The Smell of Death: Evidence that Putrescine Elicits Threat Management Mechanisms

Abstract

The ability to detect and respond to chemosensory threat cues in the environment plays a vital role in survival across species. However, little is known about which chemical compounds can act as olfactory threat signals in humans. We hypothesized that brief exposure to putrescine, a chemical compound produced by the breakdown of fatty acids in the decaying tissue of dead bodies, can function as a chemosensory warning signal, activating threat management responses (e.g., heightened alertness, fight-or-flight responses). This hypothesis was tested by gauging people’s responses to conscious and non-conscious exposure to putrescine. In Experiment 1, putrescine increased vigilance, as measured by a reaction time task. In Experiments 2 and 3, brief exposure to putrescine (vs. ammonia and a scentless control condition) prompted participants to walk away faster from the exposure site. Experiment 3 also showed that putrescine elicited implicit cognitions related to escape and threat. Experiment 4 found that exposure to putrescine, presented here below the threshold of conscious awareness, increased hostility toward an out-group member. Together, the results are the first to indicate that humans can process putrescine as a warning signal that mobilizes protective responses to deal with relevant threats. The implications of these results are briefly discussed.

Full text

The Smell of Death: Evidence that Putrescine Elicits Threat Management
Mechanisms
Arnaud Wisman and Ilan Shrira
Journal Name: Frontiers in Psychology
ISSN: 1664-1078
Article type: Original Research Article
Received on: 27 May 2015
Accepted on: 10 Aug 2015
Provisional PDF published on: 10 Aug 2015
Frontiers website link: www.frontiersin.org
Citation: Wisman A and Shrira I(2015) The Smell of Death: Evidence that
Putrescine Elicits Threat Management Mechanisms. Front. Psychol.
6:1274. doi:10.3389/fpsyg.2015.01274
Copyright statement: © 2015 Wisman and Shrira. This is an open-access article
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Emotion Science

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The Smell of Death:
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Evidence that Putrescine Elicits Threat Management Mechanisms
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Arnaud Wisman1*, Ilan Shrira2
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1School of Psychology, University of Kent, Canterbury, Kent, UK
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2Department of Behavioral Sciences, Arkansas Tech University, Russellville, AR, USA
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*Correspondence:
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Dr. Arnaud Wisman
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School of Psychology
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University of Kent
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Canterbury
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Kent CT2 7NP
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United Kingdom
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a.wisman@kent.ac.uk
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Keywords: olfaction, putrescine, threat, threat management, chemosensory cue
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Abstract
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The ability to detect and respond to chemosensory threat cues in the environment plays a vital
24
role in survival across species. However, little is known about which chemical compounds can
25
act as olfactory threat signals in humans. We hypothesized that brief exposure to putrescine, a
26
chemical compound produced by the breakdown of fatty acids in the decaying tissue of dead
27
bodies, can function as a chemosensory warning signal, activating threat management responses
28
(e.g., heightened alertness, fight-or-flight responses). This hypothesis was tested by gauging
29
people’s responses to conscious and non-conscious exposure to putrescine. In Experiment 1,
30
putrescine increased vigilance, as measured by a reaction time task. In Experiments 2 and 3, brief
31
exposure to putrescine (vs. ammonia and a scentless control condition) prompted participants to
32
walk away faster from the exposure site. Experiment 3 also showed that putrescine elicited
33
implicit cognitions related to escape and threat. Experiment 4 found that exposure to putrescine,
34
presented here below the threshold of conscious awareness, increased hostility toward an out-
35
group member. Together, the results are the first to indicate that humans can process putrescine
36
as a warning signal that mobilizes protective responses to deal with relevant threats. The
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implications of these results are briefly discussed.
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1. Introduction
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When animals die they release an unpleasant smell. A pungent component of this scent is emitted
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by putrescine, a volatile diamine that results from the breakdown of fatty acids in the putrefying
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tissue of dead bodies (Hussain et al., 2013). Interestingly, animal research shows that putrescine
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can function as a powerful chemosensory signal that prompts the perceiver to leave or avoid the
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area (Prounis & Shields, 2013; Yao et al., 2009). The aim of the present research is to show that
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humans respond in a similar way to putrescine, and more generally, that exposure to putrescine
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triggers threat management behaviors (Blanchard et al., 2001; Neuberg et al., 2011).
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A growing body of research suggests that humans can identify threats via chemosignals
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(Ackerl et al., 2002; Chen & Haviland-Jones, 2000; de Groot et al., 2012; Mujica-Parodi et al.,
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2009; Prehn et al., 2006; Zhou & Chen, 2009). For instance, when people are exposed to sweat
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taken from donors during a fearful experience, perceivers show a heightened startle reflex (Pause
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et al., 2009; Prehn et al., 2006) and interpret ambiguous facial expressions as fearful (Zhou &
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Chen, 2009). This transmission of threat-arousing chemosignals is assumed to serve an adaptive
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function by orienting us to impending dangers. Indeed, the ability to detect and process
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chemosensory threat cues is vital for the survival of a wide range of species (Stevenson, 2010).
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However, thus far there is little evidence that humans can, like other organisms, detect olfactory
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threat cues in the environment through means other than the chemosignals (e.g., body sweat) of
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conspecifics.
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The decay of tissue and its resulting scent can function as a “necromone” cue that signals
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an animal’s death to conspecifics. Alarm and avoidance behaviors (necrophobic behaviors) in
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response to these scents are widespread in the animal kingdom and thought to have evolved at
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least 420 million years ago (Yao et al., 2009). In fact, recent research shows that necrophobic
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behavior may have innate underpinnings through the activation of trace amine-associated
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receptors (TAARs), a group of specialized scent receptors in the olfactory epithelium (Horowitz
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et al., 2014; Hussain et al., 2013; Li & Liberles, 2015). TAARs are known to detect specific
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chemicals that evoke behavioral responses, without the need for prior exposure to the scents. For
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example, in model vertebrates, certain TAARs respond to diamines (e.g., putrescine) by
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producing avoidant behaviors that likely serve to defend against immediate dangers (Yoon et al.,
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2015). Thus, it is feasible that we have a chemosensory sensitivity to diamines like putrescine (Li
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& Liberles, 2015), given that their detection can aid survival (Stevenson, 2010).
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A further advantage of examining putrescine as a threat stimulus is that we know what it
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is. Despite the impressive amount of indirect support for human chemosignals amassed in recent
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years, their chemical properties have yet to be identified (Wyatt, 2009). Focusing on a known
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compound, putrescine, enables us to directly test whether it plays a causal role in human threat
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responses. In a similar vein, although several studies have shown that chemosensory cues can
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elicit greater readiness for behavior (Bradley et al., 2001; Prehn et al., 2005), thus far there is
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little direct evidence that a specific chemical substance can cause overt behavioral changes in
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humans (Wysocki & Preti, 2004). Since exposure to putrescine elicits specific behaviors in
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animals (e.g., escape, avoidance), we can examine whether putrescine produces similar behaviors
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in humans. In sum, putrescine appears to be well-suited to test as a specific chemical compound
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that can act as a threat signal in humans.
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Chemosensory cues can convey danger in at least two fitness-relevant domains: microbial
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and predator threats (Stevenson, 2010). First, olfactory information is often central to identifying
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the presence of pathogens. For example, pathogens can alter the scent of those who become
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3
infected, which can be detected by conspecifics (Arakawa et al., 2010; Olsson et al., 2014; Tybur
1
et al., 2011). Similarly, the release of putrescine in decaying tissue co-occurs with the arrival of
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bacteria, a motivation for others to eschew physical contact with the dead body. A number of
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species exhibit necrophobic behaviors, and after detecting the scent emanating from dead bodies,
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usually respond by leaving or avoiding the area (Prounis & Shields, 2013). Second, putrescine
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released by decaying bodies can signal the risk of predation (Boissy et al., 1986). Since a large
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proportion of deaths in the wild are the result of predator attacks, putrescine would be a useful
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alarm cue to stay away (Misslin, 2003).
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In humans, responses to specific scents can develop through learned associations between
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odors and personal experiences (Degel et al., 2001; Stevenson et al., 1998). For example, based
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on the cultural expression that when “something smells fishy” it is viewed suspiciously, exposure
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to fish-like odors arouses suspicion toward others and reduces cooperation, an orientation that is
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assumed to result from conditioned reactions to this scent (Lee & Schwarz, 2012). Since
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putrescine can emanate from various sources (Yeoman et al., 2013), people may learn to
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associate the smell of putrescine with threats, and it is plausible that occasional exposure to
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putrescine, whenever it occurs, could lead to conditioned threat responses (Stevenson, 2010).
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However, we render it unlikely that modern humans have strong conscious meaningful
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associations with the scent of putrescine. Moreover, conscious scent evaluations are often
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inaccurate, context dependent, and colored by other sensory modalities (Sela & Sobel, 2010). In
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view of this, it is important to note that responses to aversive chemosensory cues do not require
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prior learning or conscious evaluation (Dielenberg et al., 2001; Li et al., 2013; Miller & Maner,
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2010). Indeed, scents can alter our perception, cognition, behavior, and physiology (e.g., heart
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rate, skin conductance) even when there is no conscious scent detection (Krusemark & Li, 2012;
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Li et al., 2007; Pause et al., 2009; Sela & Sobel, 2010), and even after olfactory adaptation has
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set in (de Groot et al., 2012; Smeets & Dijksterhuis, 2014). Thus, neither prior associations with
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olfactory signals, nor conscious processing, are necessary conditions for people to process them
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as threatening (Pause, 2012; Williams et al., 2006; Köster, 2002; Sela & Sobel, 2010; Smeets &
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Dijksterhuis, 2014).
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At the most basic level, threat detection increases vigilance and sharpens our reactions to
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events in the environment (Williams et al., 2006). For instance, detection of a predator’s scent
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will interrupt foraging and increase behaviors (e.g., scanning the environment) that facilitate
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predator detection (Woody & Szechtman, 2011). Once the threat management system is
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engaged, it produces readiness for fight-or-flight behaviors (Blanchard et al., 1986; Cannon,
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1927; Gray & McNaughton, 2003; Mobbs et al., 2009). Flight responses seek to escape the
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situation, whereas fight responses—whether physical or verbal aggression—are typically only
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used when escape is not possible. In contrast to popular belief that the dominant response to
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threats is to fight, flight is actually far more common (Misslin, 2003), presumably because nature
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selects more strongly for strategies that minimize risk. In one study, for example, when people
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were confronted by a threatening out-group member, they responded with aggressive readiness
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(fight), but only when there was little possibility of escaping; when given the option, though,
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participants chose to distance themselves (flight) from the other person (Cesario et al., 2010).
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2. Overview and Hypotheses
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Coming full circle, we propose that putrescine can serve as a (non-conscious) signal that initiates
1
threat management responses. Specifically, we hypothesize that brief exposure to putrescine
2
increases vigilance, followed by the readiness to either escape (flight), or engage in aggressive
3
readiness (fight) when escape is not possible. Experiment 1 assessed whether putrescine (vs.
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ammonia and a neutral scent) increased vigilance as measured by faster responses in a simple
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reaction time task. Experiments 2 and 3 assessed whether brief exposure to putrescine (vs.
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ammonia and neutral scent) caused participants to walk away faster from the exposure site after
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completing the experiment (outdoors). Experiment 3 also tested whether putrescine evoked
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cognitions related to escape and threat. Finally, Experiment 4 examined whether non-conscious
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exposure to putrescine increased aggressive readiness (e.g., defensiveness toward an out-group
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member). All four experiments adhered to the Declaration of Helsinki guidelines, and gained the
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prior approval by the University Research Ethics Committee. Written consent was obtained from
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all participants involved in these experiments, and all were fully debriefed.
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3. Experiment 1: The effect of putrescine on vigilance
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In Experiment 1, we tested whether brief exposure to putrescine increased vigilance. To measure
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vigilance, we employed a task closely modeled after the shortened version of the Psychomotor
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Vigilance Task (PVT; Dinges & Powell, 1985) that assessed participants’ reaction times to a red
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dot that was presented at random intervals on a computer screen.
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In addition, Experiment 1 was designed to determine whether ammonia served as an
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appropriate aversive control condition. Our pilot testing revealed that ammonia, unlike other
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aversive scents we had examined (i.e., skatole
1
and indole), was rated similarly to putrescine on
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repugnance, familiarity, and intensity. Moreover, previous research has used ammonia (NH3;
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ammonium hydroxide) as an aversive scent prime (Rieser et al., 1976; Wise et al., 2005).
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Furthermore, ammonia can increase trigeminal nerve activation associated with vigilance and
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sensory rejection, via activation of the sympathetic nervous system (Hummel & Kobal, 1992;
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Sekizawa & Tsubone, 1994). However, some research suggests that unpleasant ambient odors
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can also decrease reaction times on simple tasks like the current PVT (Millot et al., 2002). In
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view of this, we made no specific prediction about whether ammonia, like putrescine, would
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enhance vigilance relative to our scentless control condition.
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3.1. Method
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3.1.1. Participants and Procedure
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A sample of sixty participants (43 females; Mage = 21.20, SD = 3.20) completed the study in
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return for a financial incentive of 3 pounds (approximately $5).
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In line with previous research (Wheatley & Haidt, 2005), we pilot-tested the so-called “fart spray” along with
skatole, indole, and ammonia, for suitability as an aversive control condition. These ratings are presented in Table 1.
As can be seen, ammonia and fart spray were rated similarly to putrescine on all three dimensions of repugnance,
familiarity, and intensity, whereas indole and skatole diverged from putrescine on at least one dimension. A
disadvantage of fart spray, however, is that we could not ascertain its precise chemical compounds—its
manufacturers were reluctant to disclose this information.

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Participants were randomly assigned to one of three conditions: putrescine (C4H12N2;
1
Sigma-Aldrich), ammonia (5%; NH3; Sigma-Aldrich), or water. One hour before the start of the
2
experiment, cotton wool pads were blotted with 2 ml of one of the three compounds, and stored
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separately in small (100 ml) sealable amber jars. Participants were run in our lab individually,
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and seated in different cubicles to avoid carryover effects of scents. The refreshment rate in each
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cubicle was 4 to 5 air changes (cycles) per hour. Furthermore, participants were booked at least
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30 minutes apart in order to ventilate the rooms—by opening the lab room’s window—between
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sessions. When preparing materials for the experiment, one of the researchers marked the bottom
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of each jar with a number code, so that the experimenters were unaware of the meaning of these
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codes. This basic procedure was repeated in our subsequent experiments to keep the
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experimenters blind to the conditions.
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Participants were seated in front of a standard PC (equipped with Authorware 7.1
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software) with a 17-inch screen. They were given instructions (on-screen) to open the jar, sniff
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the scent inside for 10 seconds, and close the jar. After that, they rated the scent on its intensity
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(“This scent is intense”; 1 = strongly disagree and 9 = strongly agree), repugnance (“This scent
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is repugnant”; 1 = strongly disagree and 9 = strongly agree), and familiarity (“This scent is
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familiar”; 1 = strongly disagree and 9 = strongly agree). Repugnance was included as evaluative
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rating (alongside the standard measures of intensity and familiarity) because repugnance (or
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disgust) is often a central component of aversive scents. Participants were then introduced to the
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adapted PVT, which lasted about five minutes (see Loh et al., 2004). The task instructed them to
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click on a red dot as quickly as possible whenever they saw the dot on the screen. Ten dots (each
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measuring 1 cm) were shown at different locations on the screen, and the time between
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appearances was randomized at variable intervals (2-45 sec). As soon participants clicked on the
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red dot with the mouse, a screen appeared for five seconds with the message: “prepare for next
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trial”. Participants received two practice trials first, to get them familiar with the main task of ten
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trials. Finally, after completing the PVT and filling out a standard demographic questionnaire,
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they were fully debriefed and thanked for their participation.
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3.2. Results and Discussion
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3.2.1. Hedonic Evaluations
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We began by testing our prediction, based on our pilot testing, that putrescine and ammonia
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would not differ from each other on repugnance, familiarity and intensity. As predicted, separate
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one-way between-subjects ANOVAs revealed that there was no significant difference between
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ammonia and putrescine on repugnance, F(1, 38) = 0.38, p = .54, η² = .01, familiarity, F(1, 38) =
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0.26, p = .26, η² = .03, or intensity, F(1, 38) = 0.14, p = .71, η² = .004 (see Table 2, for
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descriptive statistics). Moreover, the analyses reported below were not altered when entering all
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hedonic evaluations as covariates.
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3.2.2. Reaction Times
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We examined our main prediction that putrescine, relative to the neutral control condition
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(water), would elicit faster reaction times. In line with previous PVT research, we applied
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reciprocal transformation to the raw data (i.e., 1/RT). This type of transformation is standard
45
within the PVT paradigm, as it reduces the impact of extreme scores and brings them into an
46

6
acceptable range (Dinges et al., 1987; Dorrian et al., 2004). A one-way between-subjects
1
ANOVA revealed a difference between the scent conditions, F(2, 57) = 4.32, p = .018, η² = .13.
2
Post hoc comparisons, with the raw means reported here, showed that putrescine produced faster
3
reaction times (M = 1.04, SD = .10) than the neutral scent (M = 1.24, SD = .35; p = .013), but not
4
compared to ammonia (M = 1.12, SD = .20; p = .28). No difference was found between the
5
neutral and ammonia conditions (p = .14).
6
7
In sum, only putrescine caused participants to react more quickly compared to the neutral
8
condition, supporting our hypothesis that putrescine increases vigilance. At the same time,
9
ammonia did not increase vigilance relative to the scentless control condition. Importantly, the
10
findings show that, consistent with our pilot study, ammonia and putrescine are evaluated similar
11
on repugnance, familiarity, and intensity, and were similar in the degree of vigilance they
12
elicited. Consequently, together with previous research (Rieser et al., 1976; Wise et al., 2005),
13
Experiment 1 indicated that ammonia would serve as an appropriate aversive control condition.
14
Experiments 2 and 3 investigated our hypothesis that putrescine activates the motivation to
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escape the situation (flight).
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4. Experiment 2: The effect of putrescine on escape behavior
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Similar to Experiment 1, Experiment 2 first asked participants to rate a scent prime (putrescine
20
vs. ammonia vs. neutral) on three dimensions: intensity, familiarity, and repugnance, then we
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observed whether it influenced the tendency to escape the situation. To avoid the biases
22
associated with some operationalizations of flight in prior research (e.g., self-reported intentions,
23
Gilbert & Gilbert, 2003), we employed an overt behavioral measure of escape (e.g., Ellsworth et
24
al., 1972; Wisman & Koole, 2003). Specifically, we assessed whether putrescine would cause
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participants (who were under the impression the study was finished) to walk away more quickly
26
over a predetermined distance of 80 meters.
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4.1. Method
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30
4.1.1. Participants and Procedure
31
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Forty-five participants (21 females and 24 males; Mage = 27.51, SD = 9.72) completed the study
33
on campus. We filled three empty felt-tip pens, each with one of the three compounds
34
(putrescine, ammonia, or water). To fill each pen, 10ml of liquid odor was injected onto the
35
pen’s fiber rod inside the pen. The pens were then re-assembled and left to stand upside down for
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24 hours in order to allow the liquid to soak into the fiber rod. Just before the start of the
37
experiment, scent blotters were marked with the scent marker pens and stored in separate
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sealable containers.
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Participants were approached on a fixed spot on the campus and asked if they had time to
41
participate in a brief scent test of approximately ten minutes. Participants were tested
42
individually and randomly assigned to one of three conditions (putrescine, ammonia, or water).
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The experimenter, blind to the conditions, presented one of the three containers to the
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participant, who rated the scent on intensity (“This scent is strong”; 1 = strongly disagree and 5
45
= strongly agree), repugnance (“This scent is repugnant”; 1 = strongly disagree and 5 = strongly
46

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agree), and familiarity (“This scent is familiar”; 1 = strongly disagree and 5 = strongly agree).
1
After finishing and being thanked for their participation, a second experimenter blind to the
2
condition and hypotheses of the experiment and out of sight of the participants— used a standard
3
stopwatch to time how many seconds it took participants to walk away over a distance of 80
4
meters (pre-measured before the experiment began). The recorded time constituted our
5
dependent variable. After they reached this distance, participants were re-approached, fully
6
debriefed and thanked again.
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4.2. Results and Discussion
9
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4.2.1. Hedonic Evaluations
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Consistent with Experiment 1, separate one-way between-subjects ANOVAs revealed that there
13
was no significant difference between ammonia and putrescine on repugnance, F(1, 28) = 2.30, p
14
= .14, η² = .07, and familiarity, F(1, 28) = 0.04, p = .75, η² = .01. However, ammonia was rated
15
as relatively more intense (M = 4.73; SD = 0.46) compared to putrescine (M = 4.27; SD = 0.70; p
16
= .04; see Table 3). Once again, the results reported below were not altered when we entered the
17
intensity (nor the other hedonic) ratings into the analyses as covariates. We also note that the
18
results are similar whether participants rate how “intense” or “strong” the scent smells (see
19
Experiment 3 below).
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4.2.2. Escape Behavior
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To test our hypothesis that putrescine elicited an escape motivation, we compared our scent
24
conditions in a one-way ANOVA, using gender as a covariate
2
. The results yielded a significant
25
effect of the scent prime on the time it took to walk 80 meters, F(2, 41) = 19.03, p < .001, η² =
26
.48. The only significant differences occurred between putrescine (M = 56.40 seconds; SD =
27
4.19) and ammonia (M = 59.93, SD = 5.04), and between putrescine and the neutral scent prime
28
(M = 60.00, SD = 4.42; both ps < .005; see Figure 1). Thus, putrescine caused participants to
29
walk away more quickly, supporting our assumption that putrescine evoked a stronger
30
motivation to escape. Experiment 3 was conducted to replicate this finding, and furthermore to
31
test whether putrescine elicited implicit cognitions related to escape and threat.
32
33
5. Experiment 3: The effect of putrescine on escape behavior and thoughts
34
35
The procedure for Experiment 3 was similar to Experiment 2’s. First, we asked participants to
36
evaluate the scents on the different dimensions (repugnance, familiarity, intensity). In addition,
37
we gauged participants’ implicit threat-related associations using a word stem-completion task.
38
Specifically, this task measured the implicit accessibility of thoughts related to “escape” and
39
“threat.” We predicted that only putrescine would increase the accessibility of these cognitions.
40
Finally, we assessed whether putrescine would cause participants to walk away more quickly
41
over a predetermined distance of 60 meters.
42
2
Because previous research has shown that men and women tend to walk at different speeds (Chumanov, Wall-
Scheffler, & Heiderscheit, 2008), the results of Experiments 2 and 3 included gender as a covariate. In addition, we
analyzed the results of Experiments 2 and 3 with gender as a separate factor and this did not alter the significance of
the results.

8
1
5.1. Method
2
3
5.1.1. Participants and Procedure
4
5
Sixty participants (32 females and 28 males, Mage = 21.57, SD = 1.12) completed the study on
6
campus. Individuals were approached just outside campus on a path sloping downhill and asked
7
if they had time to participate in a brief scent test for about 15 minutes.
8
9
Participants were randomly assigned to one of the three scent conditions, then they rated
10
the scent on intensity, repugnance, and familiarity (“This scent is intense”; 1 = strongly disagree
11
and 9 = strongly agree), repugnance (“This scent is repugnant”; 1 = strongly disagree and 9 =
12
strongly agree), and familiarity (“This scent is familiar”; 1 = strongly disagree and 9 = strongly
13
agree). Then, to assess cognitions relevant to the concepts of “escape” and “threat,” participants
14
completed the word-stem completion task, a widely used and well-established measurement that
15
gauged the thought accessibility of these two concepts (Arndt et al., 1997; Greenberg et al.,
16
1994; Lozito & Mulligan, 2010; Migo et al., 2010). Participants were asked to complete 30 word
17
fragments, 20 of which were neutral (e.g., B_ NK could be BANK or BUNK) in terms of any
18
particular theme, five of which could be words related to “escape” (e.g., the fragment RU_ could
19
be completed as RUN or RUB, the latter a neutral word), and another five could be completed
20
with a word related to “threat” (e.g., _ _ RROR could be TERROR or MIRROR). We summed
21
the number of escape- (M = 2.73, SD = 1.07) and threat-related words (M = 1.90, SD = .66) that
22
participants completed to assess the thought accessibility of these concepts. Finally, participants
23
were again timed by a second experimenter, who was blind to the conditions and the hypotheses,
24
for how long it took them to walk away over a distance of 60 meters (Due to natural constraints a
25
slightly shorter distance than in Experiment 2).
26
27
5.2. Results and Discussion
28
29
5.2.1. Hedonic Evaluations
30
31
Separate one-way between-subjects ANOVAs revealed no difference between the chemosensory
32
primes on repugnance, F(1, 38) = .35 , p = .56, η² = .01, familiarity, F(1, 38) = .04, p = .85, η² =
33
.001, and intensity, F(1, 38) = 0.29, p =.59, η² = .008 (see Table 4). Thus, participants rated
34
ammonia and putrescine similarly to one another on intensity, repugnance, and familiarity.
35
Again, the results reported below were did not differ when we entered the hedonic evaluations
36
into the analyses as covariates.
37
38
5.2.2. Escape- and Threat-Related Cognitions
39
40
To test our hypothesis that putrescine elicited implicit cognitions related to escape and threat, we
41
analyzed the escape and threat word-completion results separately. The results revealed a
42
significant effect of scent prime on escape thought accessibility, F(2, 57) = 10.90, p < .001, η² =
43
.28 (see Table 5). Putrescine caused participants to complete word stems more frequently with
44
escape related words (M = 3.45, SD = .69) than both the ammonia (M = 2.45, SD = 1.05) and the
45
neutral scent (M = 2.15, SD = .99) primes (both ps < .005). Similarly, the scent primes affected
46

9
the accessibility of threat-related thoughts, F(2, 57) = 8.39, p < .001, η² = .23. Putrescine led to
1
more threat word-stem completions (M = 2.55, SD = .94) than ammonia (M = 1.73, SD = .64)
2
and the neutral scent (M = 1.68, SD = .65; all ps < .005).
3
4
5.2.3. Escape Behavior
5
6
Like Experiment 2, the analyses showed a significant effect of chemosensory primes on walking
7
speed, F(2, 56) = 9.11, p < .001, η² = .24 (see Figure 2). The pattern of results again showed that
8
putrescine (M = 33.38, SD = 2.99) caused people to walk more quickly than ammonia (M =
9
35.92, SD = 3.38) and the neutral scent prime (M = 37.67, SD = 3.13; p < .05). Again, no
10
difference was found between the ammonia and the neutral scent condition (p = .87).
11
12
Experiment 3 revealed that putrescine elicited implicit cognitions of escape and threat. In
13
addition, Experiment 3 replicated the finding that putrescine increased walking speed. Thus,
14
taken together, the results of Experiments 2 and 3 indicated that putrescine motivated (automatic)
15
escape behavior. An important feature of the settings in Experiments 2 and 3 was that
16
participants were outdoors and in a context that facilitated the possibility that they could distance
17
themselves from the scent.
18
19
6. Experiment 4: The effects of putrescine on defensive responses toward an out-group
20
21
Experiment 4 sought to extend our understanding of the effects of putrescine in two important
22
respects. First, we tested the hypothesis that non-conscious (unobtrusive) exposure to putrescine
23
could elicit threat management responses. As we highlighted in the Introduction, this possibility
24
is consistent with evidence that scent primes, even when presented at sub-threshold levels, can
25
influence brain activation (Sobel et al., 1999), learning (Koster et al., 2002), and physiological
26
state (Stern & McClintock, 1998). This applies similarly to aversive scent primes, which for
27
example, have the ability to alter skin conductance (Jacquot et al., 2004), social preferences (Li
28
et al., 2007), and cognitive performance (Epple & Herz, 1999) in ways that correspond to
29
supraliminal exposure to aversive stimuli (Sela & Sobel, 2010). Thus, we predicted that
30
subliminal presentation of putrescine would be capable of activating threat responses.
31
32
Second, Experiment 4 focused on the fight rather than the flight component of alarm
33
responses. Consistent with previous research showing that implicit threat cues increase
34
intolerance toward out-group members (Holbrook et al., 2011) and defensive responses
35
(Blanchard et al., 2001; Wheatley & Haidt, 2005), we hypothesized that putrescine would
36
increase defensiveness toward an out-group member, in a situation where there was no
37
immediate opportunity to escape (Cesario et al., 2010). Like Experiment 1, we conducted this
38
experiment in a laboratory setting. After priming the participants with one of the scents, they
39
filled out a standard positive and negative affect scale that gauged their mood. Although, our
40
pilot study (see Table 1) and some research (e.g., Knasko, 1993) revealed that aversive scent
41
primes do not alter mood on a conscious level, we intended to rule out the possibility that the
42
subliminal primes influenced participants’ feelings at a conscious level. After that, they read
43
about an out-group member—a foreign student who criticized the participants’ value system—
44
and were asked to evaluate the target. This evaluation was designed to assess how much hostility
45
participants felt toward the target.
46

10
1
6.1. Method
2
3
6.1.1. Participants and Procedure
4
5
Sixty-nine participants (39 females and 30 males, Mage = 24.00, SD = 8.38) were run in our lab
6
individually, in different cubicles (randomized) to avoid carryover effects of scents. Furthermore,
7
participants were booked at least 30 minutes apart in order to ventilate the rooms between
8
sessions. Upon arrival, participants were given the first of two questionnaire packets to complete.
9
This first questionnaire consisted of demographic questions and a number of filler items. We
10
then randomly assigned participants to their condition by marking one of the three liquid scents
11
(putrescine, ammonia, water) to the top of each page (0.5 ml) of the second questionnaire
12
participants were given. In the putrescine and ammonia conditions, this amounted to a very
13
subtle scent prime that was not meant to be detected. At the conclusion of the experiment, we
14
funnel debriefed participants to determine whether they noticed or smelled anything unusual
15
during the study. None of them reported being aware of the scents.
16
17
The second questionnaire assessed participants’ mood, and our dependent variables. First,
18
to rule out the possibility that our results could be explained by generalized affect, participants
19
began the second part of the questionnaire by completing the 20-item Positive and Negative
20
Affect Scale (PANAS; Tellegen et al., 1988). This scale measured the extent to which each of 10
21
positive affect descriptors (α = .86) and 10 negative affect descriptors (α = .85) reflected how
22
they felt at that moment (1 = very slightly or not at all, 5 = extremely). We computed the average
23
positive affect (M = 3.31, SD = .68) and negative affect (M = 1.61, SD =.59) scores for
24
everybody.
25
26
This was followed by the description and evaluation of the out-group member (Greenberg
27
et al., 2001; Navarrete et al., 2004; Norenzayan et al., 2007). Specifically, participants read an
28
essay supposedly written by a college student from the Middle East who was visiting the United
29
Kingdom to study English. In this essay, the student went on to criticize Western values,
30
predicting its eventual decline (see Norenzayan et al., 2007). Participants were then asked to
31
evaluate the author and his message by responding to four questions on a 9-point Likert scale
32
(“To what extent do you like the author”; ‘To what extent would you like to be friends with the
33
author”; “How much would you oppose the author teaching your (future) children”; and “How
34
much do you want the ideas of the author to be publicized”; 1 = very much, 9 = not at all). We
35
derived an overall out-group hostility index (M = 5.82, SD = 1.63) by averaging all items
36
together (α = .77), such that larger values indicated greater hostility. Finally, we measured
37
motivation to escape the situation by timing how long it took participants to complete the second
38
(scented) questionnaire followed by a standard demographic questionnaire (91% of the
39
participants were native to England, 3% Greece, 4% Ireland, and 1% to the United States).
40
41
6.2. Results and Discussion
42
43
6.2.1. Ancillary Analyses
44
45

11
A one-way ANOVA tested whether the chemosensory primes elicited different levels of self-
1
reported affect across the three conditions. However, the primes had no impact on positive affect
2
F(2, 66) = 1.87, p >.16, nor negative affect, F(2, 66) = .36, p > .70. Moreover, the analyses
3
below were no different when we used these affect measures as covariates, showing that any
4
effect of our primes on out-group defense was not mediated by mood.
5
6
6.2.2. Out-group Defense
7
8
As predicted, we found a significant effect of scent prime on defensiveness toward the author of
9
the essay, F(2, 66) = 11.83, p < .001, η² = .26 (see Figure 3). Post-hoc analyses found that
10
putrescine led to greater hostility (M = 6.98, SD = 1.42) compared to ammonia (M = 5.05, SD =
11
1.54) and the neutral conditions (M = 5.43, SD = 1.30; both ps < .005). There was no significant
12
difference between the ammonia and control conditions, p > .6.
13
14
Experiment 4 supported the hypothesis that non-conscious exposure to putrescine evoked
15
defensive responses toward an out-group member, and this effect was not due to conscious
16
awareness of the scents, mood, or to the motivation to escape the aversive scent primes
3
.
17
Although these results suggest that the scent primes elicited an odor percept (non-consciously),
18
future studies may wish to control the precise intensities of the stimulus odors that are presented
19
(e.g., using an olfactometer).
20
21
7. General Discussion
22
23
This research was designed to test the hypothesis that putrescine could serve as a warning signal
24
that mobilizes protective responses to deal with threats. In four experiments, we found support
25
for this idea: conscious and non-conscious exposure to putrescine elicited distancing and
26
defensive reactions (e.g., fight and flight responses). Putrescine increased vigilance (Experiment
27
1), heightened the accessibility of escape- and threat-relevant cognitions (Experiment 3), and
28
increased the speed participants walked away from the location of the scent (Experiments 2 and
29
3). Experiment 4 created a situation where immediate escape was not likely and gave participants
30
the opportunity to evaluate an out-group member. Subtle exposure to putrescine produced greater
31
defensiveness toward the out-group member, suggesting an aggressive readiness in participants
32
(Cesario et al., 2010). As a whole, the findings indicate that even brief exposure to putrescine
33
mobilizes threat management responses designed to cope with environmental threats.
34
35
These are the first results to show that a specific chemical compound (putrescine) can be
36
processed as a threat signal. Thus far, nearly all the evidence for threat chemosignals has come
37
from those that are transmitted by body sweat (de Groot et al., 2012; Pause et al., 2012).
38
Moreover, these are among the first studies that show that a specific chemical compound can
39
cause overt behavior in humans (Wysocki & Preti, 2004). Furthermore, an advantage of isolating
40
putrescine in threat management processes is that it may help in determining which sensory and
41
brain pathways are involved in chemosensory threat detection and processing. For instance,
42
research suggests that the central nucleus of the amygdala projects to the midbrain
43
3
When the amount of time participants took to complete the questionnaire was used as a covariate, the results
remained significant, F(2, 65) = 13.13, p < .001, η² = .29).

12
periaqueductal gray, the hypothalamus and the brainstem, which together coordinate to prepare
1
fight-or-flight responses to threatening stimuli (Misslin, 2003). We speculate that putrescine
2
activates a similar neurological pathway. Future research could include physiological
3
measurements (e.g., systolic blood pressure, heart rate) to test the thesis that the observed effects
4
of putrescine are modulated by processes originating in the sympathetic nervous system.
5
6
An important direction for future research will be to understand the precise nature of the
7
threat produced by putrescine (e.g., microbial, predatory). Our view is that putrescine is relevant
8
to both of these domains, though the immediate context should determine which type of threat is
9
more primary. Recent work on trace amine receptors (TAARs) has the potential to shed light on
10
some of these mechanisms, as the activation of different receptors may function to detect specific
11
threats, such as predators and pathogens (Li & Liberles, 2015; Pérez-Gómez et al., 2015). In
12
addition, this research suggests that cadaverine (a compound with a similar chemical structure as
13
putrescine; both are diamines) activates a similar pathway and produces similar escape and
14
avoidance responses (Hussain et al., 2013; Oliveira et al., 2014) in animals. Thus, we render it
15
likely that cadaverine evokes a similar threat response as putrescine (see Li & Liberles, 2015).
16
17
It would also be interesting to examine how putrescine detection affects sensitivity to
18
particular types of threat and whether it produces elevated responses to certain stimuli more than
19
others (e.g., fear- vs. disgust-based sensitivities). For instance, further research could elucidate
20
how putrescine activates sensory acquisition (typically associated with fear experiences) and
21
sensory rejection (associated with disgust) processes (Susskind et al., 2008), and whether
22
exposure to putrescine augments physiological responses (e.g., heart rate, pupil dilation) that
23
typically co-occur with adaptive responses to threats. This type of research would benefit from
24
including individual differences in both disgust and fear sensitivity (Garfinkel et al., 2014; Haidt
25
et al., 1994). By the same token, future work could clarify whether putrescine elicits discrete
26
emotions (e.g., fear versus disgust) or less specific affective states associated with negative
27
valence and high arousal (see also Li & Liberles, 2015; Smeets & Dijksterhuis, 2014). Our
28
findings, which showed that responses to putrescine were automatic, occurred after various
29
lengths of delay (Experiments 1-3), and when presented at sub-threshold levels (Experiment 4),
30
suggested that conscious evaluations are not at the heart of the observed responses to putrescine.
31
This is consistent with our theorizing and ample work showing that chemosensory cues influence
32
psychological and physiological operations outside conscious awareness (for extended reviews,
33
see Sela & Sobel, 2010; Smeets & Dijksterhuis, 2014). However, we hasten to add that more
34
research is needed to specify the exact nature of the effects produced by the sub-threshold
35
priming of putrescine, for instance, by varying the exposure times to putrescine, the delay after
36
the primes, and the intensity of the putrescine stimulus.
37
38
Another important question is how specific threat management responses develop.
39
Within non-olfactory sensory channels, for example, there may be an innate bias for humans to
40
detect certain biologically-relevant stimuli as threatening, such as the sight of snakes and spiders
41
(Ohman & Mineka, 2001). Although controversial in human research, some work suggests that
42
responses to chemosensory stimuli are innate (Hussain et al., 2013; Misslin, 2003; Dielenberg et
43
al., 2001). For instance, Soussignan et al. (1997) showed that soon after birth, butyric acid (a
44
malodorous scent) evoked disgust reactions in neonates, a finding they claim is consistent with
45
an innate predisposition toward ecologically-relevant scents. To test for possibility of innate
46

13
biases toward threatening chemosensory cues, it would be interesting to examine whether
1
putrescine triggers facial expressions associated with fear in infants. In fact, research indicates
2
that adults do not habituate so readily to the scent of putrescine emitted from rotting flesh
3
(Roberson et al., 2008), suggesting that there might be a bias to respond warily to it.
4
5
Although the innateness of responses to chemosignals is still controversial, humans’
6
ability to incorporate learned information into cultural practices is beyond question (Boyd &
7
Richerson, 2005). Consequently, the magnitude of specific chemosensory threat responses could
8
be different in cultures where people are exposed to putrescine more frequently. Likewise,
9
reactions to putrescine may differ between cultures with different burial practices (e.g.,
10
embalming practices, the duration before burial). These factors should remind us that the context
11
is critical to how people react to putrescine. How olfactory information modulates other sensory
12
inputs (Zhou et al., 2012) is no doubt central to whether it will be interpreted as threatening.
13
14
One alternative theoretical perspective of our findings on the effects of putrescine is
15
Terror Management Theory (TMT; Greenberg et al., 1994). According to this theory, reminders
16
of death are regulated by a “cultural anxiety buffer” that consists of beliefs and values that imbue
17
life with meaning and the promise of immortality. Interestingly, TMT argues that a great deal of
18
the darker side of human behavior (e.g., aggression, out-group prejudice, religious intolerance)
19
stems from the need to maintain and defend the integrity of this cultural anxiety buffer, due to its
20
vital role in managing existential angst. In this view, putrescine could function as a reminder of
21
mortality, and subsequently elicits similar defensive processes, as activated by reminders of
22
death. We do not rule out this possibility, but render it unlikely that chemosensory threats trigger
23
the same type of processes as those that originate from the unique human ability to reflect on the
24
conundrum of life and death (Landau et al., 2007). Nevertheless, examining whether putrescine
25
can be used as a subtle reminder of death, and whether it influences cultural beliefs, values, and
26
practices, would open up fascinating directions of research.
27
28
Most research has shown that humans process threats either visually or audibly, while
29
other animals inhabit the inaccessible world of scents. At the same time, we know that humans
30
are guided by many of the same olfactory processes, especially when they involve fitness-
31
relevant information. We believe that by identifying putrescine as one of these signals, a further
32
understanding of its mechanisms can shed light on more general processes that modulate
33
chemosensory signaling and threat management responses.
34
35
36

14
8. Conflict of Interest Statement
1
2
The authors declare that the research was conducted in the absence of any commercial or
3
financial relationships that could be construed as a potential conflict of interest.
4
5
6

15
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20
Table 1
1
Hedonic evaluations of putrescine, ammonia, indole, “fart spray,” and skatole1 (Pilot study).
2
3
Scent primes Putrescine Ammonia Indole Skatole Fart spray
4
5
Intensity2
6
M 5.98b 6.60b 5.25a 7.23c 5.52b
7
SD 2.50 2.46 2.15 2.08 2.07
8
Familiarity
9
M 4.98a 5.10a 6.88b 5.21a 4.90a
10
SD 2.71 2.95 2.46 2.56 2.69
11
Repugnance
12
M 5.94b 5.94b 3.65a 6.54b 5.31b
13
SD 2.65 2.55 1.78 2.94 2.63
14
Positivity
15
M 2.63b 2.69b 3.81a 2.50b 2.67b
16
SD 1.55 1.78 2.05 1.87 1.77
17
N 48 48 48 48 48
18
19
20
1 “How intense is this scent?”, 1 = Not at all and 10 = Very much; “How familiar is this
21
scent?”, 1 = Not at all and 10 = Very much; “How repugnant is this scent?”, 1 = Not at all and 10
22
= Very much; “How positive does this scent make you feel?”, 1 = Not at all and 10 = Very much.
23
2 Different subscripts on a hedonic dimension (within a row) indicate a significant
24
difference of p < .05.
25
26
27

21
Table 2
1
Scent ratings for the chemosensory primes (Experiment 1)
2
3
Chemosensory primes Neutral Ammonia Putrescine
4
5
Intensity
6
M 3.30 4.73 4.27
7
SD 1.81 1.45 1.92
8
9
Familiarity
10
M 6.00 5.10 4.40
11
SD .86 2.25 1.60
12
13
Repugnance
14
M 2.35 5.90 5.65
15
SD 1.46 1.34 1.23
16
N 20 20 20
17
18

22
Table 3
1
Scent ratings for the chemosensory primes (Experiment 2)
2
3
Chemosensory primes Neutral Ammonia Putrescine
4
5
Intensity
6
M 1.53 4.73 4.27
7
SD .64 .46 .70
8
9
Familiarity
10
M 4.75 1.60 1.67
11
SD .46 .51 .62
12
13
Repugnance
14
M 1.73 4.47 4.80
15
SD .70 .74 .41
16
N 15 15 15
17
18

23
Table 4
1
Scent ratings for the chemosensory primes (Experiment 3)
2
3
Chemosensory primes Neutral Ammonia Putrescine
4
5
Intensity
6
M 1.85 3.20 3.40
7
SD .99 1.32 .99
8
9
Familiarity
10
M 2.95 2.20 2.15
11
SD .83 .89 .75
12
13
Repugnance
14
M 2.60 3.70 3.50
15
SD .60 .98 1.15
16
N 20 20 20
17
18
19

24
Table 5
1
The ratings of escape-related and threat-related cognitions for the chemosensory primes
2
(Experiment 3).
3
4
Chemosensory primes Neutral Ammonia Putrescine
5
6
Escape cognitions
7
M 2.15 2.45 3.45
8
SD 0.99 1.05 0.69
9
10
Threat cognitions
11
M 1.68 1.73 2.55
12
SD 0.65 0.64 0.94
13
N 20 20 20
14
15

25
1
2
3
Figure 1. The number of seconds it took participants to walk 80 meters after exposure to the
4
scent prime (Experiment 2). Asterisks denote that two groups differ at **p < .005.
5
6
7
8
**
**
50
55
60
65
neutral ammonia putrescine
Walking time in seconds

26
1
2
3
4
Figure 2. The number of seconds it took participants to walk 60 meters after exposure to the
5
scent prime (Experiment 3). Asterisks denote that two groups differ at *p < .05.
6
7
8
9
10
11
12
*
*
25
30
35
40
neutral ammonia putrescine
walking time in seconds

27
1
2
3
Figure 3. Mean scores on the worldview defense scale for all three conditions (Experiment 4).
4
Higher scores reflect greater hostility toward the target. Asterisks denote two groups differ at **p
5
< .005.
6
7
8
9
10
11
**
**
0
1
2
3
4
5
6
7
8
9
neutral ammonia putrescine
outgroup defense