A study finds that dogs can smell your stress — and make decisions accordingly

New research out of the United Kingdom suggests that the smell of human stress affects dogs’ emotions as well as their decisions, leading them to make more pessimistic choices.

The study, published Monday in the journal Scientific Reports, was the result of a partnership between the University of Bristol, Cardiff University and the British charity Medical Detection Dogs.

It involved putting humans in the hot seat and then putting sweaty rags and food bowls in front of more than a dozen dogs to see how they would react to the scent of stress.

“Some people have looked at, can dogs actually detect differences in odor. And they have. But nobody had really looked at how that affects the dog's emotions,” says Dr. Zoe Parr-Cortes, a veterinarian and Ph.D. student at Bristol Veterinary School who was the lead author on the study.

It’s well-established that dogs can be trained to detect changes in levels of cortisol, a hormone that floods the body in times of stress, as service dogs do for people with certain health conditions.

But researchers wondered how sniffing stress-related changes in cortisol might impact dogs’ emotional state.

“Being a species that we've lived and co-evolved with for thousands of years, it kind of makes sense that dogs would learn to read our emotions because it might be helpful to them to know if there's something threatening in the environment or some stressor that they need to be aware of,” Parr-Cortes explains.

How researchers — and dogs — sniffed for clues

To find out, researchers first put human volunteers — who, importantly, were not known to the participating pups — through a stress test.

They were forced to prepare and deliver a five-minute speech on the spot and — it gets worse — do a high-pressure math task, as researchers “maintained serious expressions throughout to increase social anxiety.” They were then rewarded by getting to cozy up on a beanbag chair and watch a 20-minute video of forest and seaside scenes.

Researchers measured multiple stress indicators, including cortisol levels, heart rate and self-reported anxiety, during both sets of activities. They also collected samples of peoples’ breath and sweat by sticking pieces of cloth under their armpits.

Meanwhile, 18 dogs of varying breeds went through their own trials, as they were carefully trained to recognize the position and contents of several bowls in the study room.

Parr-Cortes says that setup is based on the famous test in which a person is shown a partially filled glass and asked to discern if it’s half full or half empty.

“Their response might change depending on their mood at that moment, or maybe their outlook in life at that time,” she explains.

First the dogs were trained to learn that a food bowl on one side of the room always contained a food reward, while a bowl on the opposite side was always empty. Over time, the dogs were fast to approach the full bowl and slow to approach the empty one.

Then researchers changed the scenario, removing the two bowls and placing a third right between the two locations, creating what they call an ambiguous scenario.

“Do they approach quickly, optimistic that there's going to be a food reward in there, or do they approach more slowly, with a more pessimistic outlook that there might not be food in that bowl?” Parr-Cortes says of the dogs.

This is where the sweaty cloths come in. The dogs’ owners, acting as handlers, would open up a jar with one of the rags and have the dog sniff it, before a bowl was put in front of them. Researchers ran the test multiple times, with both stressed and relaxed smells and in different orders and the bowls in all three locations.

They found that the dogs were more hesitant to approach the bowl in the ambiguous location after smelling the odor of a stressed stranger — meaning they were more pessimistic that it would have any food in it. The relaxed scent, in contrast, didn’t have a measurable effect.

“It basically shows that the stress smell might affect how [dogs] respond to ambiguous situations,” Parr-Cortes explains. “They might be less likely to try something risky if they think they’re going to be disappointed.”

Molly Byrne, a Ph.D. student at Boston College who studies comparative cognition — and is not affiliated with the study — is impressed by the findings but cautions that there’s still a lot we don’t know about how dogs perceive things, and all sorts of factors, including their life experiences, could influence the decisions they make.

To her, the study confirms that dogs might be less likely to think a reward is coming if they know their human is in a bad mood — which makes sense.

“When your owner is training you, they’re probably not giving you maybe as many treats if they’re really stressed,” Byrne says.

What the results mean for dog lovers

 We already knew that positive training, heavy on the rewards, is a good thing for owner-dog relationships, Parr-Cortes says. But this study suggests that the reverse is also true: Approaching the process while stressed could have a negative effect on how a dog feels and learns.

“Importantly, it highlights how in-tune dogs are at picking up on mood,” she adds. “So keeping your relationship with your dog … based on positive reinforcement and happy, fun engagement is the best way to have a good relationship and a happy dog.”

Byrne points out that frustration during training can be a source of stress, adding that “a lot of times the problem is only a problem because the person is upset.”

The fact that the study relies on volunteers who aren’t known to the dogs — which Parr-Cortes says shows that the dogs’ response is universal rather than learned — holds lessons even for people who aren’t actively training or parenting dogs.

Byrne says it’s a good idea to keep in mind that situations that are stressful for humans are probably also stressful for dogs. If you get anxious in crowds, for example, they very well could too.

“If you're stressed, you're probably more likely to be tense and not as patient,” she adds. “[And it] might literally make them have different behaviors. I think that's really, really important to know.”

All dogs have the potential to be affected by stress, Parr-Cortes says, even if not all of them show it. She points to her own dog, a mellow retired racing greyhound named Darwin.

“I'm just aware that even though he looks pretty chilled and laid back, there's probably things going on in his mind that are still affected by my stress and other things going on,” she says.

Humans, who tend to rely most on sight to make sense of their environments, may well forget that dogs’ most dominant sense is actually smell, which gives them a very different perspective on the world around them, Parr-Cortes adds.

While it can be easier said than done, she says that’s just one of many reasons to shed your stress — around dogs and in general.