So I'm a dad now.

And my wife got me these socks.

They're covered in my son's face.

Pretty much the cutest socks ever.

Now that I'm a dad, I realize I'm basically only going to watch animated movies for the next decade.

And as I started thinking back about all the great Disney movies through the years, I noticed something weird has been happening to the princesses.

In the earliest Disney films, the princesses more or less look like real, human women.

But through the years, something strange happens.

Heads get bigger compared to their bodies, and their eyes get bigger compared to their heads.

By the time we get to Elsa, it's clear this 22-year old Frozen princess doesn't look like an adult female.

She has the body ratios of an 8 year old.

Moana is supposedly 16 years old, but she has the body ratios of a 4 year old.

Disney princesses have been looking more and more like children.

And this case of the Benjamin Buttons isn't just happening to princesses.

In fact, this is true all over Toon-Town.

The designs of almost all cartoon characters have changed over time, and almost always in the same way.

As they get older, they age in reverse.

As we develop, we get a literal "head start." Our arms, legs, and bodies catch up as we get older.

That's why a newborn's eyes are already 75% of their adult diameter, and our brains hit 55% of their adult volume by 3 months of age.

Small bodies, big heads, big eyes.

Biologist Konrad Lorenz speculated that these babyish features trigger an instinct in adult mammals to give love and attention.

When I show you this photo of a kitten, something happens in your brain that makes you want to cuddle and feed it--unless you're some kind of unfeeling monster.

In other words, "cuteness" is nature's secret weapon to persuade adults into caring for babies.

Disney is just using the same biological trick to encourage audiences to root for their characters.

That's why cartoon protagonists tend to have juvenile characteristics, and the villains… not so much.

But this doesn't just happen in Fantasyland.

Which would you rather cuddle?

This cavalier king charles spaniel?

Or this wolf?

This floppy-eared ball of snuggles has been selectively bred to be cute.

It retains juvenile features into adulthood, or, what biologists call neoteny.

We see neoteny in many domesticated animals.

Although selecting for cuteness can explain the spaniel, animals like pigs show neoteny too.

Why would ancient humans care about the cuddle-ability of something they were raising for bacon?

Well, maybe cuteness is just a side-effect?

The most important trait in becoming a domesticated animal is… tameness.

Whether it's a companion, a worker, or a food source, you can't have a productive relationship with fearful or aggressive animals.

That fight or flight response is something that most animals only acquire as they get older--baby animals are pretty chill with humans.

So an animal that somehow never "grows up" in that sense might make the best candidate for domestication.

Beginning in the 1950s, Soviet scientist Dmitry Belyaev began a breeding experiment to study this idea, using wild silver foxes.

The foxes were tested for their reaction to human contact, and only the foxes that were friendly toward experimenters were allowed to breed.

After just 20 generations, his foxes had not only changed in behavior, but also in appearance.

Floppy ears, smaller jaws, and shorter tails that now wagged whenever humans were around.

Belyaev noted changes in hormones and brain chemistry that he suspected were capable of reshaping the foxes' external features.

What does this tell us?

If you select for one childish trait, a bunch of others tend to come along with it.

Evolutionary biologist Stephen J. Gould noticed that the same was true of the world's most famous cartoon character: Mickey Mouse.

OG Mickey was... kind of a jerk.

But as his personality got softer and sweeter, so did his appearance.

By the 1950s, Mickey had not only become the child-like mouse we know today, he'd become a nice guy.

But beyond pets or cartoons, you can also see neoteny in yourself.

Most biologists agree that humans are, in many ways, big babies.

Compared to other adult primates, we grow less body hair, have shorter limbs, and flatter faces.

And if you compare how much a chimp's skull morphs as it matures, you can see that our skull shape changes much less.

Our neoteny offered us lots of evolutionary advantages.

Less body hair meant we could run farther in that African heat, and our faces were more visible to each other as social interaction became more important.

Also, suppressing our own fight or flight response meant we could cooperate and organize in larger numbers.

And most importantly, these big brains need a lot of room and time to develop, which is why we rely on our parents for much longer than most mammals.

It might not be a coincidence that the more complex our society gets, the more time humans need to become independent.

Childhood is a time for experimentation and learning--most animals get locked into pretty rigid programs by the time they're adults.

By extending our childhood into adulthood, we can learn and change as long as we live.

Maybe that's why so many of us still love cartoons.

Because we may get old, but we never grow up. That's what makes us human.

Stay curious.