There is conflicting evidence about whether living with pets results in better mental and physical health outcomes, with the majority of the empirical research evidence being inconclusive due to methodological limitations. We briefly review the research evidence, including the hypothesized mechanisms through which pet ownership may influence health outcomes. This study examines how pet and non-pet owners differ across a variety of socio-demographic and health measures, which has implications for the proper interpretation of a large number of correlational studies that attempt to draw causal attributions. We use a large, population-based survey from California administered in 2003 (n = 42,044) and find that pet owners and non-pet owners differ across many traits, including gender, age, race/ethnicity, living arrangements, and income. We include a discussion about how the factors associated with the selection into the pet ownership group are related to a range of mental and physical health outcomes. Finally, we provide guidance on how to properly model the effects of pet ownership on health to accurately estimate this relationship in the general population.

Introduction

Approximately sixty-two percent of the American population live with a pet [1], and it is generally believed that these pets provide mental and physical health benefits to their human companions [2]. There is a modest but growing research literature examining the health impact of human animal interaction, which is largely inconclusive due to contradictory findings and methodological weaknesses [3]. Several studies have found that owning and/or interacting with a pet (mostly a dog) has benefits for the individual, including mental health outcomes such as decreased anxiety, and physical health outcomes such as improved immune response and physical activity [4–8]. Other studies have documented negative effects of pets including dog bites, spreading of disease, and have shown that pet ownership is associated with asthma and other allergies [4–8], and associations with a higher incidence of heart attacks and readmissions in heart attack patients [9]. And still other studies have found no link between pet owners and health outcomes [10].

If pet ownership is demonstrated to provide mental, social, and/or physical health benefits for adults, children, or adolescents at the population level, it could provide a relatively cost-effective way to promote health. While the published scholarly studies do not provide strong support for a link between pet ownership and health, some evidence does points in that direction, and researchers are calling for stronger methodological studies [3]. There are key limitations common to this body of work that prevent causal links between human animal interaction and health outcomes, even when associations are found. Most problematic, these studies use convenience samples that may not be representative of the general population, examine a narrow range of outcome variables, and use cross sectional designs that do not consider long-term health outcomes. This is not surprising, as experimental designs where people or families are randomly assigned to be pet- or non-pet owners, would be extremely challenging.

Our goals for this paper are twofold: (1) Describe how pet owners and non-pet owners differ. (2) Describe why this difference needs to be accounted for in observational research on pet ownership and health. In this paper, we will examine the factors associated with pet ownership to provide empirical evidence about how dog and cat owners differ from the general population. We also describe how these differences are also associated with health outcomes, which may lead researchers to under- or over-estimate the impact of pet ownership on health in any observational studies that do not use suitable statistical controls. We then provide guidance into how to strengthen the research basis, recommending some recent methodological innovations that help overcome the limitations associated with selection bias.

Unclear evidence of the relationship between pet ownership and health mechanisms of the potential effect When examining the relationship between pet ownership and health, it is helpful to first consider the mechanisms through which we believe the effect might work. For example, do pets promote health through companionship and emotional support; do they encourage healthy behavior; or is there something else about them that could improve mental or physical health? Understanding these mechanisms is vital for understanding how pets might impact health so that we can translate findings into broader public health policy. One research line has focused on how pet ownership may improve the physical health of owners. The leading theory is that pets encourage physical activity [11]. Most dog owners exercise their dogs, and although not the primary aim, exercising one’s dog also usually involves exercising oneself. In an Australian sample the majority of dog owners walked their dog with almost a quarter of them walking together five or more times per week, however dog owners were significantly more likely to exercise at least 90 minutes per week [12]. This benefit may extend to children as well; research found that the odds of being overweight were lower for any young children who lived in a dog owning household [13]. Researchers have also hypothesized the mechanisms through which pets affect psychological wellbeing. Two theories exist regarding the effects of social support—the ‘main effect’ hypothesis suggests that the beneficial effects are diffuse, the ‘buffering’ hypothesis suggests that social support effects are notable only in the presence of stressors. Two experiments provide evidence of these mechanisms, but how these results translate into long term differences in animal owners’ health is unknown. In a test of the buffering effect hypothesis, researchers tested whether animals could moderate the anxiety inducing effects of a stressful situation [14]. The researchers randomly assigned participants to one of five groups: they were asked to pet a rabbit, a turtle, a toy rabbit, a toy turtle, or they were assigned to a control group. The results showed that petting a toy animal was not significantly better than petting no animal at reducing anxiety; however, petting a real animal did significantly reduce anxiety. It should be noted that the sample size was small (58 individuals), focused on a limited set of proximal outcomes, which may not translate into public health outcomes. However, this study suggests that interaction with, and feedback from, the animal may be important in emotional regulation, and the authors hypothesized that the interaction provided a form of social support. Allen, Blascovich, and Mendes [15] tested the buffering hypothesis by subjecting individuals to stressful situations, examining the effects of social support from pet cats, dogs, spouses and friends. The results showed that individuals who had a pet had lower heart rate and blood pressure at rest than those without pets. Both studies provide strong evidence for stress buffering effects, but their methods limit the researchers’ abilities in concluding that these effects lead to any long term psychological of physical health benefits in a sample drawn from the general population. There is clear evidence of short-term differences in psychological adjustment that may be attributed to animal interactions; however, how these changes translate into public health outcomes for pet owners or within the general population has yet to be determined. Additionally, each study looked at a small range of dependent measures, thus limiting their ability to detect costs or benefits beyond the scope of the study and thus not providing any measure of “net effects.” Recent work by Beetz and colleagues [16] pulled together the evidence on the impact of human-animal interaction research to present a unified theory on the causal mechanism for the wide array of consistent impacts (e.g., social attention and behavior, interpersonal interactions, mood, heart rate, blood pressure, fear and anxiety, mental and physical health and cardiovascular function) and inconsistent effects (on stress and epinephrine/norepinephrine, immune system functioning, pain management, aggression, empathy, learning). They hypothesize that interacting with animals releases oxytocin, a hormone that is associated with a variety of heath promoting effects, and that the intensity, duration, and type of interaction mediates the relationship between interaction and health outcomes. They support their theory using the findings from 69 empirical studies that ranged in population, methods, measures, and design; however, the theory has yet to be specifically tested.

Empirical evidence of the effect The best evidence of the positive effect of animals on physical, mental, and emotional health has focused on a therapeutic environment, termed animal assisted therapy, because the studies use experimental designs that do not suffer from the problems inherent in observational studies [17, 18]. Such research demonstrating the benefits of animals with clinical populations has been carried out primarily with dogs [19], but has also examined the impact of cats [20], horses [21], dolphins [10], guinea pigs [22], and the robotic dog Aibo [23]. The mental health benefits of interacting with animals outside the therapeutic environment have been studied less—in part because of the difficulties of carrying out methodologically rigorous research outside the controlled environment of therapy. The extent to which these studies of the therapeutic environment can be generalized to the presence of pets in the home, and of public health outcomes is somewhat dubious. The health benefits of human-animal interaction has been also been studied in clinical patients and the findings are contradictory. For example, with adults recovering from illness, some studies have found pet owners do better while others have found that they do worse. Friedmann and colleagues [24] investigated patients who had been discharged from hospital after a heart attack. Patients who owned a dog had a much higher rate of one year survival– 6% of dog owning patients did not survive their first year, compared to 28% of non-dog owning patients. In a follow up study, they further explored this result, finding that there were differences in heart rate variability between pet owners and non-owners who had survived a heart attack, and suggested that this may be a mediating factor in the effect of pets on survival [25]. However, another study found that heart attack patients with dogs were more likely to have another attack or hospital readmission than dog owners [9]. These results demonstrate the potential health outcome differences between pet- and non-pet owners, but since they were performed on small, distinct, and self-selecting populations, they cannot be applied to the general population and we cannot infer that the difference in survival was caused by the dog ownership. Using non clinical populations, there is more compelling evidence that pet owners may be healthier. One study found that they make fewer visits to the doctor and take less medication [26]. In one of the most methodologically sophisticated studies examining this phenomenon, Headey and Grabka [27] employed propensity score matching to ensure, as far as possible, equivalence in owners and non-owners in Germany. This study represents the most rigorous causal test of pet ownership on overall health, using doctor visits as a proxy. The effect size of the association between pet ownership and doctor visits was reduced after matching across 11 variables—the mean difference between the groups was reduced from .44 visits to .28 visits after selection bias was taken into account. This demonstrates that when the impact of pet ownership is isolated from other related variables that also impact health, the relationship was appears to be diminished (although the authors do not specifically test whether the difference in effect sizes is statistically significant). Overall, they estimated the treatment effect of pet ownership resulted in a 24% reduction in annual doctor visits. They also conducted analyses on an Australia population and estimated an 11% reduction in doctor visits, after controlling for several other demographics that also impact health. While this study clearly demonstrates that isolating pet ownership from confounds is important for precise estimates of its causal impact, it falls short of a definitive answer since there are other differences limiting its generalizability, such as cultural differences between German and Australia samples, the use of doctor’s visits as a proxy for health, potential missing variable bias, among others. The recent review of the research literature on the impact of pet ownership on health concluded that there is not enough evidence to make any conclusions [3]. The piece reviews the methodological challenges preventing the extant literature from building a strong research base, including problems with small samples, convenience samples, lack of methodological rigor, and self-report measures, and the “file drawer” effect. All the research to date suffers from several limitations that prevent any strong conclusions about the health effects of pet ownership from being made.