Whereas a variety of measures and interpretations of the uncanny valley's affective component exist, a recent meta-analysis on the uncanny valley's methodology suggests that specific anxiety-related semantic items like eerie, creepy, and uncanny, or anomaly-related items like strange and weird to be effective measures to capture the effect [31]. ...
... The affective or perceptual component of the uncanny valley has been described as a specific sensational response related to eeriness, creepiness, strangeness, and coldness [2,[30][31][32][33]. Whereas a variety of measures and interpretations of the uncanny valley's affective component exist, a recent meta-analysis on the uncanny valley's methodology suggests that specific anxiety-related semantic items like eerie, creepy, and uncanny, or anomaly-related items like strange and weird to be effective measures to capture the effect [31]. This study will focus on the experiences of uncanniness and abnormality and their proposed causes. ...
... While some researchers have proposed that cognitive disfluency underlies the uncanny valley, possibly caused by categorization difficulty [41][42][43], categorization confusion or difficulty and uncanniness ratings follow different trajectories across a range of stimuli varying on the degree of human likeness [1,44]. Furthermore, some researchers have argued that general cognitive theories like disfluency or dissonance are insufficient in explaining the uncanny valley as they have not been related to specific sensations of eeriness or uncanniness in previous research [7,31]. ...
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Humanlike entities deviating from the norm of human appearance are perceived as strange or uncanny. Explanations for the eeriness of deviating humanlike entities include ideas specific to human or animal stimuli like mate selection, avoidance of threat or disease, or dehumanization; however, deviation from highly familiar categories may provide a better explanation. Here it is tested whether experts and novices in a novel (greeble) category show different patterns of abnormality, attractiveness, and uncanniness responses to distorted and averaged greebles. Greeble-trained participants assessed the abnormality, attractiveness, uncanniness of normal, averaged, and distorted greebles and their responses were compared to participants who had not previously seen greebles. The data show that distorted greebles were more uncanny than normal greebles only in the training condition, and distorted greebles were more uncanny in the training compared to the control condition. In addition, averaged greebles were not more attractive than normal greebles regardless of condition. The results suggest uncanniness is elicited by deviations from stimulus categories of expertise rather than being a purely biological human- or animal-specific response.
... The term uncanny valley describes negative emotional appraisal of near humanlike entities compared with less humanlike entities or humans (Mori, 2012). In uncanny valley research, the effect of manipulating a stimulus' human likeness or realism (which is often measured on rating scales) is plotted against affective responses towards the stimulus (Diel, Weigelt, & MacDorman, 2022). Affinity increases with human likeness until it dips into the negative and increases back to the positive at fully human likeness, producing a N-shaped function. ...
... Affinity increases with human likeness until it dips into the negative and increases back to the positive at fully human likeness, producing a N-shaped function. The negative emotional experience has been described as eeriness, creepiness, or uncanniness (Diel et al., 2022;Ho & MacDorman, 2010, 2017Mangan, 2015). The effect is not specific to human entities: artificial animals (Löffler, Dörrenbächer, & Hassenzahl, 2020;Schwind, Leicht, Jäger, Wolf, & Henze, 2018) and manipulations of realistic animals (Diel & MacDorman, 2021;Yamada, Kawabe, & Ihaya, 2012) elicit observable uncanny valleys. ...
... First, plotting uncanniness against place realism should create a quadratic (U-shaped) or cubic (N-shaped) function (uncanny valley hypothesis) akin to previous uncanny valley research (Diel et al., 2022). ...
Full-text available
Certain built environments can decrease aesthetic appeal. For humans and objects, deviation from typical appearances leads to nonlinear appraisal characterised as the uncanny valley. The first time, it was explored whether an uncanny valley can be found for built environments. In Experiment 1, a cubic N-shaped function of uncanniness plotted against realism of built environments was found, indicating an uncanny valley. Quantitative and qualitative data indicate an association between uncanniness and structural anomalies. Experiment 2 explored distortions leading to uncanniness of indoor places. In Experiment 3, human presence decreased uncanniness of distorted indoor public places but increased uncanniness of private rooms. Taken together, the evidence indicates that deviations from familiar configural patterns drive uncanniness of built physical places. Thus, strong deviations from a built environment's predictable pattern decreases its aesthetic appeal.
... Recent advancement of robotics technologies and the proliferation of robots in societies have spurred abundant research linking human-like appearance of robots to various domains of psychology, Human-Robot Interaction (HRI), and Human-Computer Interaction (HCI), including perceptual (Martini et al., 2016;Mathur et al., 2020;Powers & Kiesler, 2006;, cognitive (Gray & Wegner, 2012;Rosenthal-von der Pütten & Krämer, 2015;Zhao, Cusimano, & Malle, 2016), and behavioral domains (Haring, Watanabe, Silvera-Tawil, Velonaki, & Matsumoto, 2015, May;2021. Amidst these interests in the influence of robot human-likeness on people's perceptions, the topic of whether the uncanny valley exists has received much attention (Diel, Weigelt, & MacDorman, 2022;Fink, 2012;Kätsyri et al., 2015;Mathur et al., 2020;Mori et al., 2012;Pollick, 2010;Wang et al., 2015;Zlotowski et al., 2013). ...
... These items were selected to measure both positive and negative emotional responses (i.e., shinwakan and bukimi) that the uncanny valley hypothesis describes (Jentsch, 1906(Jentsch, /1997Mori, 1970;Mori et al., 2012). 1 We chose a self-report measure to capture uncanny reactions because it can be an efficient way to derive easily interpretable and comparable results from participants' responses to many stimuli. Further, asking directly about participants' emotion is considered as a more reliable measure of affective responses than relying solely on behavioral measures (Diel, Weigelt, & MacDorman, 2022). ...
... (Phillips et al., 2018). The images of the robots were standardized (Diel, Weigelt, & MacDorman, 2022) in that they were depicted against a white or transparent background, in a standing, neutral, forward-facing pose with a neutral or mildly positive facial expression, whenever possible. Fig. 3 shows all the robots currently present in the database and used as stimuli in the present research. ...
Full-text available
The uncanny valley hypothesis describes how increased human-likeness of artificial entities, ironically, could elicit a surge of negative reactions from people. Much research has studied the uncanny valley hypothesis, but little research has sought to examine people's reactions to a broad range of human-likeness manifested in real-world robots. We focused on examining people's emotional responses to real-world, as opposed to hypothetical, robots because these robots impact real-life human–robot interactions. We measured both positive and negative emotional responses to a large collection of full-body images of robots (N = 251) with various human-like features. We found evidence for the existence of not one, but two uncanny valleys. Mori's uncanny valley emerged for high human-like robots and a second uncanny valley emerged for moderately low human-like robots. We attributed these valleys to unique combinations of perceptual mismatches between human-like features, specified by a match between surface and facial feature dimensions accompanied by a mismatch with the body-manipulator dimension. We also found that patterns of the uncanny valleys differed between positive (shinwakan) and negative (bukimi) emotional responses. Lastly, the word uncanny appeared to be an unreliable measure of the uncanny valley. Implications for robot design and the uncanny valley research are discussed.
... For example, research has found that people are uncomfortable with robots that look very (but not perfectly) humanlike: such robots are said to fall into the "uncanny valley" [18]. A recent meta-analysis find that this effect is large and robust to different operationalizations of human likeness and affective reactions [5]. There are many potential explanations for this phenomenon, many of which rely on the belief that robots and humans belong in separate categories, such that highly humanlike robots blur categorical boundaries, threaten human uniqueness, and create discomfort [7,31]. ...
... We reverse-scored this measure to create our primary dependent variable which we label "comfort" (α 0.92). We recognize that there is ambiguity and debate regarding the proper antonym for uncanny, such that alternative labels for our measure such as likeable or appealing may also be appropriate [5]. We also asked participants how much they would be interested in purchasing the robot, using the same scale. ...
Full-text available
The uncanny valley hypothesis describes how people are often less comfortable with highly humanlike robots. However, this discomfort may vary cross-culturally. This research tests how increasing robots’ physical and mental human likeness affects people’s comfort with robots in the United States and Japan, countries whose cultural and religious contexts differ in ways that are relevant to the evaluation of humanlike robots. We find that increasing physical and mental human likeness decreases comfort among Americans but not among Japanese participants. One potential explanation for these differences it that Japanese participants perceived robots to be more animate, having more of a mind, a soul, and consciousness, relative to American participants.
... Research that used morphed images found support for the uncanny valley hypothesis (e.g., Lischetzke et al., 2017;MacDorman & Ishiguro, 2006;Mathur & Reichling, 2009 but this line of research was criticized for the lack of external validity (Diel et al., 2022;Palomäki et al., 2018). A recent review and meta-analysis (Mara et al., 2022) demonstrated that higher scores on human likeness were absent in experiments that used realistic human-like robots. ...
Full-text available
Equipping robots with sophisticated mental abilities can result in reduced likeability (uncanny valley of mind). Other work shows that exposing robots to harm increases empathy and likeability. Connecting both lines of research, we hypothesized that eliciting empathy could mitigate or even reverse the negative response to robots with mind. In two online experiments, we manipulated the attributes of a robot (with or without mind) and presented the robot in situations in which it was either exposed to harm or not. Perceived empathy for the robot and robot likeability served as dependent variables. Experiment 1 (N = 559) used text vignettes to manipulate robot mind and a video that involved either physical harm or no harm to the machine. In a second experiment (N = 396), both experimental factors were manipulated via the shown video. Across both experiments, we observed a significant indirect effect of presenting the robot in a harmful situation on likeability, with empathy serving as a mediating variable. Moreover, a residual negative influence of showing the robot in a harmful situation was detected. We conclude that the uncanny valley of mind observed in our studies could be based on the robot's human-like imperfection, rather than descriptions of its supposed mind.
... Surprisingly, even monkeys behave with such an uncanny feeling with realistic but fake monkey faces (Steckenfinger and Ghazanfar, 2009). The uncanny effect has been replicated in numerous laboratories (e.g., Kätsyri et al., 2015;Lay et al., 2016;Mathur and Reichling, 2016;Mathur et al., 2020;Wang et al., 2015;Zhang et al., 2020) and is known to have a large effect size, as shown by a large-scale meta-analysis of 72 studies in 56 papers (Diel et al., 2022). ...
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The uncanny valley stands for the feeling of eeriness triggered by something that looks almost, but not exactly, like a real human. This study, thus, examined whether other-race bias modulates the uncanny valley phenomenon; both effects are based on familiarity with different face categories. We asked participants from Japan and Norway to rate the unpleasantness of computer-generated East Asian and European faces with progressively scaled eye sizes (from unnaturally small to unnaturally large). Simultaneously, we monitored their pupil sizes with an eye tracker. Pupillary diameter can be used as an objective measure of the uncanny feeling elicited by faces. We found that even when the changes in the images eye size were small, both Japanese and Norwegian participants rated the faces of their own race as more unpleasant than the faces of the different races, indicating the presence of other-race bias in the context of the uncanny valley, at least with computer-generated faces. Similar to the rating data, the pupils of Japanese participants dilated more for East Asian faces than for European faces. In contrast, the pupils of Norwegian participants dilated more for East Asian faces than for European faces. These differences can be attributed to unequal exposure to the faces from different races within each culture, thus, demonstrating other-race bias in the uncanny valley.
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El campo de la robótica social antropomórfica constituye uno de los territorios más interesantes para la reflexión filosófica contemporánea, por cuanto aúna en un mismo frente cuestiones de orden antropológico, ético y estético. Tomando como base la leyenda medieval del autómata de san Alberto Magno, el presente trabajo señala los riesgos asociados a la mimetización robótica del ser humano cuando las presunciones teóricas sobre el mismo son deflacionarias de su complejidad. Presento, primero, un resumen del fenómeno del “valle inquietante” como respuesta estética de rechazo ante diseños robóticos que devalúan la complejidad formal y comportamental humana; seguidamente se estudian las soluciones más aceptadas desde el punto de vista ingenieril, basadas en diseños de apariencia conforme a un principio abstractivo más que imitativo; por último, tomando un ejemplo de performatividad robótica, argumento el irreductible carácter de la belleza y la creatividad humanas frente a sus conatos de imitación robótica.
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In a multi-talker situation, listeners have the challenge of identifying a target speech source out of a mixture of interfering background noises. In the current study, it was investigated how listeners analyze audio-visual scenes with varying complexity in terms of number of talkers and reverberation. The visual information of the room was either congruent with the acoustic room or incongruent. The listeners' task was to locate an ongoing speech source in a mixture of other speech sources. The three-dimensional audio-visual scenarios were presented using a loudspeaker array and virtual reality glasses. It was shown that room reverberation, as well as the number of talkers in a scene, influence the ability to analyze an auditory scene in terms of accuracy and response time. Incongruent visual information of the room did not affect this ability. When few talkers were presented simultaneously, listeners were able to detect a target talker quickly and accurately even in adverse room acoustical conditions. Reverberation started to affect the response time when four or more talkers were presented. The number of talkers became a significant factor for five or more simultaneous talkers.
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Deviating from human norms in human-looking artificial entities can elicit uncanny sensations, described as the uncanny valley. This study investigates in three tasks whether configural deviation in written text also increases uncanniness. It further evaluates whether the uncanniness of text is better explained by perceptual disfluency and especially deviations from specialized categories, or conceptual disfluency caused by ambiguity. In the first task, lower sentence readability predicted uncanniness, but deviating sentences were more uncanny than typical sentences despite being just as readable. Furthermore, familiarity with a language increased the effect of configural deviation on uncanniness but not the effect of non-configural deviation (blur). In the second and third tasks, semantically ambiguous words and sentences were not uncannier than typical sentences, but deviating, non-ambiguous sentences were. Deviations from categories with specialized processing mechanisms thus better fit the observed results as an explanation of the uncanny valley than ambiguity-based explanations.
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Parent–child story time is an important ritual of contemporary parenting. Recently, robots with artificial intelligence (AI) have become common. Parental acceptance of children’s storytelling robots, however, has received scant attention. To address this, we conducted a qualitative study with 18 parents using the research technique design fiction. Overall, parents held mixed, though generally positive, attitudes toward children’s storytelling robots. In their estimation, these robots would outperform screen-based technologies for children’s story time. However, the robots’ potential to adapt and to express emotion caused some parents to feel ambivalent about the robots, which might hinder their adoption. We found three predictors of parental acceptance of these robots: context of use, perceived agency, and perceived intelligence. Parents’ speculation revealed an uncanny valley of AI: a nonlinear relation between the human likeness of the artificial agent’s mind and affinity for the agent. Finally, we consider the implications of children’s storytelling robots, including how they could enhance equity in children’s access to education, and propose directions for research on their design to benefit family well-being.
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In 1970, Masahiro Mori proposed the uncanny valley (UV), a region in a human-likeness continuum where an entity risks eliciting a cold, eerie, repellent feeling. Recent studies have shown that this feeling can be elicited by entities modeled not only on humans but also nonhuman animals. The perceptual and cognitive mechanisms underlying the UV effect are not well understood, although many theories have been proposed to explain them. To test the predictions of nine classes of theories, a within-subjects experiment was conducted with 136 participants. The theories' predictions were compared with ratings of 10 classes of stimuli on eeriness and coldness indices. One type of theory, configural processing, predicted eight out of nine significant effects. Atypicality, in its extended form, in which the uncanny valley effect is amplified by the stimulus appearing more human, also predicted eight. Threat avoidance predicted seven; atypicality, perceptual mismatch, and mismatch+ predicted six; category+, novelty avoidance, mate selection, and psychopathy avoidance predicted five; and category uncertainty predicted three. Empathy's main prediction was not supported. Given that the number of significant effects predicted depends partly on our choice of hypotheses, a detailed consideration of each result is advised. We do, however, note the methodological value of examining many competing theories in the same experiment.
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Background: The increasing involvement of social robots in human lives raises the question as to how humans perceive social robots. Little is known about human perception of synthesized voices. Aim: To investigate which synthesized voice parameters predict the speaker's eeriness and voice likability; to determine if individual listener characteristics (e.g., personality, attitude toward robots, age) influence synthesized voice evaluations; and to explore which paralinguistic features subjectively distinguish humans from robots/artificial agents. Methods: 95 adults (62 females) listened to randomly presented audio-clips of three categories: synthesized (Watson, IBM), humanoid (robot Sophia, Hanson Robotics), and human voices (five clips/category). Voices were rated on intelligibility, prosody, trustworthiness, confidence, enthusiasm, pleasantness, human-likeness, likability, and naturalness. Speakers were rated on appeal, credibility, human-likeness, and eeriness. Participants' personality traits, attitudes to robots, and demographics were obtained. Results: The human voice and human speaker characteristics received reliably higher scores on all dimensions except for eeriness. Synthesized voice ratings were positively related to participants' agreeableness and neuroticism. Females rated synthesized voices more positively on most dimensions. Surprisingly, interest in social robots and attitudes toward robots played almost no role in voice evaluation. Contrary to the expectations of an uncanny valley, when the ratings of human-likeness for both the voice and the speaker characteristics were higher, they seemed less eerie to the participants. Moreover, when the speaker's voice was more humanlike, it was more liked by the participants. This latter point was only applicable to one of the synthesized voices. Finally, pleasantness and trustworthiness of the synthesized voice predicted the likability of the speaker's voice. Qualitative content analysis identified intonation, sound, emotion, and imageability/embodiment as diagnostic features. Discussion: Humans clearly prefer human voices, but manipulating diagnostic speech features might increase acceptance of synthesized voices and thereby support human-robot interaction. There is limited evidence that human-likeness of a voice is negatively linked to the perceived eeriness of the speaker.
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Virtual agents are animated life-like characters generally used in virtual learning environments to facilitate learning tasks. With virtual agent, students can hold meaningful interactions throughout the learning process for more effective cognition. Hence, the effectiveness of virtual agent in term of promoting positive emotions is very much related to character realism influence. The level of realism of virtual agent may cause distress to the users, especially when the character mimics like human which was based on the uncanny valley phenomenon. For that, four different realism designs of virtual agents (realistic, semi-realistic, stylized and cartoon-like agents) in Quiz based Multimedia Learning Environment (QMLE) had been developed as experimental items and tested as experimental items to analyze emotions in the dimension of valence and arousal. Quasi-experimental design was used to answer the research questions derived and the data obtained was analysed using ANOVA and post hoc. The experiment was carried out on 600 electric engineering students from seven polytechnics of Malaysia. The experiment was carried out with 600 Electrical Engineering students from seven polytechnics in Malaysia. Students were divided into four different groups where each group consisted of 150 students who underwent four different realism designs of the virtual agents respectively. It has been found that the four different realism designs of virtual agents fall at high affective state with high arousal and positive valence. Consequently, the designed virtual agents escaped from uncanny valley effect.
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Human replicas highly resembling people tend to elicit eerie sensations-a phenomenon known as the uncanny valley. To test whether this effect is attributable to people's ascription of mind to (i.e., mind perception hypothesis) or subtraction of mind from androids (i.e., dehumanization hypothesis), in Study 1, we examined the effect of face exposure time on the perceived animacy of human, android, and mechanical-looking robot faces. In Study 2, in addition to exposure time, we also manipulated the spatial frequency of faces, by preserving either their fine (high spatial frequency) or coarse (low spatial frequency) information, to examine its effect on faces' perceived animacy and uncanniness. We found that perceived animacy decreased as a function of exposure time only in android but not in human or mechanical-looking robot faces (Study 1). In addition, the manipulation of spatial frequency eliminated the decrease in android faces' perceived animacy and reduced their perceived uncanniness (Study 2). These findings link perceived uncanniness in androids to the temporal dynamics of face animacy perception. We discuss these findings in relation to the dehumanization hypothesis and alternative hypotheses of the uncanny valley phenomenon.
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This paper extends Mori’s (IEEE Robot Autom Mag 19:98–100, 2012) uncanny valley-hypothesis to include technologies that fail its basic criterion that uncanniness arises when the subject experiences a discrepancy in a machine’s human likeness. In so doing, the paper considers Mori’s hypothesis about the uncanny valley as an instance of what Heidegger calls the ‘challenging revealing’ nature of modern technology. It introduces seeming autonomy and heteronomy as phenomenological categories that ground human being-in-the-world including our experience of things and people. It is suggested that this categorical distinction is more foundational than Heidegger’s existential structures and phenomenological categories. Having introduced this novel phenomenological distinction, the paper considers the limits of Mori’s hypothesis by drawing on an example from science fiction that showcases that uncanniness need not only be caused by machines that resemble human beings. In so doing, it explores how the seeming autonomy-heteronomy distinction clarifies (at least some of) the uncanniness that can arise when humans encounter advanced technology which is irreducible to the anthropocentrism that shapes Mori’s original hypothesis.
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Depend on the development of science and technology, the demands for robots are not only limited to the use of functions but also pay more attention to the emotional experience brought by the products. However, as the robot’s appearance approach human-likeness, it makes people uncomfortable, which is called the Uncanny Valley (UV). In this paper, we systematically review the hypothesis and internal mechanisms of UV. Then we focus on the methodological limitations of previous studies, including terms, assessment, and materials. At last, we summarize the applications in interaction design to avoid the uncanny valley and propose future directions.
The present study investigated self-face perception in 12-month-old infants using the morphing technique. Twenty-four 12-month-old infants participated in both the main and control experiments. In the main experiment, we used the participant's own face, an unfamiliar infant's face (age-and gender-matched), and a morphed face comprising 50 % each of the self and unfamiliar faces as stimuli. The control experiment followed the same procedure, except that the self-face was replaced with another unfamiliar face. In both experiments, two of these stimuli were presented side by side on a monitor in each trial, and infants' fixation duration was measured. Results showed that shorter fixation durations were found for the morphed face compared with the self-face and the unfamiliar face in the main experiment, but there were no significant preferences for any comparisons in the control experiment. The results suggest that 12-month-old infants could detect subtle differences in facial features between the self-face and the other faces, and infants might show less preference for the self-resembling morphed face due to increased processing costs, which can be interpreted using the uncanny valley hypothesis. Overall, representations of the self-face seem to a certain extent to be formed by the end of the first year of life through daily visual experience.