All dog owners know how difficult it is to stay upset with their beloved pets, even if Rover is behaving badly. Something about those puppy dog eyes just melt away the anger. Researchers at Duke University have now worked out a potential explanation for why we fall for “those” looks from dogs but less so from wolves, their close relatives.

The domestication of dogs took place around 33,000 years ago. The Duke researchers identified a muscle in dogs' facial anatomies that is used to raise the inner eyebrow. Wolves do not have this muscle, which suggests that humans selected for it during domestication.

When dogs use this facial muscle, it actually makes them appear more relatable and sympathetic to our emotions. This phenomenon where the dogs mimic human behaviors is called “paedomorphism," and it gives animals that have it a selective advantage over their counterparts. This means that, over thousands and thousands of years, we humans preferentially bred dogs that had the muscle, and today all of them do.

Unlike some forms of artificial selection, like farmers purposely creating varieties of corn that are adapted to very wet or dry soils, humans probably didn't select animals specifically because they had a strong inner eyebrow muscle. But, according to the Duke researchers, this feature triggers a subconscious impulse in us to nurture them, making them cuter and more snuggable in our eyes, which is likely why it exists only in today's dogs and not in wolves.