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Gwen Dewar, Ph.D.

Gwen Dewar Ph.D.

Why Do Humans Have to Cut Their Hair?

Chimpanzees don't need haircuts. Why do we?

Mohawk woman by Bengt Nyman wikimedia commons

This week at the American Association for the Advancement of Science in Boston, anthropologist Nina Jablonski presented her argument for the evolution of hairless skin. Humans have relatively little body hair because our active, tropical ancestors were in danger of overheating. But once naked, the human body became a handy canvas for decoration — with body paint, tatoos, scarification, and cosmetics.

Which reminds me of another puzzle. Why is the hair atop our heads so unusual?

I’ve always thought it interesting that humans, alone among mammals, have a hair problem. It doesn’t stop growing. Not for a very long time. So whereas your cat’s hair knows precisely when to stop to create the effect of a perfectly-contoured, full-body buzz cut, your own hair is clueless. Regardless of your race, age, sex, or stylistic preferences, you hair will, left to grow on its own, become difficult.

Which isn’t to say it will look bad. According to the Guinness Book of World Records, Aevin Dugas has the world’s largest Afro. How big? As of 2011, the circumference of her hair — when “blown and picked out”— measured 5 feet. She is 5’ 2” tall.

She’s a beautiful woman with movie star looks, and I’ll bet that a lot of people think her hair looks great. But when she tells us about the downside, it’s pretty clear why people cut their hair.

As Dugas has told the Huffington Post, “I have no peripheral [vision]. I can’t react quickly to anything.”

Worn as a big Afro, her hair gets hooked on trees. She has gotten it caught in the car door. Not so convenient.

But Dugas has a choice. She can grow long hair, or cut it short. She can pick it out into a perfect Afro, or weave it into plaits. She can even shave her head.

The question is: Why? Why do humans have these choices? Why doesn’t our DNA instruct our hair follicles to stop before we get to the hooked-on-trees stage?

Some evolutionary psychologists have argued that long hair has a special benefit for humans: It functions as an honest signal of your long-term good health.

If you have long hair on display — and it looks reasonably nice — we can presume you haven’t suffered any devastating illnesses in the last few years. So people with long hair can advertise a history of good health to potential mates.

Maybe so. But that doesn’t explain why children have the capacity to grow long hair.

Himba boy by Thomas Schock

It’s pretty easy to imagine humans evolving two stages of hair growth: Easy-care childhood hair that stops growing after a couple of inches. Post-pubescent hair that can achieve much greater lengths.

That sort of dimorphism has evolved many times. Think of harp seals — white as babies, brown as adults. Or the development of conspicuous body hair in humans. If the ability to grow long hair is merely about sexual attractiveness, we might expect that it wouldn’t develop until adolescence.

Besides, it’s obvious that humans fuss a great deal with their hair, and the results are culturally meaningful. Hair is often used to signal group membership. Are you with us or them? Throughout human history, people have used hair to tell each other where they belong.

Tymoshenko by Euku

So I’m thinking that we’re stuck with ever-growing hair because it helps us communicate our allegiances. It’s the raw material for creating a huge array of ethnic symbols, the original way to brand your organization, tribe, or clan.

Which might not help when you’re trying to get raisins or sticky rice or finger paint out of a child’s unruly mane of hair. Or when you’re agonizing about whether or not to chop it all off.

But it reassures me when I wonder why we care so much. It’s not just an irrational, arbitrary obsession with cosmetics. It’s also a signal of affiliation and territoriality. And so much nicer than peeing on a fire hydrant.


More reading

Most of the text of this blog is reprinted from an earlier posting on the BabyCenter Blog, "Why must we cut our children's hair?" For other blog posts about the evolution and anthropology of human behavior, check out "Parents have always been subsidized," and “Why don’t you eat your placenta?”

Text © 2012-2013 Gwen Dewar, Ph.D., all rights reserved

image of Mohawk woman by Bengt Nyman/ wikimedia commons
image of Himba boy by Thomas Schock / wikimedia commons
image of Tymoshenko by Euku / wikimedia commons