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56 | HAI B
Human-Animal Interaction Bulletin
2015, Vol. 3, No. 2, 56-71
Oxytocin Responses After Dog and Cat Interactions Depend on
Pet Ownership and May Affect Interpersonal Trust
Benjamin A. Curry1, Brianne Donaldson2, Moana Vercoe1,
Matthew Filippo3, & Paul J. Zak1,4
1Claremont Graduate University, 2Monmouth College,
3Western University of Health Sciences, & 4Loma Linda University Medical Center
Although many of us interact daily with animals, we have little understanding of how this affects
our interactions with people. This study assessed the physiological effects of human-animal
interactions and tested if this affected interpersonaltrust. Participants (N=141) were assigned to play
with a friendly but unfamiliar cat or dog for 10 minutes or to rest quietly in a private room. Blood
was obtained from human participants before and after animal interactions or rest, and videos of
animal interactions were coded for encounter styles. Participants then made interpersonal monetary
decisions to quantify trust and trustworthiness toward strangers. Although oxytocin (OT) fell on
average after interactions with both dogs and cats, there was a positive and significant correlation
between the change in OT after interacting with a dog and lifetime pet exposure. Participants who
had lived with four or more dogs in their lifetimes had a positive increase in OT after interacting
with an unknown dog. We found a negative correlation between the change in OT after interacting
with a cat and cat ownership. Participants who had a reduction in stress hormones after a dog
interaction showed increased trust in strangers. Specifically, a one-percentage-point decrease in the
stress hormone adrenocorticotropin hormone increased trust in a stranger by 24 percent. Our
findings show that the human OT response to animals depends on previous pet exposure.
Keywords: Hormones, stress, attachment, neuroeconomics, trust
Correspondence concerning this article should be addressed to Dr. Paul J. Zak, Center for
Neuroeconomics Studies, Claremont Graduate University, 160 E. 10th St., Claremont, CA 91711-
6165. E-mail: paul.zak@cgu.edu
People live with pets for a variety of
reasons and interact with them in differe nt
ways, but there is no universal human
response to animals. Some people are
inseparable from their cats and dogs, while
others don’t like animals at all. Studies have
shown that patients with companion animals
have lower blood pressures, heart rates, and
stress levels, as well as improved emotiona l
well-being (Jorgenson, 1997; Odendaal,
2000; Barker & Wolen, 2008; Barker et al.,
2012). Interacting with a dog for as little as
five minutes can lead to a reduction in the
stress hormone cortisol, suggesting that
animal-assisted therapy may be an effective
anti-stress treatment (Odendaal, 2000; Barker
& Wolen, 2008).
Chronic stress has been associated with
cardiovascular disease, cancer, and
depression (Maddock & Pariante, 2001), and
the costs of work-related stress are estimated
to be as high as four percent of GDP (Hoel et
al., 2001). Some organizations have begun
permitting employees to bring pets to work as
a stress-reduction technique. Indeed,
employee perceptions of pets in the office are
positive, with the most commonly cited
benefit being a perceived lowering of stress
(Wells & Perrine, 2001). Barker et al. (2012)
found a significant reduction in perceived
stress in pet owners who brought their dogs
to work compared to those who had pets but
did not bring them to work, and compared to
111
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2015
Creative Commons Attribution 4.0 International License
57 | HAI B
OXYTOCIN RESPONSES
employees who had no pets at all (Barker et
al., 2012).
Interactions with animals may affect
more than stress. For example, people who
have dogs are judged to be more trustworthy
than those who do not (Gueguen & Cicotti,
2008). Of particular interest is the finding
that human-dog interactions have been
associated with increases in oxytocin (OT;
Odendaal, 2000; Odendaal & Meintjes, 2003;
Nagasawa, et al., 2009), although this effect
may be non-robust due to factors like limited
sample sizes and poor experimental controls
(Beetz et al., 2012; Miller et al., 2009).
Neurochemicals like OT fluctuate to
guide human and nonhuman anima ls’
responses to their internal and external
environments. Many hormones in human
beings, including OT, adrenocorticotrop in
hormone (ACTH), cortisol (CORT), and
testosterone (T), respond to social
interactions. As social creatures, humans
seek to balance the costs of social interact io ns
(such as fear and danger) with its benefits
(such as mating and alliance-buildi ng).
Changes in neurochemicals affect this
calculus, working at different time
frequencies that range from milliseconds to
hours.
OT is known to be released in women
during labor and breastfeeding, and by both
sexes during sex. Beyond peri-reproductive
behaviors, over a decade of research on the
behavioral effects of OT have shown that it
increases when one is trusted (Zak, et al.,
2004; 2005), touched (Morhenn et al., 2008,
2012), watches an emotional movie (Barraza
& Zak, 2009), or engages in a variety of
group rituals (Zak, 2012). These studies have
been confirmed by intranasal OT infus io n
studies showing that OT generally increases
prosocial or moral behaviors (Zak, 2012;
2011; 2007; 2005; 2004; Kosfeld et al., 2005;
Barraza & Zak, 2009; Barraza et al., 2011).
OT is a rapid and unconscious signal that
those around us appear to be safe, familiar, or
trustworthy.
The effects of OT occur both on the
central nervous system (primarily affecting
activity in the amygdala, hypothalamus, and
anterior cingulate cortex; Bethlehem, et al.,
2013; Kirsch et al, 2005), and the peripheral
nervous system reducing sympathetic tone
via the vagus nerve and inhibition of stress
hormones such as ACTH and CORT (Meyer-
Lindenberg et al., 2011; Norman et al., 2011).
OT is synthesized in the hypothalamus within
1-2 seconds after a stimulus (Zak et al., 2004;
2005) and has a 3-5 minute half-life (Chard
et al., 1970). Unlike most neurochemicals, a
stimulus causes OT to be released in both the
brain and, via the pituitary gland, the blood
(Landgraf & Neumann, 2004). This means
that a change in OT measured in the blood
reflects a change in OT in the brain.
Nevertheless, basal (unstimulated) plasma
OT and OT in cerebrospinal fluid are
unrelated (Dogterom et al., 1977).
Humans are gregariously social, and our
neuroanatomy reflects this. Humans appear
to have a larger number of frontal cortex OT
receptors than other animals (Loup et al.,
1991) though definitive cross-species studies
have not yet been done. Human behaviors
and brain structure suggest that people might
use the same attachment system that evolved
to care for offspring to attach to animals. The
present study seeks to test this hypothesis
using a large sample and an accepted
methodology to measure the change in OT.
Others have tested this hypothesis but in
much narrower ways than we do here.
Odendaal (2000) tested N=16 participants
and found a significant positive relations hip
between interacting with one’s own dog and
changes in plasma OT in both species. A
similar study using N=10 females also found
an increase in plasma OT in human
participants when petting one’s own dog
(Handlin et al., 2011). Nagasawa et al.,
(2009) reported that a long gaze (but not a
58 | HAI B
OXYTOCIN RESPONSES
short gaze) at one’s dog increased human
urinary OT for N=55. Because of the short
half-life of OT and the lack of published
kinetics from OT synthesis in the brain to
excretion in urine, Nagasawa et al.’s results
are in question. Acute changes in OT are best
measured through serial blood draws. The
cited works also study only pets and their
human caretakers. As a result, these studies
are conditioned on a potentially long period
of attachment between animals and humans
than can be difficult to quantify. Conversely,
our approach had people interact with
unfamiliar but friendly dogs as a stark test of
the neurochemical effects of dogs on humans.
Our study used different dogs so that the
findings were not dependent on interact io ns
with a single animal.
Nothing in the brain happens in
isolation. In the present study we also tested
whether animal interactions affect the stress
hormones ACTH and CORT. Odendaal &
Meintjes, 2003) report such a finding for
human CORT levels. Service dogs appear to
decrease CORT in autistic children (Viau et
al., 2010) and health care professiona ls
(Barker et al., 2005). Because our
experimental design used a short-duratio n
(10 minute) interaction, we measured CORT
along with the faster acting stress hormone
ACTH. To rule out spurious findings from
noisy physiologic measures, we also included
an active behavioral task to assess if
neuroendocrine changes in humans affect
prosocial behaviors. That is, we tested if the
neurophysiologic effect of interacting with an
animal would carryover to interpersona l
human interactions.
We also included a larger sample size
than other studies as well as an active control
condition. A total of N=62 participants
interacted with unfamiliar dogs and N=47
interacted with unfamiliar cats. Including
cats was a natural control since nearly as
many households have cats (30.4%) as have
dogs (36.5%; American Veterinary Medical
Association, 2012). We also included a
second control condition (N=32) of no
animal interaction at all. Animal- human
interactions were videotaped to test if the
quantity and quality of these encounters were
associated with physiologic changes.
We hypothesized that interactions with
dogs, but not cats or quiet rest, would
increase OT, decrease ACTH and CORT, and
increase interpersonal trust.
Methods
Timeline
This study adapts the protocol in Zak et
al. (2005) to accommodate animals in the
experimental design. Six participants were
recruited per session and, after baseline blood
draws, were randomly assigned to one of
three conditions: Dog, Cat, or Rest. In the
animal conditions, participants interacted
alone with an animal in small, closed rooms
for 10 minutes. The Rest condition put
participants in the same rooms but had them
sit quietly for 10 minutes.
Immediately after the animal interactio n
or rest, a second blood draw was obtained and
participants answered survey questions
regarding their moods, pet experience
histories, and attitudes. After completion of
the surveys, participants were given
instruction in the economic decision task and
then made monetary transfer choices. After
all decisions were made, participants were
paid their earnings in private and dismissed.
There was no deception of any type in this
experiment.
59 | HAI B
OXYTOCIN RESPONSES
Participants
Participants were recruited from the
Claremont Colleges using fliers posted on
campus, through social networking sites, and
through word of mouth. Recruitment fliers
and emails told potential participants that the
study would examine the biological bases of
decision-making and might involve
interactions with dogs or cats. The
Institutional Review Boards of Scripps
College and Claremont Graduate Univers ity
approved the study. All participants gave
written informed consent prior to inclus io n
and were assigned an identity- masking code
used to ensure anonymity. Experime ntal
sessions were conducted in the early evening
at the Center for Neuroeconomics Studies at
Claremont Graduate University, and each
session lasted approximately 60 minutes. A
total of 141 participants were recruited, with
50 percent of them female. The average age
of participants was 21.65 (SD = 4.64).
Blood Draws and Assays
Participants were taken to a private room
where 28 ml of blood was drawn from an
antecubital vein by a qualified phlebotomist.
Two 8-ml EDTA whole-blood tubes and one
12-ml serum-separator tube were drawn
maintaining a sterile field using Vacutainer®
blood collection kits. The initial blood draw
established basal OT, CORT, and ACTH.
A second 28-ml blood draw was
obtained from each participant immedia te ly
following his or her 10-minute interactio n
with the animal or at the conclusion of the 10-
minute rest period. All second blood draws
occurred within two minutes after the rest or
animal interaction ended. This design takes
into account OT's short half-life and seeks to
capture participants' physiologic states
during the animal interaction.
After phlebotomy, each blood tube was
immediately placed on ice before being put
into to a refrigerated centrifuge and spun at
1500 rpm at 4°C for 12 minutes. Plasma and
serum were withdrawn and placed into 2-ml
polypropylene microtubes with screw caps.
The microtubes were immediately placed on
dry ice and then transferred to an -80°C
freezer until analysis.
Hormones were assayed using either
radioimmunoassays (RIA) or enzyme- linked
immunosorbent assays (ELISA). Assays
were performed at Biomarkers Core at
Emory University in Atlanta, Georgia.
ACTH and CORT were assayed using an
RIA kit from Diagnostic Systems
Laboratories (Webster, Texas), while OT was
assayed using an ELISA from R&D Systems
(Minneapolis, Minnesota). The inter- and
intra-assay coefficients of variations were
less than 15 percent for all tests. While recent
advances in assay design have shown that an
RIA with an extraction step measures OT
more precisely than an ELISA (McCullough
et al., 2013), this study was done in 2010
before the differences between the ELISA
and RIA had been established. Since we
focus on the change in OT after a stimulus
and not OT levels, side-products that the
ELISA may measure are likely to disappear
when changes in OT are analyzed. As a result,
we only report changes in OT.
Economic Decision Task
Participants were instructed in and made
a single decision using, a variation of the trust
game (TG) developed by Berg et al. (1995).
Each participant was randomly assigned to a
dyad with another participant and then
randomly assigned the role of Decision-
Maker 1 (DM1) or Decision-Maker 2 (DM2).
Both DMs were endowed with $10 at the start
of the task and were extensively instructed
about how their decisions would affect their
earnings and the earnings of the other dyad
member. Interactions between the
60 | HAI B
OXYTOCIN RESPONSES
participants were
conducted
exclusive ly
through a computer interface.
The instructions stated that the amount
transferred by DM1 would be removed from
his or her account and would be tripled and
placed in DM2's account. DM2 was then
informed by the software of the tripled
transfer received from the DM1 in his or her
dyad and the total in his or her account. Then,
DM2 was prompted by the software to enter
an amount, including $0, that he or she would
like to transfer back to the DM1 in the dyad.
The amount sent by DM2 to DM1 would be
subtracted from DM2's account and would
transfer one-for-one into DM1's account.
The instructions included several examples
of DM1 and DM2 transfers and subsequent
earnings. Participants were also prompted to
ask questions prior to making decisions.
The transfer from DM1 to DM2 in this
task is considered a measure of trust, and the
amount returned by DM2 to DM1 is
considered a measure of trustworthiness or
reciprocity (Smith & Walker, 1993). The
classical prediction in economics is that DM2
will return nothing to DM1 regardless of the
amount she or he receives. However, in most
studies in developed countries, 90-95% of
participants show at least some trust and
reciprocity (Zak, 2011; Camerer, 2003).
Video
Animal-human interactions were
recorded on Sony digital cameras using
tripods. Cameras were set up in such a way
that both the animal and the human
participant were in frame for the majority of
the recording. Videos were transferred onto
DVDs and given to four independent raters
who had an inter-rater reliability of b = 0.634,
p < 0.001. The coding quantified touch,
visual gaze, and the use of toys during animal
interactions.
Animals
Animals were recruited from lab
members based on their friendliness toward
strangers. Two animals were used at a time.
The most-used dogs were named Rowdy, a
Brittany, and Herriot, a beagle, though other
dogs of various sizes and breeds were used as
well. Cat interactions were conducted using
two common housecats. The interactio n
rooms included toys and treats so that
participants had several ways to engage with
the animals. Multiple animals were used so
that the results did not depend on the behavior
of a single dog or cat.
In every experimental session, animals
were accompanied by their owners or an
animal rights observer. Our focus was on
human physiology and behavior after animal
interactions, not animal responses. As a result,
blood samples were only obtained from
people, not animals. Breaks were provided
after every experimental session for anima ls
to eat, drink, and use the litter box (cats) or
walk in a grassy area (dogs). One dog in the
first session appeared to be mildly distressed
and was not used in subsequent sessions. All
other animals appeared relaxed before,
during, and after experimental sessions.
Claremont Graduate University’s Internal
Review Board approved the animal handling
protocol; IACUC approval was not necessary
as the animals were not being studied.
Surveys
Trusting behaviors can be influenced by
a number of underlying psychological factors.
In order to untangle these interactions, we
administered a number of surveys. These
included Satisfaction With Life (SWL; Pavot
61 | HAI B
OXYTOCIN RESPONSES
Figure 1: Endocrine changes after animal interactions. Playing with an unfamiliar dog or cat caused a significant
reduction in OT and insignificant changes in ACTH and CORT.
& Diener, 1993) that provides a global
assessment of one’s flourishing, and the
Affect Intensity Measure (AIM; Larsen,
1984) that measures the strength and valence
of emotions. We also included a number of
questions taken from the General Social
Survey regarding attitudes about others
(Gleaser et al., 2000) was well as
demographic information. Questions
regarding one’s feelings about animals and
pet ownership history were created for this
study and can be found in the Appendix.
Results
Hormones
The physiologic effect of animal
encounters was highly variable. The change
in OT for those who interacted with dogs
varied from a +81% to -40%, while cat
interactions produced changed in OT from
+72% to -57%. As shown in Figure 1, the
average percentage change in OT was
negative when participants interacted with
both dogs (M=-5.3%, p=0.05) and cats (M=-
Figure 2a: Percent change in OT and the total number of lifetime pets for people who interacted with a dog.
Increases in oxytocin were associated with a higher number of pets owned in a participant’s lifetime.
10.0%
8.0%
5.0%
4.1%
0.6%
0.0%
OT % Change
-5.0%
CORT % Change ACTH % Change
-3.3%
-5.3%
-10.0%
-9.5%
-15.0%
Dogs Cats
100%
80%
60%
40%
20%
0%
-20% 0
-40%
-60%
5
10
15
20
25
30
35
All Pets
Percent Change in OT
62 | HAI B
OXYTOCIN RESPONSES
Figure 2b: There was a positive association between the increase in OT and the total number of lifetime dog
cohabitations for people who interacted with a dog.
Figure 2c: The number of cats a participant had lived with was positively associated with an increase in OT
after interacting with a dog.
9.5%, p=0.003). Average percentage changes
in stress hormones did not statistically differ
from zero after playing with dogs (two-tailed
paired t-tests: CORT M=0.6%, p=0.89;
ACTH M=8.0%, p=0.18) or cats (CORT M=-
3.3%, p=0.42; ACTH M=4.1%, p=0.45).
Pet History
We found a positive and significa nt
correlation between the percentage change in
OT after interacting with a dog and the
number of dogs (r=0.275, p=0.04), cats
(r=0.410, p=0.002), and total number of pets
one has resided with in one's lifet ime
(r=0.403, p=0.002; Figures 2a-c). The
significant relationship between the lifet ime
number of pets owned and the change in OT
continues to hold after controlling for age and
removing outliers (for instance, one
individual reported 30 lifetime pets).
The opposite pattern was found for
participants who interacted with cats: There
was a negative relationship between the
percentage change in OT and the number of
cats a participant had lived with in his or her
lifetime (r=-0.334, p=0.03; Figure 3). The
relationship between the lifetime number of
100%
80%
60%
40%
20%
0%
-20%
0
-40%
-60%
2
4
6
8
10
12
Dogs
100%
80%
60%
40%
20%
0%
-20%
0
-40%
-60%
2
4
6
8
10
12
Cats
Percent Change in OT
Percent Change in OT
63 | HAI B
OXYTOCIN RESPONSES
Figure 3: Percent change in OT and the totalnumber of cats one has had for participants who interacted with a cat.
A larger number of cats was associated with a greater decrease in OT.
cats owned and the percentage change in OT
continued to be significant even after
controlling for age (b = -3.9%, p = 0.02). The
relationship between lifetime number of pets,
lifetime number of dogs, and the change in
OT after interacting with a cat did not reach
statistical significance (Pets owned: r=-0.21
p=0.19; Dogs owned: r=0.21 p=0.18).
Interaction Types
While we coded for over two-dozen
animal-human interaction variables, only a
handful of them were significantly associated
with changes in hormones. Table 1 shows the
correlation and p-values for all coded
variables. Significant correlations were found
between petting a dog’s belly and an increase
in ACTH (r=0.34, p=0.01); the dog lying
down away from the person and an increase
in OT (r=0.26, p=0.05); and eye contact
between the participant and the dog and an
increase in OT (r=0.29, p=0.02). The
correlation between the number of times a
participant made eye contact with a dog and
changes in OT replicates earlier work
(Nagasawa et al., 2009). For participants who
interacted with a cat, offering the cat a toy
was associated with an increase in OT
(r=0.30, p=0.05).
Trusting Behaviors
Overall, there were no significa nt
differences among the three differe nt
treatment groups and the amount transferred
by DM1s in the trust game (Dog: $6.97; Cat:
$6.71; Control: $6.28; F=0.14, p=0.86).
Return transfers by DM2s also were
statistically the same across groups (Dogs:
50.5%; Cats: 41.4%; Control: 57%; F=1.77,
p=0.18).
When we looked at the relations hip
between stress hormones and trusting
behaviors, we found that for DM1s who
interacted with dogs, a reduction in ACTH
marginally increased trust in a stranger (r=-
0.349, p = 0.06; Figure 4). For those in the
dog group, the changes in OT and CORT
were unrelated to trust (ps>0.05). The
negative relationship between the change in
ACTH and trust for those who interacted with
dogs persists after controlling for changes in
OT, age, and gender (b = -2.77, p=0.02,
R2=0.32).
60%
40%
20%
0%
-20%
-40%
-60%
-80%
0
2
4
6
8
10
Cats
Percent Change in OT
64 | HAI B
OXYTOCIN RESPONSES
Dog Interaction Correlations
Cat Interaction Correlations
% Change
in
OT
CORT
ACTH
% Change
in
OT
CORT
ACTH
Petting - Total
r
0.09
0.04
0.22
Petting - Total
r
-0.05
-0.01
0.09
# of times
p
0.49
0.75
0.10
# of times
p
0.74
0.96
0.60
Petting - Head r 0.09 0.15 0.07 Petting - Head r 0.02 -0.01 0.03
and face
p
0.49
0.27
0.60
and face
p
0.90
0.94
0.87
Petting - Side r 0.06 -0.03 0.17 Petting - Side r -0.03 -0.01 0.16
and back
p
0.66
0.84
0.21
and back
p
0.84
0.96
0.32
Petting - Belly r 0.06 -0.08 0.34 Petting - Belly r -0.25 0.03 -0.01
p
0.64
0.58
0.01
p
0.12
0.83
0.95
Toys (offers)
r
-0.12
0.12
0.12
Toys (offers)
r
0.30
0.17
0.04
p
0.38
0.39
0.40
p
0.05
0.28
0.80
Treats (offers) r -0.10 -0.24 -0.13 Treats (offers) r -0.02 -0.25 0.02
p
0.47
0.07
0.34
p
0.89
0.12
0.89
Lying down
r
0.00
0.15
0.16
Lying down
r
-0.19
0.03
0.08
(to engage person)
p
0.99
0.25
0.25
(to engage person)
p
0.24
0.87
0.62
Lying down
r
0.26
0.06
0.05
Lying down
r
0.01
-0.03
0.14
away from person
p
0.05
0.68
0.71
away from person
p
0.94
0.83
0.38
Whining
r
-0.05
0.11
0.19
Whining
r
0.26
0.06
-0.04
# of times
p
0.71
0.44
0.16
# of times
p
0.09
0.70
0.82
Facing door r 0.18 0.15 -0.15 Facing door r 0.14 -0.18 -0.26
or window
p
0.18
0.27
0.28
or window
p
0.38
0.26
0.10
Person says name r -0.07 -0.01 0.03 Person says name r 0.00 -0.19 0.09
p
0.60
0.96
0.85
p
1.00
0.25
0.58
Response to name r -0.12 -0.03 -0.20 Response to name r 0.09 -0.01 0.00
p
0.39
0.83
0.14
p
0.56
0.96
0.99
Response to non- r 0.04 -0.16 -0.16 Response to non- r 0.09 -0.23 0.05
name verbal
p
0.77
0.24
0.24
name verbal
p
0.59
0.15
0.76
Response to r 0.02 -0.15 0.05 Response to r 0.01 -0.15 0.08
gesture
p
0.86
0.28
0.73
gesture
p
0.95
0.35
0.61
-
Person changes
r
0.06
0.01
-0.14
Person changes
r
-0.22
0.05
0.08
position
p
0.66
0.96
0.32
position
p
0.17
0.78
0.64
Animal initiates
r
0.03
0.08
0.05
Animal initiates
r
0.05
-0.18
0.21
contact
p
0.81
0.57
0.70
contact
p
0.77
0.27
0.20
Eye Contact
r
0.29
0.21
0.21
Eye Contact
r
0.12
0.04
-0.15
p
0.02
0.12
0.12
p
0.44
0.81
0.35
Table 1: Endocrine changes associates with human-animal interactions. All p values are two-tailed tests. For dogs, Eye
contact, petting, offering treats, and the number of times the dog lied down away from the participant were significantly
correlated with hormone changes. For cat interactions, offering a toy during the session was positively correlated with
changes in OT.
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OXYTOCIN RESPONSES
Figure 4: Percent change in ACTH and DM1 transfer amount after dog interactions. For participants who
interacted with a dog, larger reductions in ACTH were associated with higher DM1 transfer amounts.
Figure 5: Percent change in CORT and DM1 transfer amount after cat interactions. For those participants who
interacted with a cat, larger reductions in CORT were associated with higher DM1 transfer amounts.
For those who interacted with cats, a
reduction in CORT increased trust (r=-0.495,
p = 0.02; Figure 5). Neither the change in OT
nor the change in ACTH were associated
with trusting behaviors for those who
interacted with cats (ps>.05). The negative
relationship between changes in CORT and
trust for participants who interacted with cats
is also robust after controlling for the same
variables as in the dog analysis (b=-7.10,
p=0.06, R2=0.30).
These findings caused us to further
examine the relationship between stress
hormones and trust. We found that 65% of
DM1s had negative CORT changes after
interacting with either animal, with an
average percentage change of -17%. The
increase in CORT for the other 35% only
averaged 1.7%. Participants with a decrease
in CORT did not exhibit a difference in trust
from those whose CORT increased (Increase:
$7.25; Decrease: $6.94; p=0.11). Simila r ly,
51% of DM1s had negative ACTH changes
from animal interactions that averaged -28%,
while the 49% of those with an increase in
ACTH averaged just 2%. There was no
150%
100%
50%
0%
$0.00
-50%
$2.00
$4.00 $6.00 $8.00 $10.00
-100%
DM1 Transfer Amount
60%
40%
20%
0%
$0.00
-20%
$2.00
$4.00 $6.00 $8.00 $10.00
-40%
-60%
DM1 Transfer Amount
Percent Change
in ACTH
Percent Change
in CORT
66 | HAI B
OXYTOCIN RESPONSES
difference in trust between groups (Increase:
$7.15; Decrease: $6.48; p=0.40). These
inconclusive results could be the result of
measurement error from the assay. Indeed,
we found a lack of the expected correlation
between ACTH and CORT in participants
who interacted with animals in our sample
(r= -.15, p=.12). In order to derive robust
results, we assessed trust in the 32% of
participants who had reductions in both
ACTH and CORT after animal interactio ns.
We found that participants with reductions in
both stress hormones demonstrated 25%
higher trust compared to those who did not
have an unambiguous reduction in stress
($8.11 vs. $6.48; p=0.02).
For DM2s who interacted with either
type of animal, there were no significa nt
correlations between hormone changes and
trustworthiness. (Cats: CORT r=-0.13,
p=0.57; OT r=-0.08, p=0.72; ACTH r=0.07,
p=0.76; Dogs: CORT r=0.02, p=0.91; OT
r=0.03, p=0.87; ACTH r=0.13, p-0.51). This
was expected since the signal of trust DM2s
receive from DM1s is likely to overwhelm
the effects of dog or cat interactions (Zak et
al., 2005; 2011). There was also no
significant difference in DM2
trustworthiness for those with negative
changes in CORT (Negative: 51%; Positive :
40%; p=0.11), ACTH (Negative: 46%;
Positive: 46%, p=0.98), or both ACTH and
CORT (Negative: 50%; Positive: 45%;
p=0.59).
As in previous research (Zak et al., 2005),
the trait and attitude surveys showed little
association with trusting behaviors. Only
those who thought money was very important
showed less
trust
in strangers (r=-0.40,
p=0.01). On the other hand, trustworthiness
was positively
correlated
with emotiona l
lability as measured by the AIM (r=0.51,
p=0.001) and negatively associated with a
desire to make money any way possible (r=-
0.46, p=0.00) and a stated
distrust
of
strangers (r=-0.29, p=0.06).
Discussion
This study sought to characterize the
physiologic effects of human-anima l
interactions and to test if physiologic changes
due to animals affected human-huma n
interactions. As expected, our findings were
more nuanced than were our hypotheses.
Rather than interactions with dogs unifor mly
increasing OT, we found that only a subset of
participants (27%) responded this way. If one
owned a sufficient number of dogs, cats, or
pets in one's lifetime, this increased the
likelihood that interacting with an unfamiliar
dog increased OT. For our data, if an average
participant had resided with four or more
dogs in his or her lifetime, then interacting
with a dog in the lab produced an OT increase.
This suggests that pet experience may prime
the brain for physiologic attachment (an
increase in OT) when one interacts with an
unknown dog. There is evidence in rodents
that rich social environments stimulate
oxytocin receptor binding and result in
increased social behaviors (Champagne &
Meaney, 2007). It is possible that having pets
in one's household has a similar effect on
human beings.
We also tested the effect of human-
animal interactions on interpersona l
behaviors by asking participants to make a
single monetary decision in which they could
exhibit trust or trustworthiness through a
computer-aided monetary transfer to a
stranger. We found that a reduction in stress
after playing with a dog linearly increased
trust in a stranger. In our sample, a one
percentage point decrease in ACTH
increased trust by 24%, a substantial size
effect. A further analysis revealed that a
robust measure of stress reduction (a decline
in both CORT and ACTH) from interacting
with either a dog or cat was associated with
25% greater trust in a stranger. Playing with
an animal reduced stress in approximate ly
67 | HAI B
OXYTOCIN RESPONSES
one-third of participants. Compared to those
who did not get a stress reduction, this group
of participants were more likely to self-
identify as “dog people” (r=0.19, p=0.05) and
report higher levels of religiosity in a several
questions (ps<.05). Future studies should
seek to understand the circumstances in
which interacting with an unknown pet
reliably reduces stress.
We did find several lines of evidence
regarding the type of interactions with
animals that affected OT. Eye contact with a
dog was found to correlate with an increase
in OT, confirming earlier work (Nagasawa, et
al., 2009). Given that both humans and dogs
are social animals, intermittent eye contact
can be interpreted as a display of trust and an
engagement in a bonding activity. The dog
lying down away from the participant also
positively correlated with changes in OT.
This behavior could indicate a degree of
comfort the dog has toward the person in the
room, enabling the human to feel more
attached to the animal. In interact io ns
between participants and cats, we found
positive correlations between changes in OT
and how many times a participant offered a
toy to the animal. The lack of an increase in
OT with other play styles may be due to the
limited motivation by cats to socialize based
on their generally solitary nature. By offering
the cat a toy, a participant appears to be
forming a connection with the animal,
resulting in an increase in OT. The
conditional nature of our findings is
consistent with reports showing that OT
infusion on human social interactions is
conditional on set and setting (de Dreu et al.,
2010).
For the scientific community, our
discovery that an increase in OT after
interacting with a dog was conditional on
previous pet experience may help explain the
inability of other labs to replicate Odendaal’s
(2000) findings of an increase in OT after
interacting with one's own dog. The present
study had a large number of participants
interacting with unfamiliar dogs, setting the
bar higher to find an OT effect from cross-
species interactions. It is also interesting that
previous cat ownership had a linear ly
decreasing effect on OT after interacting with
a cat. This could be due the reduced quantity
and quality of participant interactions in cats
versus dogs. We hope that this research
stimulates other studies on cross-species
attachment, a phenomenon that may be more
common than once thought (Zak, 2014).
Our findings may also prove useful to
professionals using animal-assisted therapy.
Encouraging patients to engage in eye
contact, petting, and other bonding behaviors
with service animals may be the most
effective way to reduce stress and increase
oxytocin. Our analysis also suggests that
animal therapy may have a cumulative effect
as we found for pet ownership on oxytocin
release. Programs that provide live-in therapy
dogs to warfighters who need emotiona l
support, for example, due to post-traumatic
stress disorder, may have a stronger
therapeutic effect than short-duration visits
with dogs (Altschuler, 1999). Patients who
have a history of pet ownership may also
achieve the benefits of animal-assisted
therapy more rapidly than those who have not
resided with pets. Our findings also support
this use of animal therapy for other anxiety-
related maladies such as autism spectrum
disorder (ASD). Children with ASD have a
diminished stress response when they hold
guinea pigs (Kršková, Talarovičová, &
Olexová, 2010; O'Haire, McKenzie, Beck, &
Slaughter, 2015). Dog therapy for those with
ASD may have even stronger effects
(Solomon, 2010).
Dogs appear to have a special
relationship with humans. This is reflected in
the intense physiologic response to dogs for
those who have a history of pet exposure. Our
research elucidates the mechanism that is at
least partially responsible for the positive
68 | HAI B
OXYTOCIN RESPONSES
effects of animal-assisted therapy and bring-
your-dog-to-work programs. The benefits of
dogs in our lives appear to grow the more we
are exposed to them.
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Human-Animal InteractionBulletin
2015, Vol. 3, No. 2, 56-71
Appendix: Survey on Disposition Towards Animals
Have you even been threatened or physically assaulted by a dog? (y/n)
Have you ever been attacked by a cat? (y/n)
Have you ever interacted with a farmed animal (pig, chicken, goat, sheep, etc.)? (y/n)
If yes, on how many occasions?
Have you ever been to a zoo? (y/n)
If yes, on approximately how many occasions?
Have you ever observed animals in the wild? (y/n)
If yes, on approximately how many occasions?
Have you ever been to a circus? (y/n)
If yes, on approximately how many occasions?
Please indicate whether you agree with the following statements (1-5)
On the whole, I consider myself to be a “dog person.”
On the whole, I consider myself to be a “cat person.”
I would choose the company of animals over most people.
I have positive memories of dogs in my life.
I have positive memories of cats in my life.
I feel that dogs have unique personalities.
I feel like cats have unique personalities.
I feel that humans can treat animals anyway we want.
I think there is a connection between human violence towards animals and human violence towards
other humans.
I think compassion towards animals can strengthen society.
I believe animals should receive moral consideration.
I feel that we should not inflict unnecessary suffering on animals.
I feel that animals should have legal protection.
In general, I feel that our society treats animals well.
I consider the treatment of animals when purchasing food.
I consider the treatment of animals when purchasing cleaning/beauty products.
I find it easier to interact with dogs than cats.
I find it easier to interact with cats than dogs.
I find it easier to interact with humans than animals.
I feel that dogs show emotional complexity.
I feel that cats display individual preferences.
I feel that cats can express their own likes and dislikes.
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