Ask any dog owner if their pet is considered family, and you are likely to hear a resounding “yes.” Now, new brain scan research gives that intuition biological grounding.
In 2014, Massachusetts General Hospital (MGH) researchers found that similar regions in the brain were activated when mothers looked at photos of their dogs and when they looked at photos of their own children.
The discovery not only validated the feelings of pet owners, it provided a starting point for investigating the science behind the human-animal bond.
Inside the Brain Scan Study
The study, led by Luke Stoeckel and Lori Palley (and published in the journal PLOS ONE), involved 14 mothers with children and dogs. Each participant had at least one child, age 2-10, and at least one dog that had lived in the family for two or more years.
The research involved two steps. First, the MGH team went to each mother’s home, took photos of her children and her dogs, and asked them to discuss their relationships with each.
Then each mother came to MGH’s Athinoula A. Martinos Center for Biomedical Imaging for an fMRI (functional magnetic resonance imaging) scan.
Using a type of MRI scan that measures blood flow, which is an indirect indicator of brain activity, the researchers observed which brain regions were active when each participant looked at a continuously rotating set of photos on a screen.
This set included photos of her own child, her own dog, another child, and another dog, all from the study pool.
According to the Harvard-affiliated hospital’s news service, the results showed similar patterns of brain activity when mothers viewed photos of their own children and photos of their own dogs.
Specifically, the brain regions involved in emotion processing and reward, affiliative behavior and social cognition, and vision and movement were all activated when the mothers viewed photos of their own child or dog. However, some regions within these networks were more strongly activated in response to one than to the other.
The brain region called the substantia nigra/ventral tegmental area, which has been associated with bonding, was activated only when the mothers looked at photos of their children. Meanwhile, the fusiform gyrus, which is involved in face processing, was activated more strongly by photos of dogs than by photos of children.
When asked to rate how exciting or pleasant each photo was, the mothers reported similar levels of excitement and pleasure when viewing photos of their own child or dog.
However, the gap between how mothers rated their own child versus an unfamiliar child was larger than the gap between how they rated their own dog versus an unfamiliar dog.
“The importance of pets in the lives of many people is unquestionable,” noted Palley, a doctor of veterinary medicine at MGH’s Center for Comparative Medicine, who participated in the study. “Our findings provide biological evidence for the devotion and companionship that people feel toward their pets.”
The Chemistry Behind the Human-Animal Bond
While the MGH team used brain scans to explore this bond, another research team has investigated the role of chemical messengers in the body.
About a year after the MGH study appeared, researchers at Azabu University in Japan, led by Miho Nagasawa, identified a potential biochemical basis for the attachment between pets and their owners.
In a paper published in the journal Science, the team reported that dogs and their owners experience an increase in oxytocin, sometimes called the “bonding hormone,” when they spend time looking into each other’s eyes.
Nagasawa and her colleagues found that dogs and their owners both experienced elevations in oxytocin when they made prolonged eye contact. Further, the more time a dog spent staring at its owner, the more oxytocin was produced, which in turn made the owner more affectionate, encouraging the dog to continue staring.
This “oxytocin feedback loop” was not observed in similar experiments using wolves and their owners.
The researchers speculate that this mechanism, which has not been seen in other animals, emerged as dogs and humans spent thousands of years living together. This explanation fits with research on oxytocin’s role in mother-baby bonds, which also involve intense eye contact.
Further research is needed before scientists can conclude that this mechanism influences dogs’ behavior toward humans, says Nagasawa. Some researchers have argued that the study’s sample size was small and that it should have included more wolves for comparison.
Additionally, the gender of the owner may influence oxytocin levels; the study’s owners were both male and female.
Even so, these findings, too, fit with the MGH study, suggesting that similar biochemical processes underlie human bonds with other humans and with dogs.
Why This Matters
The research findings from both studies align with the experiences of pet owners, as documented in numerous public health surveys. In particular, the American Heart Association (AHA) has published a series of scientific statements on the relationship between pet ownership and heart health.
In 2013, the AHA published a statement about pets and cardiovascular health, authored by Glenn Levine, MD, of Baylor College of Medicine, and his colleagues.
Levine, who chaired the statement on behalf of the AHA’s Council on Clinical Cardiology and Council on Cardiovascular and Stroke Nursing, noted that pet ownership “is likely to be associated with reduced risk of incident cardiovascular disease.”
His team wrote that pet ownership, and particularly dog ownership, was linked to lower blood pressure and better lipid profiles, as well as increased physical activity and reduced stress response. Studies reviewed by the National Alliance on Mental Illness (NAMI) also linked pet ownership to decreased feelings of isolation and loneliness.
In fact, most pet owners would probably agree, according to a 2024 survey from Morning Consult conducted for the American Psychiatric Association and released jointly with the American Veterinary Medical Association.
The survey queried pet owners about the effect of pets on their mental health. Among the key findings was that most pet owners believe their pets positively affect their emotional well-being.
APA President Petros Levounis, MD, MA, commented on the survey results, saying that pets provide “a source of companionship, comfort, love, and friendship” and that he often encourages his patients struggling with addiction or other mental health issues to consider getting a pet.
Meanwhile, the Human Animal Bond Research Institute tracks the physiological effect of human-animal interactions. As revealed by this institute’s research, spending time with a pet can lower stress hormones such as cortisol and increase oxytocin and dopamine, as well as lower heart rate and blood pressure.
None of this would be possible if pet ownership did not activate some of the same physiological pathways as human social interactions, including those that strengthen interpersonal bonding.
What These Studies Mean
None of this research suggests that a pet is an exact substitute for a child, and the authors of both studies note the limitations of their research. For one, the MGH study used fMRI technology, which can identify which brain regions are involved in a certain activity but cannot establish causality.
In other words, the scans can show that a particular brain region “lights up” when you look at photos of your child and photos of your dog, but they cannot show that one causes the other.
Moreover, while the MGH study included 14 participants, the sample size was small, and the study only included women who were parents. The study design did not take into account whether similar effects would occur in men or in people who did not have children. A larger study with a more diverse group of participants would be needed to investigate that question.
Meanwhile, Nagasawa’s team agrees that further research is needed to explore how oxytocin underlies people’s connections with their pets.
In particular, the study needs to be replicated with a larger sample size, and additional research is needed to understand how this mechanism might differ in dogs and in wolves. Some researchers argue that the results could be skewed because the study included only a few wolves.
Even with these limitations, the results of the studies have stood the test of time. Researchers who have studied the human-animal bond for years believe that their findings reflect real connections.
Brain scans, oxytocin measurements, and public health surveys all indicate that the powerful sense of companionship that people feel toward their pets activates biological processes similar to those triggered by time spent with human loved ones.
In fact, this research merely formalizes what pet owners have long known.
Sources
Patterns of Brain Activation when Mothers View Their Own Child and Dog: An fMRI Study
Mothers’ brains show similar responses to her baby and her dog
Oxytocin-gaze positive loop and the coevolution of human-dog bonds
Pet Ownership and Cardiovascular Risk: A Scientific Statement From the American Heart Association
The Remarkable Human-Animal Bond
‘The pet effect’: Exploring research on the human-animal bond