Light eyes aren’t common in domestic canines. A dog with piercing blue eyes might be described as strikingly beautiful (or sometimes even unsettling). Rarely do we call their eye color cute, though, and apparently past generations of dog breeders agreed. And new research suggests we may have selectively bred dogs to have darker eyes to make them appear younger & friendlier.

Last year, a team of Japan-based researchers compared eye color in gray wolves to that of domestic canines. After analyzing photos of 22 wolves & 81 dogs across 35 different breeds, they found the dogs’ irises were notably darker on average. While dark brown is the standard eye color in dogs, wolf eyes are more likely to be yellow.

It’s possible that difference was intentionally selected by humans. After analyzing canine eye color, the researchers asked participants to look at pictures of dogs
& make assumptions about their personalities, with some images altered to make the pets’ irises lighter or darker. Dogs with dark eyes scored higher for friendly & nonthreatening traits like kindness, sociability, and trustworthiness, while pale-eyed dogs were more likely to be labeled aggressive & mature. What that means is we humans find brown eyes cute & puppy-like, which could explain why breeders gave preference to the feature.

But it’s not really the eye color that people find adorable…it’s the perceived size of the pupils. Humans are born with relatively large pupils that get smaller as the eye muscles that control them get weaker with age. We associate big pupils with youth, and the darker the iris is, the harder it is to see where the pupil begins. The study authors suspect that brown eyes make pupils appear bigger than they really are, tricking us into thinking our pets are helpless babies that need our protection, even when they’re 200 lb. Mastiffs.

And color isn’t the only trait that makes a dog’s eyes appear bigger & more lovable. Domestic canines have evolved in their ability to raise their eyebrows when looking at their owners, a trick otherwise known as puppy dog eyes.

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