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Science has proven that domesticated animals have significantly smaller brains than wild animals. Whether it’s a dog versus a wolf, or even farmed trout compared to wild trout, the brains of human-bred animals are just tinier. The statistics are dramatic: Domestic pigs have brains 35% smaller than wild boars, while domesticated cats have 30% less brain mass than their wild counterparts.

The first variable to rule out when explaining this difference is the size of the animals themselves—within species, smaller animals tend to have smaller brains than larger animals. But if you look at the brain of a dog next to a wolf of the same size, the dog generally has a smaller brain. This holds true no matter the size of the species.

Why is this? When thinking about comparative brain size, it’s useful to look at which parts of the brain are different. It turns out that in domesticated animals, the parts of the brain involved in flight-or-flight and aggression responses are significantly smaller. This makes sense—when humans began domesticating animals, they would have chosen those who were already the most docile. As those selected animals mated over millennia, their characteristics were exaggerated. And that’s how we got the cuddly, friendly, and frankly, slightly dumb animals we know and love today.