Dogs Can Smell Human Stress, Study Finds Scent Triggers Pessimistic Decisions

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

According to the UK research team, this is the first scientific evidence that odours produced by human stress can affect dogs’ emotions and learning. The findings could also offer important insight into the longstanding relationship between humans and dogs.

Although anyone living with a dog may not be surprised that dogs can pick up on human moods, the study indicates that this sensitivity may be more powerful than many assume.

“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 researcher in human-animal interactions at the University of Bristol.

Earlier research in people has shown that we can pick up clues to others’ emotional states through smell, unconsciously identifying chemosignals in sweat. These concealed signals can, in turn, subtly influence our own feelings and decisions.

Recent findings have also demonstrated that dogs can identify these signals from humans. Rooney and her colleagues wanted to establish how the odours associated with our stress influence dogs.

How dogs responded to human stress scents

As dogs are adept at interpreting both verbal and non-verbal human signals, the researchers chose not to expose them directly to stressed people.

Instead, the dogs encountered sweat and breath samples taken from three unfamiliar human volunteers while they were either relaxed or undertaking a stressful task.

For the relaxed condition, volunteers watched a nature video. The stress task, meanwhile, involved frustrating instructions connected with maths and public speaking.

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

Once they had learned this, dogs approached faster when the bowl appeared at positive location P, which was linked to treats, than at negative location N, where no food was available.

The researchers explain that this enthusiastic rushing reflects “optimism”: a behavioural indicator of an animal’s emotional state. Previous research has associated positive and negative emotions in people with “optimistic” and “pessimistic” decisions respectively.

Dogs’ pessimistic decisions in the bowl test

Following the initial training, the researchers placed bowls at unfamiliar positions between the original two locations, testing how willing the dogs were to approach them.

Three new positions were used, defined by their closeness to either original site: near-positive NP, middle M, and near-negative NN.

The trials were repeated while dogs were exposed to odour samples from stressed or relaxed people, or to no scent at all.

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

The stress scent was less discouraging when the bowl stood in the middle or near-positive location. Yet when it was paired with a position close to the food-free area, the odour apparently reduced the dogs’ expectation of a reward.

Dogs appeared less deterred by the same near-negative bowl position when no stress odour was present. This indicates that they were weighing up surrounding smells alongside the bowl’s location when judging the chance of finding 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 more restrained behaviour of dogs exposed to human stress odour meets the definition of pessimism and suggests a negative emotional state. This could be adaptive, potentially enabling dogs to save resources or prevent frustration.

Implications for dog welfare and training

However, much about this process remains unclear, and further studies will be required to determine precisely how human odours shape dogs’ feelings and learning.

Given dogs’ wide-ranging importance to people around the world-as colleagues, partners and friends-it would be sensible to pursue evidence that could strengthen the connection between our species.

“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.