Dogs know when you’re angry, but they’re not so good at knowing when you’re happy.

In fact, when dogs see you smile, they may misinterpret it as aggression: new research indicates that dogs understand people’s facial expressions much better than previously thought. They just don’t always read them accurately.

That’s according to a new study by researchers at the University of Bari Aldo Moro in Italy, published this month in the journal Learning & Behavior.

The researchers showed photographs of human faces, one male and one female, to 26 feeding dogs. In each photo, the models displayed one of the six basic human emotions: anger, fear, happiness, sadness, surprise, or disgust. The dogs were also shown an additional photo of each model wearing a neutral expression. The models were told to make each face “with the greatest intensity possible.”

Researchers placed the photos strategically to the side of the dogs’ line of sight while they ate, and recorded their reactions when they stopped eating to look at the photos, evaluating which way they turned their heads, their cardiac activity, and their behavior.

The dogs showed greater response and cardiac activity when shown photographs that expressed arousing emotional states such as anger, fear, or happiness. They also took longer to start eating again after seeing those images. And the increased heartrates indicated that they were experiencing higher levels of stress, as well.

In addition, dogs turned their heads to the left when they saw human faces expressing strong emotions such as anger, fear, or happiness. But they turned their heads to the right when the faces looked surprised. And they didn’t show much reaction at all when shown faces expressing disgust or sadness.

The researchers say these findings support the idea that dogs access different regions of their brains to process basic human emotions. “Clearly arousing, negative emotions seem to be processed by the right hemisphere of a dog’s brain, and more positive emotions by the left side,” said lead author Marcello Siniscalchi, PhD.

And because the dogs turned their heads to the right when the faces looked surprised, the researchers theorize that dogs interpret a surprised expression as relaxed and nonthreatening.

Which begs an interesting question: Since dogs process a smiling face with the same part of the brain that they use to process an angry expression, does that mean they interpret happiness as a negative emotion?

Possibly, if the smile isn’t accompanied by auditory clues such as laughter or a happy, upbeat tone of voice.

The researchers say their findings suggest that dogs process smiling faces differently than people do, and note that the bared teeth and lifted lips that characterize human smiles could elicit a negative response from dogs, because when dogs communicate with each other, bared teeth and lifted lips are a clear message to back off, a message often followed by further aggression.

So the next time you smile at a strange dog, you might want throw in a courtesy laugh, just to let them know you mean it.

Photo credit: © Kosamtu