But when viewing the angry faces, the researchers noticed something odd. Dog performance was affected by whether they saw happy or angry expressions. During the initial training, dogs seeing the angry expression took longer to learn to approach and nose-touch the image for a treat than dogs who saw the happy expression. In other words, dogs were less inclined to approach and nose-touch angry faces, even though doing so would yield a treat.

“Why would I approach an angry person? That makes no sense,” a dog might think. Through past experiences with people, dogs could come to view the angry expression as aversive. The researchers suggest that dogs “had to overcome their natural tendency to move away from aversive (or threatening) stimuli…”