We’ve all seen it. That sad look our dogs give when trying to convince us not to leave for work, or not to get mad at them for making a snack out of a new pair of shoes. It’s so hard to resist the appeal of “puppy dog eyes.” The urge to cave and give them what they want becomes overwhelming. Dogs seem to know how to use this to their advantage, and it turns out genetics have played a role in giving pups this superpower.

Dogs have muscles around their eyes that allow them to raise their eyebrows and make that heartwarming expression. But that wasn’t always the case. Dogs’ ancestor, the wolf, doesn’t have these muscles. Recent research indicates that dogs developed the muscles over the course of 33,000 years as a way of enhancing communication with humans. In short, as dogs became domesticated humans developed a subconscious preference for dogs that moved their eyebrows more. That longing look mimics the expressions humans make when they are sad, and thus it triggers in humans a desire to care for and protect the dog. This gave dogs that moved their eyebrows a “selection advantage” over other dogs and the trait became reinforced as dogs evolved genetically and honed their social interactions with people.

Genes play a role in “puppy dog eyes” beyond just the muscles used to create eyebrow movement. For example, dogs such as Scottish Terriers and Schnauzers have “facial furnishings”—a genetic trait that gives these dogs prominent beards, mustaches and eyebrows. Similarly, dogs such as Bernese Mountain Dogs and Rottweilers that express the tan point gene have distinct patches of color above their eyes called “pips”. These markings and furnishings are seen in mixed breed dogs as well, and are determined by the coat traits they inherit from their parents. Both furnishings and pips can increase the prominence of a dog’s facial expression. The more noticeable a dog’s eyebrows are in general, the more noticeable it is when those eyebrows move.

Subtle or prominent, one thing is for certain: “puppy dog eyes” have a way of melting our hearts. And we wouldn’t have it any other way.