When it comes to dogs, it's a lot like dating: People all have their types.

Over the centuries, humans have bred dogs to create the looks and personalities they want. We've bred border collies for herding, bloodhounds for tracking, and golden retrievers for retrieving game — with the latter eventually evolving into the most chill family pet.

All this time we've been messing with appearance and skills, it turns out our tinkering has gone much deeper. A new study that looks at dogs' brain scans shows that humans haven't just changed the way dogs look and act; we've actually changed the shape of the canine brain.

To see what impact all that breeding had on gray matter, scientists looked at MRI brain scans from 62 purebred dogs from 33 different breeds.

"The first question we wanted to ask was, are the brains of different breeds of dogs different?" lead author Erin Hecht, a neuroscientist studying dog cognition at Harvard University, told The Washington Post.

And that's exactly what they found. Researchers saw a wide variety in brain structure that wasn't simply related to the size of the dogs or the shapes of their heads.

Breed and brain diversity

Researchers identified six networks of regions in the brain that were various sizes in different dogs, and found that each of those networks was associated with at least one behavioral trait. Areas linked to sight and smell, for example, were different in dogs that were bred to be alert, such as dobermans, versus other breeds. Breeds bred for fighting had changes in the network that correlated with anxiety, stress and fear responses.

"Brain anatomy varies across dog breeds," Hecht told Science, "and it appears that at least some of this variation is due to selective breeding for particular behaviors like hunting, herding and guarding."

The results were published in the Journal of Neuroscience.

Interestingly, these brain changes were there even though all the dogs studied were pets. They weren't professional herders or retrievers or otherwise working dogs.

"It's kind of amazing that we can see these differences in their brains even though they're not actively performing the behaviors," Hecht tells Science.

The fact that we're changing dogs so much that it affects their brain structure is "deeply profound," Hecht says. "I think it is a call to be responsible about how we're doing that and how we’re treating the animals that we've done it to."

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

How we've changed dogs' brains
Breeding has changed no only the way dogs look and act but also the shape of their brains.