National Geographic:

Researchers don’t yet know whether the dogs’ ability to discriminate between the two expressions is because of past experiences or the result of the domestication process.

Whatever the reason, it’s not so surprising that dogs can tell facial expressions apart, Müller says. “Because they spend so much time with humans, they have a lot of opportunities to see human expressions.”

Marc Bekoff, a behavioral ecologist who specializes in canines at the University of Colorado, Boulder, agrees. People and dogs have forged an incredibly close connection over thousands of years together, says Bekoff, who wasn’t involved in the study. Along the way, dogs have been bred for certain traits, and “one of the traits would be the ability to read us.” (Read “How to Build a Dog” in National Geographic magazine.)

The question now is whether dogs that spend a lot of time with people would be as good at picking up our expressions as dogs without a lot of people experience, Bekoff adds.