Have you ever wondered if your stress affects your dog? A new study published in Scientific Reports suggests that it might. The study found that the scent of human stress can influence dogs’ behavior, potentially making them more cautious and less likely to take risks, even when it comes to something as simple as approaching a food bowl.
Dogs and humans have shared a close bond for thousands of years, with dogs often being called “man’s best friend.” This deep relationship has led dogs to become incredibly attuned to human emotions, reading our body language, facial expressions, and even our tone of voice. However, researchers wanted to explore another, less obvious way dogs might be picking up on our emotions—through their sense of smell. Specifically, they sought to determine whether dogs could detect the smell of human stress and how it might affect their behavior.
The idea of “emotional contagion,” where individuals in a group can pick up on and mirror the emotions of others, is well-documented in humans. But how does this concept apply to the human-dog relationship? Researchers theorized that if dogs could detect stress through smell, it might influence their mood and decision-making. This study is the first to test how the odor of human stress affects dogs’ learning and emotional state.
“Most dog owners will tell you their dogs can sense when they’re stressed or upset. When interpreting our emotions, dogs likely use a combination of cues, such as our body language, voice tone, etc., but we didn’t know if they responded only to scent, without behavior or vocal cues present,” said study author Zoe Parr-Cortes, a qualified veterinarian and PhD student in clinical veterinary science at the University of Bristol and Cardiff University.
“We also know that dogs can be trained to detect changes in cortisol in humans. For example, dogs trained by Medical Detection Dogs in the UK can detect when a person’s cortisol levels in people with Addison’s disease drop too low. So, since cortisol tends to increase during stress, we wondered if dogs could detect cortisol during stress and further how the smell affects dogs’ own emotions.”
“What’s interesting about this study is that these odors were from unfamiliar people, so it suggests a common ‘stress’ smell among individuals that dogs can identify, rather than just learning the smell of their stressed owner from previous experience.”
The researchers conducted a two-phase study involving both human and canine participants. The first phase focused on collecting odor samples from human volunteers, while the second phase involved testing dogs with these odors to see how they would react.
Eleven volunteers were recruited to provide odor samples under two conditions: stress and relaxation. During the stress condition, participants were subjected to a well-known stress-inducing test, which involved public speaking and mental arithmetic in front of evaluators. In the relaxation condition, they watched calming videos of nature scenes. Throughout both sessions, the participants wore cotton cloths under their arms to absorb sweat and exhaled breath, which were later used as odor samples.
The second phase involved 18 dogs of various breeds and ages. These dogs were put through a cognitive bias test, a type of test that assesses an animal’s emotional state by measuring their response to ambiguous stimuli. In this case, the ambiguous stimuli were bowls placed at different locations in a testing room, some of which previously contained food (positive location) and others that did not (negative location). The key question was how quickly and eagerly the dogs would approach the bowls when they were placed in new, intermediate locations—closer to the positive, negative, or somewhere in between.
Each dog was exposed to three conditions over three separate sessions: a stress odor from a human participant, a relaxed odor from the same participant, and a control condition with no added odor. The researchers measured how long it took for the dogs to approach the bowls and whether they approached them at all, particularly focusing on the bowls placed in ambiguous locations.
The study revealed that dogs were less likely to approach a bowl placed in a location near where they had previously found no food (near-negative location) when exposed to the stress odor compared to when there was no odor or a relaxed odor. This suggests that the smell of human stress made the dogs more cautious or risk-averse, even though the bowl might have contained food.
“While we found that being stressed around your dog may affect their mood and how they learn and respond to training, we found that the relaxed odor did not have this effect,” Parr-Cortes told PsyPost. “So being aware of this and keeping calm or even doing a relaxing activity before training your dog could reduce this effect if you’re stressed.”
“We also know that using positive training with lots of rewards is good for your relationship with your dog and can have positive effects on their welfare. So, keeping your relationship with your dog based on positive reinforcement and fun engagement is the best way to keep your dog happy and look after their well-being.”
Interestingly, this effect was most pronounced during the third session when the stress odor was introduced after the dogs had already been exposed to the relaxed odor. This finding hints at a possible interaction between the odor and the learning process, as the dogs seemed to become increasingly cautious as the testing progressed.
“Originally, we were just looking at how the smell of stress affected the dog’s cognitive bias (optimism/pessimism),” Parr-Cortes said. “So, when we found an effect of the stress odor on dogs’ learning and how they perceive rewards as well, it was an interesting additional finding.”
The researchers also found that dogs were quicker to approach the positive location (where they expected to find food) and slower to approach the negative location (where they expected no food) as the sessions went on. This suggests that dogs were not only influenced by the odor but also by their previous experiences during the testing, learning to differentiate between the likely and unlikely rewards.
But the study, like all research, does have some limitations. First, the sample size of both human and dog participants was relatively small, which means that the results may not be fully generalizable to all dogs. Future studies with larger groups could help confirm and expand on these findings.
“As with many first studies, the sample size could always be bigger,” Parr-Cortes said. “So, it would be great to repeat this study with more dogs. Nonetheless we’re extremely pleased we were able to show differences with the odors we tested. But it opens the door to so many new questions and avenues for future research.”
Additionally, the study focused on unfamiliar human odors, so it remains unclear how dogs might react to stress odors from their owners, with whom they have a closer bond. Future research could explore how familiar versus unfamiliar stress odors affect dogs differently.
The findings open up several interesting avenues for future research. “Moving forward, we would like to look at the effect of other emotions, such as happiness or deep relaxation, and also look at the effect of repeated exposure to odors of stress and other emotions rather than just a one-off test,” Parr-Cortes explained.
“While the findings may seem negative, I think it’s an important reminder of how important dogs are in our lives, as well as how important we are in our dog’s lives,” she added. “This research should make us consider how we interact with and act around dogs and should encourage people to be more compassionate with the dogs they live or work with. It’s an opportunity to see things from the dog’s perspective and ensure our interactions with our dogs are as positive as possible so we can minimize any negative effects our stress or emotions have on our dogs.”
The study, “The odour of an unfamiliar stressed or relaxed person affects dogs’ responses to a cognitive bias test,” was authored by Z. Parr-Cortes, C. T. Müller, L. Talas, M. Mendl, C. Guest, and N. J. Rooney.
