Human Stress Scent Makes Dogs Make ‘Pessimistic’ Decisions, Study Finds

Dogs can detect human stress, and fresh research indicates that its scent can produce a comparable emotional response in dogs, leading them towards ‘pessimistic’ choices.

How human stress odour affects dogs

According to the UK research team, this is the first scientific evidence that human stress odours can influence dogs’ emotions and learning. The findings may also offer important insight into the long-standing relationship between humans and dogs.

People who share their lives with dogs may not be surprised that they can pick up on human moods, but the research indicates that this sensitivity may be more powerful than many realise.

"Owners know how attuned their pets are to their emotions, but here we show that even the odor of a stressed, unfamiliar human affects a dog's emotional state, perception of rewards, and ability to learn," says senior author Nicola Rooney, a human-animal interactions researcher at the University of Bristol.

Earlier research in humans has found that people can unconsciously pick up emotional cues from others through chemosignals in sweat. These unseen signals may, in turn, subtly shape our own feelings and decisions.

Recent studies have likewise shown that dogs can detect these signals from humans. Rooney and her colleagues, however, wanted to establish what effect human stress odours have on dogs.

Testing dogs’ response to stress scent

Because dogs are adept at interpreting people’s spoken and unspoken signals, the researchers chose not to place them directly with stressed people.

Instead, the dogs received sweat and breath samples from three unfamiliar volunteers, collected while the volunteers were either relaxed or undertaking a stressful task.

For the relaxed condition, volunteers watched a nature programme. The stress task consisted of frustrating instructions involving maths and public speaking.

The team enlisted 18 dog-and-human pairs for trials involving the human odour samples. In training, the dogs learned that a food bowl placed at one location always held a treat, whereas a bowl in a second location was invariably empty.

Once they had learned this distinction, dogs approached bowls more rapidly at the positive P location, associated with treats, than at the negative N location, where no food was provided.

The researchers describe this enthusiastic movement as “optimism”: a behavioural indicator of an animal’s emotional state. Previous work has linked positive and negative feelings in people with “optimistic” and “pessimistic” decisions, respectively.

Pessimistic decisions near the empty bowl

After the initial training, bowls were placed in unfamiliar positions between the original two locations, allowing researchers to assess how quickly the dogs would approach them.

Three additional positions were used, defined by their distance from the original sites: near-positive NP, middle M and near-negative NN.

The tests were repeated with dogs exposed either to samples from stressed humans, samples from relaxed humans, or no odour.

When dogs could smell an unfamiliar stressed person, they were significantly less likely to approach a bowl in the near-negative position than when they encountered the scent of a relaxed stranger or a blank cloth.

A stress odour was less off-putting when the bowl stood in the middle or near-positive position. Yet when it was placed close to the area associated with no food, the scent appeared sufficient to reduce the dogs’ expectation of a reward.

Dogs were not deterred to the same extent by the near-negative bowl when no stress odour was present. This implies that they combined the surrounding odours with the bowl’s location when judging how likely they were to find food.

"Working dog handlers often describe stress traveling down the lead, but we've shown it can also travel through the air," Rooney says.

The muted behaviour of dogs exposed to human stress odour meets the definition of pessimism and suggests a negative emotional state. Such a response could be adaptive, potentially enabling dogs to preserve resources or avoid frustration.

Much remains unknown about this process, and further studies will be required to determine precisely how human odours alter dogs’ feelings and learning.

Given dogs’ worldwide importance to people-as co-workers, partners and friends-it would be sensible to pursue evidence that could strengthen the bond between us.

"Understanding how human stress affects dogs' well-being is an important consideration for dogs in kennels," says Rooney, "and when training companion dogs and dogs for working roles such as assistance dogs."

The study was published in Scientific Reports.