Living with a dog has been shown to benefit people’s well-being and reduce stress levels. This is in part due to the healthier lifestyles and habits adopted when caring for a dog. Some studies also show that interacting with a dog during stressful situations can have immediate calming effects. As such, therapy that involves interacting with dogs was created to reduce stress and improve the well-being of those participating. 

But do canine-assisted interventions reduce stress for both humans and dogs, and how is this measured? Some objective ways to measure stress include measuring biological markers released during times of stress, such as cortisol, or recording physiological changes like increases in heart rate or breathing rate. 

To explore what is known about the effects of animal-assisted interventions (AAI) on stress in humans and dogs, researchers reviewed the scientific literature to understand what biological (and other) stress measures have been used to determine the effects of AAI on humans and dogs, and the stress outcomes of AAI on both species. After multiple screening stages, the researchers identified 27 studies that measured changes in at least one biological marker in humans and/or dogs associated with AAI. Some studies also included other measures of stress such as heart rate, blood pressure, self-reports (humans) and behavior (dogs).

Effects On Humans

Of the 27 studies, 18 assessed the impact of AAI on humans, with all 18 reporting a positive effect in at least one stress measure. Eleven studies found a significant decrease in cortisol or improvement in cortisol response associated with AAI. However, five of these 11 studies also reported decreases in cortisol when participating in dog-free activities such as walking, coloring, or a visit from a psychologist. Only one study found a significant increase in cortisol associated with AAI, however, this was in the dog handlers and not the participants. The remaining six studies found no significant change in cortisol associated with AAI. 

Most studies that used a combination of stress measures had mixed results. For example, five of the nine studies that measured cortisol and heart rate found either opposite effects or changes in one but not the other.  Where self-reported stress was recorded alongside biological markers, the results were inconsistent between measures in nine instances.

Effects On Dogs

Only nine of the 27 studies looked at biological markers of stress in dogs, with all nine measuring cortisol. Four studies reported a decrease in cortisol associated with AAI, however, in two of these studies, this decrease was not statistically significant. Another three studies found no change in cortisol associated with AAI. Interestingly, one of these three studies found significantly lower cortisol in dogs that were allowed off-leash during AAI but not in those that were kept on a leash, suggesting that increasing the freedom dogs have during AAI may be important for lowering stress. The final two studies showed either an increase in cortisol following AAI or an increase on days where dogs participated in AAI versus days they did not. 

Only three of the nine studies used physiological measures alongside cortisol and only one found consistent changes in all measurements. In contrast, all four studies that measured behavior alongside cortisol found significant correlations between the two, indicating the value of behavioral analysis for measuring stress in dogs.

Conclusions And Considerations

Overall, the findings suggest that AAI generally reduces stress in humans and has neutral to positive effects on stress in dogs. However, there is a clear imbalance in the number of studies on humans versus dogs, highlighting how research prioritizes measuring the benefits of AAI to humans over their effect on dogs.

One finding that was seemingly overlooked in this review was that multiple studies reported similar benefits to humans from participating in animal-free activities. This raises the question of whether AAIs are necessary, particularly if we cannot guarantee that they are completely stress-free for dogs. Where possible we should move to using alternative, animal-free interventions to avoid the risk of unnecessary stress in dogs. 

This review reveals how different measures of stress do not always correlate with each other and underscores the importance of using multiple measures together. The literature, particularly on dogs, heavily relies on cortisol as a biological marker of stress. However, capturing a change in cortisol can be challenging and can be missed even when stress is present. We also know that cortisol can increase with positive emotions like excitement or anticipation. Therefore, we must be cautious when interpreting results based on cortisol alone. 

Unlike us, dogs cannot self-report whether they are feeling stressed or excited. But signs of stress can be picked up from their behavior, a measure that wasn’t often used despite its correlation with other stress indicators. To ensure the welfare of dogs participating in AAI, more work is needed to improve how we measure stress in these situations.