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Dear Science: Why does the hair on my head grow longer than the hair on my body?

Dear Science: Why does body hair and head hair grow to different lengths? (Video: Gillian Brockell, Rachel Feltman, Julio Negron/The Washington Post)

Dear Science: Why does hair on the body, other than the head, only grow a certain length and then stop, but if you cut it (not pull it) it immediately grows back to the original length? It is as if it knows it has been trimmed.

Here's what science has to say:

Don’t worry, your hair doesn’t have a mind of its own — not literally, anyway. Hair on the head is different from hair on other parts of the body (more on that in a minute), but it actually all grows out to a specific length. It’s thought that these lengths are largely determined by your genes.

First, here’s how hair growth works: All hair and fur grows in cycles. In the anagen phase, a protein root down in your hair follicle starts accumulating cells that form into a rope-like structure we know as hair. Your scalp’s blood supply feeds the follicle and allows it to divide into more cells. As long as the anagen phase lasts, your hair will grow longer and longer, unless you cut or break it, at a rate of about a half-inch each month.

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But the anagen phase can’t last forever, no matter how fancy your shampoo is. The growth phase lasts just a few years, and scientists think the specific length — which varies from person to person — is probably genetic.

"Hair length is mainly determined by the length of the anagen phase," Shari Lipner, a dermatologist at Weill Cornell Medicine and New York-Presbyterian Hospital, said. "The longer the anagen phase, the longer the hair will grow."

That can last from two to six years, which explains why some of us grow our hair to luscious lengths while others max out much sooner: A hair that grows for two years before stopping will be about a foot long, but one that can put in six years of growth could triple that length.

Our body hair (called androgenic hair, which replaces much of the baby peach fuzz known as vellus, when we hit puberty) is shorter than the hair on our heads. Unlike the hair on your head, the hair on your arms and the rest of your body has a growth cycle that lasts weeks, not years.

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This may be because our head hair continues to serve the greatest biological purpose: Now that we walk upright, protecting our extremities and trunks from direct sunlight isn’t a huge concern, but our heads are still pretty vulnerable to the sun’s rays. Having substantial head hair can also protect us from sweat and rain dripping into our eyes, and that sort of thing. Which is probably not what you usually think about when you go in for a trim.

Ever noticed an unusually robust hair that keeps popping up in a field of surrounding peach fuzz? "These long dark hairs are likely in a longer anagen phase than the rest of the hairs," Lipner explained. "This usually occurs more often as people age and may be due to hormones."

But all good things must come to an end, and no one's anagen phase lasts forever. After your body signals to the hair follicle that the strand it’s working on is done-zo, the follicle slowly withers and stops feeding the hair new cells. This is called the catagen phase. The hair gets pushed further up the shaft for a while, so it appears to get a little longer, but it’s not actually growing.

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Once the catagen phase is done, and the hair follicle starts to renew itself, you enter the telogen, or resting phase, when the follicle is dormant for a few weeks or months. Eventually, that dead strand either falls off or is pushed out by a new one as the new anagen phase begins.

You may have noticed that you don’t lose most of your hair every few years and start all over: That’s because humans experience staggered growth cycles. Only about 3 percent of your follicles are going through catagen phases at any given time, while about 90 percent are actively growing and the rest are chilling in telogen limbo. In some animals, this isn’t the case: Hair comes and goes in cycles or seasonally. That’s why your dog sheds in clumps, but you (hopefully) don’t leave handfuls of hair behind every time you get out of bed in the morning.

It's normal to lose about 100 hairs a day, no matter what age you are. Age-related hair loss doesn't mean your hair cycles have changed their pace. But some follicles stop working, and others produce thinner hair. That thin hair is also drier, because oil glands shrink with age, which makes it more prone to breaking when it's not at its maximum length. These factors create an overall loss of density and length, but your hair follicles are still running on the same timer.

So no, it’s not your imagination: When you trim your hair, it grows to about the same maximum length every time. That’s because your strands are only capable of growing that long. Fancy shampoos might make your hair strong enough to actually reach that maximum length — and keep it looking shiny while it gets there — but you can't cheat your genetic code.