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During two retreats in 2017 and 2020, a group of international scientists convened to explore the Human-Animal Bond. The meetings, hosted by the Wallis Annenberg PetSpace Leadership Institute, took a broad view of the human-dog relationship and how interactions between the two may benefit us medically, psychologically or through their service as working dogs (e.g. guide dogs, explosive detection, search and rescue, cancer detection). This Frontiers’ Special Topic has collated the presentations into a broad collection of 14 theoretical and review papers summarizing the latest research and practice in the historical development of our deepening bond with dogs, the physiological and psychological changes that occur during human-dog interactions (to both humans and dogs) as well as the selection, training and welfare of companion animals and working dogs. The overarching goals of this collection are to contribute to the current standard of understanding of human-animal interaction, suggest future directions in applied research, and to consider the interdisciplinary societal implications of the findings.
EDITED BY : Sandra McCune, Aubrey Howard Fine, Eric G. Strauss and
Evan MacLean
PUBLISHED IN : Frontiers in Veterinary Science and Frontiers in Psychology
Frontiers in Veterinary Science 1April 2022 | Introduction to Our Canine Connection
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ISSN 1664-8714
ISBN 978-2-88974-841-9
DOI 10.3389/978-2-88974-841-9
Frontiers in Veterinary Science 2April 2022 | Introduction to Our Canine Connection
Topic Editors:
Sandra McCune, University of Lincoln, United Kingdom
Aubrey Howard Fine, California State Polytechnic University, Pomona,
United States
Eric G. Strauss, Loyola Marymount University, United States
Evan MacLean, University of Arizona, United States
Image: Sarah DeRemer
As part of the Wallis Annenberg PetSpace Leadership Institute initiative, funding for
this publication was provided by Wallis Annenberg PetSpace.
Citation: McCune, S., Fine, A. H., Strauss, E. G., MacLean, E., eds. (2022). Our
Canine Connection: The History, Benefits and Future of Human-Dog Interactions.
Lausanne: Frontiers Media SA. doi: 10.3389/978-2-88974-841-9
Frontiers in Veterinary Science 3April 2022 | Introduction to Our Canine Connection
04 Editorial: Our Canine Connection: The History, Benefits and Future of
Human-Dog Interactions
Eric G. Strauss, Sandra McCune, Evan MacLean and Aubrey Fine
07 Variability in Human-Animal Interaction Research
Kerri E. Rodriguez, Harold Herzog and Nancy R. Gee
16 Dogs Supporting Human Health and Well-Being: A Biopsychosocial
Nancy R. Gee, Kerri E. Rodriguez, Aubrey H. Fine and Janet P. Trammell
27 Canine-Assisted Interventions in Hospitals: Best Practices for Maximizing
Human and Canine Safety
Sandra B. Barker and Nancy R. Gee
39 Commensalism or Cross-Species Adoption? A Critical Review of Theories
of Wolf Domestication
James A. Serpell
49 Considering the “Dog” in Dog–Human Interaction
Alexandra Horowitz
54 Enhancing the Selection and Performance of Working Dogs
Emily E. Bray, Cynthia M. Otto, Monique A. R. Udell, Nathaniel J. Hall,
Angie M. Johnston and Evan L. MacLean
75 Healthy, Active Aging for People and Dogs
Sandra McCune and Daniel Promislow
86 The Indispensable Dog
Clive D. L. Wynne
97 Working Dog Training for the Twenty-First Century
Nathaniel J. Hall, Angie M. Johnston, Emily E. Bray, Cynthia M. Otto,
Evan L. MacLean and Monique A. R. Udell
115 Advancing Genetic Selection and Behavioral Genomics of Working Dogs
Through Collaborative Science
Frances L. Chen, Madeline Zimmermann, Jessica P. Hekman,
Kathryn A. Lord, Brittney Logan, Jane Russenberger, Eldin A. Leighton and
Elinor K. Karlsson
129 The Animal Welfare Science of Working Dogs: Current Perspectives on
Recent Advances and Future Directions
Mia L. Cobb, Cynthia M. Otto and Aubrey H. Fine
142 The New Era of Canine Science: Reshaping Our Relationships With Dogs
Evan L. MacLean, Aubrey Fine, Harold Herzog, Eric Strauss and Mia L. Cobb
Table of Contents
published: 04 November 2021
doi: 10.3389/fvets.2021.784491
Frontiers in Veterinary Science | 1November 2021 | Volume 8 | Article 784491
Edited and reviewed by:
Marta Hernandez-Jover,
Charles Sturt University, Australia
Eric G. Strauss
Specialty section:
This article was submitted to
Veterinary Humanities and Social
a section of the journal
Frontiers in Veterinary Science
Received: 27 September 2021
Accepted: 08 October 2021
Published: 04 November 2021
Strauss EG, McCune S, MacLean E
and Fine A (2021) Editorial: Our
Canine Connection: The History,
Benefits and Future of Human-Dog
Front. Vet. Sci. 8:784491.
doi: 10.3389/fvets.2021.784491
Editorial: Our Canine Connection:
The History, Benefits and Future of
Human-Dog Interactions
Eric G. Strauss 1
*, Sandra McCune 2,3 , Evan MacLean 4,5,6, 7 and Aubrey Fine 8
1The Center for Urban Resilience, Loyola Marymount University, Los Angeles, CA, United States, 2School of Psychology,
School of Life Sciences, University of Lincoln, Lincoln, United Kingdom, 3Animal Matters Consultancy Ltd., Stamford,
United Kingdom, 4Arizona Canine Cognition Center, School of Anthropology, University of Arizona, Tucson, AZ,
United States, 5Department of Psychology, University of Arizona, Tucson, AZ, United States, 6College of Veterinary Medicine,
University of Arizona, Tucson, AZ, United States, 7Cognitive Science Program, University of Arizona, Tucson, AZ,
United States, 8California State Polytechnic University, Pomona, CA, United States
Keywords: human-animal bond, Annenberg PetSpace, canine domestication, working dogs, canine connection
Editorial on the Research Topic
Our Canine Connection: The History, Benefits and Future of Human-Dog Interactions
We are current witness to profound changes in the demography and social behavior of humans
that includes paradigm shifts in science, religion and philosophy. Coupled with the profound
impacts of plant and animal domestication, the modern forces of urbanization and technological
transformation have created complex social communities yoked to the ecology and behavior of
domestic animals whose roles were likely unimagined only a century ago. Scholars refer to the
changing place of humanity on the planet and have named the current epoch the Anthropocene,
but it would not have been possible without agricultural and animal domestication that dates back
to antiquity. Our efforts to better understand the lives of working domestic animals have great
potential to enhance the processes through which these animals are bred, selected, trained, and
cared for, resulting in a more benevolent ecosystem for this mutual relationship. In this series, we
present a suite of papers that aim to sharpen our understanding of the origins, behavior, cognition
and welfare of the domestic dog, and apply these considerations to their working roles in society.
The goal of this exercise is to provide a review of the science to date and suggest new frameworks
for future use-inspired research in the canine sciences.
The genesis of this project was made possible and funded by the Annenberg PetSpace
Foundation through the establishment of a Leadership Institute (https://www.annenbergpetspace.
org/about/leadership). The Institute seeks to foster and support the understanding of the human-
animal bond through research and program implementation. Fellowships are awarded by the
Foundation in order to bring together experts with diverse backgrounds for intellectual exchange
and visioning for future research. The first two rounds of the Fellowship Program focused on
canine research and its applications. Each round included a multi-day retreat which served as an
intellectual salon, with an open agenda for the exchange of ideas. The first retreat was held in
2017 ( and spanned a universe of expertise
from history, genetics and evolution, to behavior, welfare and human social dynamics. Emerging
from this effort was a single multi-author paper (1) that argued for carving out a space for canine
science and the domestic dog in the context of tackling the declining environmental conditions of
the globe. Titled Humanity’s Best Friend, the team of authors considered the role of domestication
and the complex relationship that we have with domestic dogs as tools to better achieve Sustainable
Development Goals (SDG) as outlined by the United Nations.
Strauss et al. Editorial: Introduction to Our Canine Connection
Building on the success of this project, the Annenberg
PetSpace Foundation hosted a second round of Leadership
Fellows in 2020 (
leadership). The strategy employed at this retreat was to narrow
the focus to the specific dimensions of working dogs, human
animal interaction (HAI) research and implications for the
understanding of domestication as a tool for both improving
human health outcomes and those of our canine companions.
What follows is the suite of 12 papers authored by select members
of both retreats and their colleagues. The papers are organized
into five broad categories, which we explore below.
Three papers address the issues of dog domestication, overall
ecological character and genetics. Serpell tackles the controversial
and dynamic story of the domestication of dogs from their wolf
ancestors. Serpell compares the evidence for two predominant
origin stories. The first suggests a commensal relationship of
wolves exploiting food scraps from humans that gradually gave
rise to a full-blown mutualism of collaborative hunting, resource
protection and social companionship. The second model, which
Serpell favors, suggests an active role for Paleolithic peoples who
actively captured wolf pups as a form of pet-keeping, otherwise
known as the cross-species adoption hypothesis. Serpell suggests
that the ecologies of both species make such a phenomenon
quite plausible.
Wynne picks up the domestication saga much later in time
and explores the complex relationships humans have had with
domestic dogs in recent history. Unlike theories that suggest
that dogs are more compliant than wolves and therefore more
likely to be domesticated, Wynne argues that the foraging
nature of dogs facilitates a strong independence, unbounded
by the complex social order of wolves. Along with this set of
traits comes the ability of humans and dogs to interact across
species boundaries, with Wynne characterizing the nature of
this relationship as one of “super-dominance,” with allusion
to Tinbergen’s ethological concept of supernormal stimuli. The
third contribution to this set of papers comes from the lab
of Elinor Karlsson and is led by Chen et al. They argue for
the implementation of modern tools of genomics to better
understand the nature of dogs in general and specifically
genetic potential for diverse working applications. They argue
for the application of statistical genomic approaches as a way
to revolutionize the field of working dogs leading to a much
higher degree of success in selection and training of puppies.
Featured is a step-by-step guide for breeders to implement
estimated breeding values in their programs through genotyping
and DNA sequencing.
Two papers address issues regarding the training and
selection of dogs for working roles. Hall et al. leads a team
that reviews current practices in working dog training
concluding that many of the techniques used today predate
major advances in our understanding of dog cognition,
many of which have occurred in the last two decades.
The authors identify specific areas in which working
dog training should be modernized for the Twenty-first
century, and identify key research questions which have yet to
be explored.
Bray et al. collaborates with a team to identify practices
through which the selection and performance of working dogs
can be optimized. They point out that while dogs provide
very complex services in their roles as working animals,
the processes by which they are selected and trained have
numerous points of failure, which collectively are expensive and
burdensome to trainers and stakeholders, and have potential
negative implications for dog welfare and human health The
authors stress the need to develop new selection criteria at the
level of the individual candidate animal and to enhance breeding
and rearing practices at the population level, in order to best serve
working dogs and the humans they assist.
Three papers explore the positive effects that working and
companion dogs have on human health, with an emphasis on
the impact of these relationships on the dogs as well. McCune
and Promislow consider the arc of aging in humans and dogs,
which differ considerably due to their variation in lifespans.
This apparent disequilibrium actually provides a myriad of
opportunities for a deeper understanding of the aging process
and the mutual companionship opportunities that exist because
of it. Using the World Health Organization’s Healthy Aging
Initiative, the authors explore the research on aging of both dogs
and humans and how we can better understand and support this
process through innovative research and program development.
Gee et al., along with Kerri Rodriguez use a biopsychosocial
framework to contextualize current research on the contribution
of human-animal interactions to human wellbeing. The authors
also focus on a wide range of social contexts in which dogs have
been involved in adjunct or complementary therapy to a diverse
community of people in need. The authors provide a framework
for analysis and help set an agenda for further research in this
transdisciplinary field.
Finally, Barker and Gee consider the state of knowledge in the
use of therapy dogs in formal hospital settings. The goal here is to
maximize the benefit to human patients while also protecting the
physical and mental health of the dogs. As they note, this places
human medicine and the veterinary sciences in a collaborative
partnership that currently lacks the appropriate understanding
and policies to ensure success in all dimensions. The paper
presents a clear rationale for canine-assisted interventions in a
hospital setting and explores the challenges presented by these
novel therapies. A two-decade program in a medical center is
used as an exemplar along with a thorough analysis of the
research in order to build the case for future expansion of
these practices.
Frontiers in Veterinary Science | 2November 2021 | Volume 8 | Article 784491
Strauss et al. Editorial: Introduction to Our Canine Connection
Two of the papers in this series deal directly with the welfare
of working dogs and also the consideration of the world from
the dog’s perspective. Cobb et al. review papers on working dogs
from the last decade (2011–2020) with an emphasis on human
interaction, ethics and the five domains of animal welfare. As
the awareness of, and research on animal welfare has increased,
the authors use this growing body of research to evaluate past
practices and current challenges for developing a sustainable
and responsible model in which working dogs are embraced
as co-workers.
Horowitz evaluates dog-human interaction under the
umbrella of HAI research from the perspective of the
welfare of the dog as the “silent partner.” In reviewing
the literature, she poses fundamental questions about
the appropriateness of dogs in research, the impact on
the species and individuals, even developing a model
for a dog’s agency in research. She poses the idea that
dogs should have a system of providing “consent” for
participation, similar to what we expect from human
research studies.
Two of the papers have a focus on research methodologies
and future challenges and opportunities of domestic canine
science. Rodriguez et al. seek to improve the quality of future
research by looking at the strengths and weaknesses of current
methodologies. They investigate ways in which experimental
design can be improved in order to sharpen the hypotheses that
are being investigated and strengthen the conclusions that can
be drawn from this work. These challenges are particularly acute
with a species as cosmopolitan as the domestic dog. The onus
on improving the research is an issue that has dogged the wild
canid research community as well, especially with respect to
managing coyotes (2). The convergence of concern in domestic
and wild canid research suggests supplying a new standard to
the research effort can amplify the effectiveness of the work
and reduce the risk of implementing weak inferences from
the findings.
The final paper from the MacLean et al. serves as a synthesis
and prospectus that unifies the suite of papers and looks toward
the future of use-inspired research in the field of anthrozoology.
Focusing on a range of topics, relevant to stakeholders, funders
and the research community, the authors delineate a path
forward for this interdisciplinary work. Drawing on similar
disciplines that investigate human-dominated natural systems
and synanthropic species, such as urban ecology, we argue that
HAI can have profound implications on human wellbeing and
that of our canine partners. The collection of articles will provide
a comprehensive synthesis of the current trends within the field.
We hope you enjoy the series and find it relevant to your work
and a catalyst to your aspirations.
All authors listed have made a substantial, direct and intellectual
contribution to the work, and approved it for publication.
The editors thank the Annenberg PetSpace Foundation for
their kind support of this project and the authors for their
commitment to the work.
1. Sykes N, Beirne P, Horowitz A, Jones I, Kalof L, Karlsson E, et al. Humanity’s
best friend: a dog-centric approach to addressing global challenges. Animals.
(2020) 10:502. doi: 10.3390/ani10030502
2. Treves A, Krofel M, McMannus J. Predator control should not be a shot in the
dark. Front Ecol Environ. (2016) 14:380–8. doi: 10.1002/fee.1312
Conflict of Interest: SM is a paid consultant, paid for by Annenberg PetSpace to
lead the development of this special topic, the manuscripts of which, came from
two workshops which they sponsored and he was employed by company Animal
Matters Consultancy Ltd. And he is a paid consultant on the Research Advisory
Board for the Waltham Centre for Pet Nutrition, Mars UK.
The remaining authors declare that the research was conducted in the absence of
any commercial or financial relationships that could be construed as a potential
conflict of interest.
Publisher’s Note: All claims expressed in this article are solely those
of the authors and do not necessarily represent those of their affiliated
organizations, or those of the publisher, the editors and the reviewers.
Any product that may be evaluated in this article, or claim that may
be made by its manufacturer, is not guaranteed or endorsed by the
Copyright © 2021 Strauss, McCune, MacLean and Fine. This is an open-access
article distributed under the terms of the Creative Commons Attribution License (CC
BY). The use, distribution or reproduction in other forums is permitted, provided
the original author(s) and the copyright owner(s) are credited and that the original
publication in this journal is cited, in accordance with accepted academic practice.
No use, distribution or reproduction is permitted which does not comply with these
Frontiers in Veterinary Science | 3November 2021 | Volume 8 | Article 784491
published: 15 January 2021
doi: 10.3389/fvets.2020.619600
Frontiers in Veterinary Science | 1January 2021 | Volume 7 | Article 619600
Edited by:
Sandra McCune,
University of Lincoln, United Kingdom
Reviewed by:
Peggy D. McCardle,
Consultant, New Haven, United States
Carmen Fuentealba,
Long Island University, United States
Kerri E. Rodriguez
Specialty section:
This article was submitted to
Veterinary Humanities and Social
a section of the journal
Frontiers in Veterinary Science
Received: 20 October 2020
Accepted: 17 December 2020
Published: 15 January 2021
Rodriguez KE, Herzog H and Gee NR
(2021) Variability in Human-Animal
Interaction Research.
Front. Vet. Sci. 7:619600.
doi: 10.3389/fvets.2020.619600
Variability in Human-Animal
Interaction Research
Kerri E. Rodriguez 1
*, Harold Herzog 2and Nancy R. Gee 3
1Human-Animal Bond in Colorado, School of Social Work, Colorado State University, Fort Collins, CO, United States,
2Department of Psychology, Western Carolina University, Cullowhee, NC, United States, 3Department of Psychiatry, Center
for Human Animal Interaction, School of Medicine, Virginia Commonwealth University, Richmond, VA, United States
The field of Human-Animal Interaction (HAI) is plagued with mixed results. Some findings
appear to indicate that interacting with a companion animal is beneficial for some aspect
of human health and well-being, while other research outcomes are inconclusive or even
indicate the opposite. The purpose of this paper is to take a closer look at this variability in
research outcomes and to provide plausible explanations and potential remedies. Some
of the reasons for mixed results are likely due to the wide variety of methodologies
implemented, intermittent use of standardized measures and manualized protocols,
variability in human and animal participants, and limited quantification of human-animal
interactions or definitions of pet ownership. Variability in research outcomes is not unique
to HAI and is, in fact, not uncommon in many more established fields such as psychology
and medicine. However, the potential reasons for the variability may be linked to the
unique nature of HAI in that, in its’ simplest form, it involves two complex organisms,
a human and an animal, interacting in dynamic ways. We argue that this complexity
makes research in this field particularly challenging and requires a broad spectrum of
theoretical and methodological considerations to improve rigor while ensuring the validity
and reliability of conclusions drawn from study results.
Keywords: human-animal interaction (HAI), methodology, animal-assisted intervention (AAI), variability in
outcomes, replication
The idea that interacting with companion animals conveys health and well-being benefits to
humans goes back for centuries. Empirical research on the impact of pets on people, however, dates
to the 1980s (1). Among the most influential early investigations were studies reporting that pet
owners had significantly lower rates of mortality following heart attacks (2) and that interacting
with dogs produced decreases in blood pressure and levels of physiological stress (3). Over the
last 20 years, research on the health and therapeutic implications of the human-animal bond,
including animal-assisted interventions (AAI), has grown exponentially. Hundreds of papers on
these topics are now published in academic journals each year, and centers devoted to the study of
human-animal relationships have been established in North America, Europe, Asia, and Australia.
In addition, nearly 50 educational institutions now offer undergraduate or graduate degrees in
human-animal relationships (4).
In recent years, the notion that pet owners are healthier and happier than non-pet owners
has gained popularity. A 2016 survey by the Human-Animal Bond Research Institute (HABRI)
found that 71% of pet owners were aware of research showing that pets improve human mental
Rodriguez et al. Variability in HAI
and physical health. Another HABRI survey found that 97% of
family doctors who responded agreed there are health benefits of
owning pets. There is a mismatch, however, between the results
of empirical investigations and public perceptions of the positive
effects of companion on human health and well-being (5). Some
studies have found, for example, that pet owners have lower rates
of mortality and obesity, higher self-esteem, are happier, and
have decreased blood pressure and stress levels (6). Yet other
studies have found no differences in these measures. Further,
some researchers have reported that pet owners are more likely
to suffer from disorders such as anxiety, insomnia, depression,
obesity, ulcers, and panic attacks (6).
Research on pet ownership and loneliness exemplifies
variations in results of studies on the impact of living with
pets on well-being. Gilbey and Tani (7) reviewed 13 studies
published between 1986 and 2014 comparing levels of loneliness
in pet-owners and non-owners. Using standardized psychological
instruments, five of the studies found that pet owners were less
lonely, seven found no differences in the degree of loneliness in
owners and non-owners, and one study reported mixed results
(7). Further, only one of eight studies published between 2014
and 2020 found that pet owners were less lonely (8). Four of
these studies reported no difference between the groups, and two
produced mixed results. When the older and newer studies are
combined, six reported beneficial associations between loneliness
and pet ownership, while twelve found no association between
pet-ownership and loneliness.
Outcomes from studies on the efficacy of animal-assisted
intervention for improving human health and well-being have
also not been uniformly positive, with similarly mixed results.
For example, several studies have suggested that therapy dog
visits may have beneficial physiological and psychological effects
on hospitalized pediatric patients (9,10). However, in one of
the largest multi-site randomized controlled trials (RCT) on
the effects of therapy dog visits on pediatric patients to date,
researchers found that children in outpatient cancer treatment
units who received 4 months of weekly therapy dog visits did not
exhibit reduced stress, reduced anxiety, or improved quality of
life compared to children randomized to treatment as usual (11).
Reviews of the literature have pointed out significant threats to
construct validity regarding the therapeutic value of the physical
animal in AAI (12) as well as the potential for inflated false
positives in findings (13) that may contribute to mixed findings
in the field.
Variations in Research Results Are
Common in Science
The high degree of variation found in the results of HAI
research is also common in more established fields. Experimental
psychology in particular has been plagued with conflicting
findings. A 2015 article in Science reported the results of
replication attempts of 100 studies published in three reputable
psychology journals (14). Only 39% of the results of the original
studies could be replicated. Indeed, the results of widely accepted
findings in behavioral research have been called into question by
inconsistent findings. These include the impact of nasal oxytocin
administration on interpersonal trust (15), changes in female
mate preferences associated with ovulation (16), and the ego-
depletion model of self-control (17).
Variability in outcomes is also common in clinical medicine.
For example, a recent review of studies published in three leading
medical journals found that standard treatments were based
on the results of nearly 400 randomized controlled trials that
failed to replicate (18). Among these were hormone therapies
for menopause, breast cancer screening, knee surgery, and CPR
techniques. A search of the phrase “variability in outcomes” in
PubMed returned 195,828 hits, with 74,977 hits when restricted
to the most recent 5-year period. The published manuscripts
from these searches covered a wide range of topics such as the
longitudinal course of posttraumatic stress disorder [PTSD; (19)],
treatment of polycystic kidney disease (20), and recovery from
arthroscopic anterior shoulder repair (21), to name but a few.
Concern for persistent variation in research results across
science (the “replication crisis”) was sparked by a 2005 paper by
John Ioannidis titled, “Why Most Published Research Findings
Are False.” Ioannidis argued that inconsistent and false findings
are particularly common in research areas that have several
characteristics. These include small sample sizes, small effect
sizes, “flexibility in designs, definitions, and outcomes” and,
finally, fields that suddenly become “hot” (22). These problems
are characteristic of many HAI research studies. Take, for
example, a recent meta-analysis of research on the efficacy of
animal-assisted psychotherapy for the treatment of trauma. The
researchers found that seven of the nine clinical trials in the
analysis were statistically underpowered; five of them had fewer
than 17 subjects (23). A meta-analysis of eleven reports on the
efficacy of prison-based dog programs found that most of the
treatment effects in these studies were low [average d=0.15;
(24)]. Finally, HAI falls into Ioannides’ “hot field” category.
According to a Google Scholar search using the term “therapy
dog,” the annual number of published papers related to canine-
assisted therapies jumped from 60 in 2010 to 237 in 2019.
A specific consideration in explaining outcome variance in HAI
research is that studies significantly vary in their methodological
design and rigor. HAI researchers use a wide variety of designs
to answer comparable questions in the field, ranging from case
studies, single-subject research, and qualitative interviews to
observational, cross-sectional, and longitudinal studies. While
early studies in HAI were largely limited by a lack of control
conditions and small sample sizes, more recent studies have
substantially improved in their methodological rigor (25).
Despite recent advances in methodology, systematic reviews of
both animal-assisted intervention and pet ownership studies
repeatedly state that is difficult to draw definitive conclusions
from the data due methodological weaknesses across studies [e.g.,
An emerging number of studies using randomized clinical
trial designs have shown promise in legitimizing the validity and
Frontiers in Veterinary Science | 2January 2021 | Volume 7 | Article 619600
Rodriguez et al. Variability in HAI
strength of evidence in the field. However, even conclusions from
RCTs can be limited by a high risk of bias from inadequate
concealment and blinding [e.g., (28)]. Even the most rigorous
studies can also vary widely in their control or comparison
conditions. For example, intervention studies may feature no
control at all, an active control (e.g., interaction with toys or
stuffed animals), or a no-treatment control (e.g., waitlist or
withdrawal periods). With a variety of control and comparison
conditions used in the field, this leads to both variability in
outcomes as well as difficulty making cross-study comparisons.
In a systematic review of eight RCTs evaluating the effects of
AAI on psychosocial outcomes, several different comparison
conditions were identified including treatment without an
animal present, active comparisons with human visitation or
quiet reading, and waitlist controls (29). The type of control
condition used may have direct impacts in study results. For
example, a moderator analysis conducted as part of a meta-
analysis on the effects of AAI in medical settings found
that studies with a social control condition (i.e., featuring
human interaction but not animal interaction) had significantly
smaller effect sizes than studies with a non-social control
condition (30).
Additionally, there is widespread variability in measurement
methods in HAI. Specifically, the use of standardized, validated
measures to quantify outcomes has been inconsistent across HAI
research (25). In a systematic review of 48 studies assessing
AAI in the form of reading to therapy dogs among school-
aged children, only 13 studies used standardized measures with
established validity and reliability to measure outcomes (31).
Rather, many studies incorporate subjective ratings, researcher-
created measures, or modified existing measures which makes it
difficult to compare findings across studies. Still, among studies
that do incorporate standardized measures, the sheer number
of measures available to quantify the constructs of interest to
HAI research (i.e., mental health, social functioning, quality
of life) has resulted in further variability in the literature. For
example, a systematic review of 14 studies on the efficacy
of AAI for children with autism found that no two studies
used the same standardized assessment tool (32). This lack of
replication of measurement across studies prevents the ability
to make informed conclusions with meta-analytic methods,
which is crucial for providing an evidence base for the
field (33).
Standardized measures also vary in the appropriateness of
content for the theoretical outcomes of HAI. For example,
a popular scale of loneliness called the UCLA Loneliness
Scale was recently evaluated for its appropriateness to quantify
beneficial social effects of pet ownership (34). Both qualitative
and quantitative evaluation suggested that only 6/20 items
were likely sensitive to change following pet ownership or
pet acquisition, concluding that despite its widespread use
the scale lacks efficacy for quantifying the effect of pets on
loneliness. Therefore, while the use of consistent measures
across studies is important for replication purposes, measures
must be chosen for their sensitivity to change following
animal interaction.
When quantifying the role that animals play in our lives, it
is important to consider the heterogeneity in how humans
may perceive, respond to, and interact with animals. Individual
differences in demographic variables such as age, gender, and
race/ethnicity may contribute significantly to outcome variance.
For example, a meta-analysis of outcomes from AAT found that
studies of young children had the most consistently positive
outcomes, while other age groups exhibited more variability
in outcomes (35). Not only may males and females have
different hormonal responses to interaction with animals (36),
but females have been found to report more positive behaviors
and attitudes toward animals (37) and toward animal-assisted
interventions in general (38). While these gender differences in
attitudes and responses may not be unique to HAI, equal care
should be taken to consider gender-specific effects in analyses
as in other fields of research. Ethnicity, cultural, and religious
differences may also contribute to attitudes and perceptions
of animals (39,40), However, neither demographic variables
nor other potentially confounding variables such as marital
status, sources of social support, and socioeconomic status are
consistently controlled for in HAI studies (41). The omission
of key explanatory variables in analyses can lead to invalid
conclusions if unmeasured confounding variables are partially
or fully explaining significant findings. For example, a recent
systematic review of the impact that pets have on child and
adolescent development found that 14 of 22 studies did not
consider any confounding variables in analyses, leading authors
to conclude that no firm conclusions can be drawn from the
literature (42). In addition to controlling for these confounders,
future large-scale research studies should also consider the extent
to which demographic or contextual variables may mediate
outcomes (43). Mediator and/or sub-group analyses may also
aid in understanding for whom and under what conditions
individuals benefit from HAI (5,44).
In addition to demographic and environmental variables,
human participants in HAI research often vary widely in their
physical and mental health. As a key research question in this field
is understanding how animal interaction may benefit individuals
of sensitive populations, HAI research often includes a range
of disabilities, disorders, and chronic conditions. Frequently,
participants are selected for participation in research based on
a single diagnosis (e.g., posttraumatic stress disorder, cerebral
palsy, etc.). However, not only is there variation across studies
in how and when this diagnosis was made, there can also be
considerable phenotypic variation among individuals with the
same condition (e.g., autism spectrum disorder, ASD). In a
systematic review of 13 studies addressing the impact of AAI on
social behaviors of children with ASD, nine different terms were
used to describe participants’ diagnosis and/or severity including
autism spectrum disorder, autism, autistic disorder, moderate
autism, early childhood autism, and atypical autism (45). Thus,
it is difficult to compare results across these studies when
participants’ symptoms and behavioral profiles are markedly
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Rodriguez et al. Variability in HAI
different. Even in phenotypically similar disorders, there is also
often participant variability in severity, progressiveness, and
duration of the condition or disability. However, these factors
are often not controlled for or considered in statistical analyses.
For example, a systematic review of the effects of AAI on
individuals with dementia found that only 13 of 32 studies
controlled for the severity of dementia in their design or analysis
(26). Disability severity and progressiveness can be important
explanatory variables in psychosocial outcomes such as quality
of life, however. In a 2006 study of the psychosocial effects of
mobility service dogs for their handlers, having a progressive
condition (e.g., muscular dystrophy, multiple sclerosis) was an
important moderator of whether having a service dog was
associated with higher positive affect (46).
Emerging research also suggests that human genetic
differences may play a key role in the study of human-animal
interactions. A recent 2019 study indicated that there may be a
genetic and heritable component for choosing to have a pet (47).
Specifically, researchers examined pet dog ownership among
over 35,000 pairs of twins in Sweden and found that more than
50% of the variability in whether an individual owned a dog at the
time of the study was explained by genetics. Although the specific
genes associated with dog ownership could not be identified,
this research suggests that a combination of environmental and
genetic influences could influence an individual’s affinity toward
animals. Genetic variability is also an important consideration in
research incorporating hormones and/or neuropeptides such as
oxytocin and cortisol. Variations in the oxytocin receptor gene
have been associated with human attachment behavior (48) and
caregiving styles (49), and recently have been demonstrated to be
associated with dog-owner attachment (50). Similarly, there are
many sources of genetic and environmental influence on cortisol
synthesis, metabolism, and reactivity (51,52).
Finally, it is also important to consider differences in human
experiences, thoughts, and behavior that may contribute to
variable outcomes from HAI. Research suggests that interactions
and relationships with companion animals can be impacted by
human personality traits (53,54). In addition, human attachment
styles (e.g., avoidant or anxious attachment) can be important
in understanding variation in the human-animal bond. For
example, studies have shown that pet owners with avoidant
attachment to their pets experience less stress-reducing benefits
from their pets (55) and report negative expectations about a
pet’s availability and responsiveness (56). Quality and quantity
of previous animal interaction, which is often unaccounted
for in HAI research (5), is another important aspect of inter-
participant variability. Future HAI research should be mindful of
these differing experiences, including previous and current pet
ownership as well as any fears or aversions toward animals, in
both the design and analysis of studies.
Not only is there unique variation in human participants that
needs to be accounted for, but also in animal participants.
Animals’ temperament, personality, training, and even
physiology are becoming increasingly important considerations
in understanding variability in HAI research. Of course, there
is wide variability in the species of animals studied in this field
(e.g., mammals, birds, exotics, farm animals) that contributes
to heterogeneity across studies (57). However, even within a
single species, there is also variability in animals’ appearance,
disposition, rearing/training, and history of human interaction
that may influence outcomes (58). As the animal itself is a key
component of HAI, detailed descriptions and considerations
of animal characteristics are critical to disentangling potential
mechanisms of benefits (12). In the case of AAI, a consideration
of animals’ varying qualities also parallels the increasing
acceptance of animals as individual agents rather than tools or
objects (59,60). Researchers should also be mindful of the fact
that the animal’s handler during an AAI session will also vary in
their experience and knowledge regarding animal welfare as an
additional source of variation (61).
As one of the most commonly studied companion animals
in HAI research, dogs in particular exhibit a wide range of
characteristics that contribute to variability. With a variety of
breeds and sizes of dogs incorporated into companion, therapy,
and assistance roles, individual differences in dogs’ morphology
and disposition are important aspects of variation in HAI
literature. For example, physical traits such as a dog’s size, coat,
eye color, and ear shape have been shown to impact the way that
humans perceive dogs (62,63). In addition, different breeds of
dogs can significantly differ in their temperament and behavior
(64,65). For example, some breeds may be more likely to make
spontaneous eye contact (66), follow human communicative
gestures (67) and be more sociable or playful with humans (68)
than others. Even dogs of the same breed category can differ in
personality characteristics including playfulness, curiosity, and
sociability (69). These individual differences may impact the way
that a dog, whether in a pet, therapy, or assistance role, interacts
with and potentially bond with humans in the short-term or long-
term (70). In the case of pet dogs, studies have found that owners
of large dogs spend more time walking their dogs (71) and engage
in more training and play with their dogs (72) while small breeds
are reported to have more behavioral problems (73). In fact,
considering breed-specific variation in analyses is an essential
step toward understanding how genetic, physical, and behavioral
differences in dogs may explain or predict human therapeutic
outcomes. For example, a recent study tracking over 180,000
heart attack victims and 150,000 stroke victims found that dog
owners had a lower risk of mortality than non-dog owners (74).
However, results varied when considering the breed and size
of dogs. Owning a pure-bred retriever breed, for example, was
associated with a 40% decrease in mortality rates among the heart
attack victims, while owning a companion/toy breed or mixed
breed dog had no association with a reduction in mortality (75).
Emerging research has also quantified how differences in dogs’
physiological profiles can influence the underlying therapeutic
mechanisms of action during HAI. For example, a recent study
showed that a population of service dogs selectively bred for
friendly and non-aggressive temperaments had higher circulating
levels of oxytocin, a neuropeptide involved in human-canine
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Rodriguez et al. Variability in HAI
social interaction and bonding, compared to pet dogs who
were not selectively bred (76). Other studies have found that
dogs’ variation in their oxytocin receptor gene is related to
certain breeds of dogs’ social behavior when greeting unfamiliar
people (77,78) as well as dogs’ attachment behaviors directed
to their owners (50). As the oxytocin pathway has been
discussed as a potential mechanism underlying positive human-
dog interactions (79,80), these individual differences across dogs
may be important for understanding variability across studies.
Thus far, we have discussed variability in both human and animal
participants that may contribute to observed variability across
HAI research findings. However, one of the most truly variable
aspects of this research lies in the nature of the human-animal
interactions themselves (i.e., the physical, emotional, and/or
psychological interactions that a human and an animal share).
In research quantifying the benefits of pet ownership, a specific
challenge lies in defining “ownership” and accounting for the
variability surrounding this term (81). For example, the human-
animal relationship and its resulting effects may differ between
those who provide a caregiving role to the animal and those who
simply cohabitate with the animal. Dogs, in particular, may also
fill several different roles across households including serving as a
companion, a surrogate child, or strictly for tasks such as hunting
or guarding (82). It is similarly important to consider the varying
length of cohabitation time and how much time a human-pet
dyad spends together on a daily basis– both of which may have a
significant impact on outcomes (83). Not only does the quantity
of time have implications for research, but so does the quality of
the interactions between an individual and a pet. For example,
there is complex variation in daily dog-owner interactions that
may contribute to the strength of the human-animal bond
ranging from co-sleeping to frequency of cooperative activities
such as play or training [e.g., (84,85)]. Dogs have also been
found to form unique attachment relationships to their owners
[e.g., avoidant or anxious; (86)] that may be impacted by their
owner’s caregiving and/or own attachment styles [e.g., (87)].
These sources of heterogeneity have prompted researchers to use
a dyadic approach to consider both the attributes of pets and
owners to holistically evaluate human-animal relationships (88).
In research assessing outcomes from AAI, interactions
can vary widely in terms of activities (e.g., structured or
unstructured), setting (e.g., hospital bed, classroom, outdoors),
human to animal ratio (e.g., group or individual interaction),
and human-animal contact (e.g., duration of petting, talking,
or walking). Because of this considerable variation, there is a
critical need for manualized protocols and/or detailed reporting
of procedures and interactions across studies (89). This will
allow for a greater understanding of the benefits from AAI are
due specifically to the animal’s presence or to other aspects
of the intervention such as novelty, attention, or human
interaction (12).
During AAI, not only do the components of the interaction
contribute to variability, but so does the “dosage” of the
intervention in terms of total time spent interacting with an
animal. For example, in a systematic review of the effects of AAI
for individuals with dementia, the duration of contact with the
therapy animal spanned from three, 10-min interactions in one
study to bimonthly interactions over 2 years in another (26). In
addition, details regarding intervention length, frequency, and
content are sometimes not reported. A systematic review of
AAI for trauma found that while most articles reported some
aspects of the procedures surrounding the participant-animal
interaction, not a single article provided enough detail to allow
for replication (90). When comparing findings across studies, the
omission of these critical details makes it impossible to determine
the potential sources of methodological variation. Therefore, it is
important for researchers to provide sufficient detail surrounding
the characteristics of human-animal interaction, especially in
AAI studies, to address this source of variability.
There is inherent variability linked to the unique nature of
HAI, in that, in its simplest form, it involves two complex
organisms, a human and a companion animal, interacting in
dynamic ways. Not only does HAI research need to account for
human psychological, sociological, physiological, and economic
variability across humans, but these same variable characteristics
apply to the animal as well. For example, a person with financial
resources, time, and or motivation to do so, may provide excellent
veterinary care, high-quality nutrition, and opportunities for life
enrichment to their companion animal that another person with
fewer financial resources, less available time, or less motivation
may provide to the same species of companion animal. It is
important to note that this example is not intended to imply that
wealthy people are better caretakers of their companion animals,
but rather that each of these variables (financial resources,
time, and motivation) are likely to play a role in the care and
life enrichment of companion animals that will contribute to
variability in pet ownership research. Further, developmental
changes must also be taken into consideration. Not only will
children interact differently with companion animals than adults
and older adults, but each developmental stage may bring a
host of unique needs or desires to human relationships with
companion animals. On the other side, we cannot neglect the
developmental changes taking place in the companion animals
as well. Not only will animals also experience physiological,
psychological, and behavioral changes as they develop, but also
may gain a better understanding of their human counterparts
or develop fears or aversions to humans. Therefore, not only do
researchers need to keep in mind the inherent variability of the
unique nature of HAI, but also how it evolves in both humans
and animals over time.
As a field, HAI is charged with understanding not one, but
two complex creatures, each with their own needs, motivations,
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Rodriguez et al. Variability in HAI
and capabilities. On the human side, individuals will vary in their
experiences, attitudes, abilities, and personalities that shape the
way that they perceive, interact with, and ultimately bond with
an animal. The animal side of the equation is further complicated
by multiple species, each with different species-specific behaviors,
welfare needs, physical and mental capabilities, housing and
enrichment requirements, and zoonotic disease concerns. Even
within the same species, there is immense additional variability
in the individual (e.g., breed, temperament, personality, and
behavior) that will influence its relationship and interactions with
humans. Therefore, research in this field must be mindful of all of
these complexities, each of which contribute to the multifaceted
nature of human-animal interaction.
While the inherent variability in HAI research contributes to a
unique complexity that makes for an interesting field of study,
it also makes the field particularly challenging. In particular,
a broad spectrum of theoretical considerations is required to
account for the variability in the human, the animal, the types
of interactions possible, the dynamics of the actual interaction,
and any potential constraints imposed by the setting of the
interaction (e.g., educational, healthcare). To achieve this, an
equally broad spectrum of research methodologies must be
incorporated to capture the subtle nuances of the interactions
(e.g., qualitative methods) and to tightly control as many aspects
of the interactions as possible (e.g., experimental methods).
In conclusion, we have described how variability in methods
and measurement, human participants, animal participants, and
interactions may contribute to mixed findings in the field
of human-animal interaction. We have also made suggestions
on how to address this variability by using appropriate
experimental designs and/or statistical analyses to account for
confounding variables, by ensuring detailed reporting of both
human and animal characteristics, and by providing thorough
descriptions of the duration, context, and structure of human-
animal interactions including replicable and/or manualized AAI
procedures when possible. However, we have also discussed the
inherent complexity of HAI in that even the simplest research
study involves considering the dynamic interaction of two
complex beings, an animal and a human.
To address the complexity of the field of HAI, researchers
must face a variety of theoretical and methodological
considerations to account for multiple sources of variability
and individual characteristics on both the animal and human
level. However, the basic tenets of science apply regardless of the
complexity of the topic under study. The field of HAI demands a
wide variety of methodologies and measurement, each of which
provides important and useful information on which to build the
field. However, whatever the approach, the experimental design
must be appropriate for the research question and conclusions
drawn must be mindful of limitations, including unaccounted
for variability that may impact or contextualize findings. It
is also incumbent upon researchers to report all results, even
nonsignificant findings, as understanding the individuals,
contexts, and conditions in which HAI is not beneficial is equally
important for the progression of the field.
Although the field of HAI has been characterized by
mixed findings, there is a wealth of promising information
available on which to expand. With the growth of research in
this field, new frameworks continue to emerge to study the
relationships between humans and companion animals such
as the dyadic approach (88), trans-species methodology (91)
and the biopsychosocial model (Gee et al., under review).
Inspiration from other fields, such as social psychology (92)
developmental psychology (93,94) and social neuroscience (95),
will also continue to inform the theoretical underpinnings of
human-animal interactions. The field will continue to benefit
from an accumulation of rigorous science while building viable
and testable theories. With increased funding opportunities
from both public and private sources, knowledge regarding
the potential therapeutic outcomes from animal interaction will
continue to strengthen by incorporating randomized clinical trial
designs and large-scale population studies (96). Although it is a
young field, HAI has a promising foundation on which to build,
and a firm commitment to scientific rigor will secure its future.
KR, HH, and NG equally contributed to the formation of
the manuscript’s conceptual ideas and framework. All authors
contributed to writing and editing the manuscript.
We thank the Annenberg Foundation for providing funds to
contribute to the open-access publication of this manuscript.
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Conflict of Interest: The authors declare that the research was conducted in the
absence of any commercial or financial relationships that could be construed as a
potential conflict of interest.
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