Why does hair on the body, other than the head, only grow a certain length and then stop?

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. "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.