Saying no when Fido gives you his best “puppy-dog eyes” can be extremely difficult. For thousands of years, dogs have been cajoling people by making this pitiful, adorable expression—an ability that may be the result of selective breeding, a new study says.

Tiny muscles around the eyes and mouth enable terrestrial mammals to form myriad facial expressions. Those muscles are more similar in dogs and humans than in dogs and wolves, says the study by researchers from Pittsburgh’s Duquesne University. In wolves, most of the facial muscles are slow-twitch fibers, which adjust less rapidly, while in humans and domesticated dogs, most of the muscles are fast-twitch fibers, which react quickly. It’s this musculature