March 14 was “International Ask a Question Day,” and over on our Facebook page, we invited our readers to, well, ask a question and be entered in a giveaway. The response was overwhelming—hundreds of you BRI stalwarts posed a trivia conundrum. Unfortunately, we can’t answer all of them and not everyone can be a winner, but we did pick five of the most intriguing one…which we’re going to answer here. One of our winning questions comes from reader Kim J., who asked…
Why do some people have curly hair, and others straight? Can pregnancy change curly hair into straight, or vice versa?
Whether your hair is curly, straight, or not quite either (we’ll call that “wavy”), it’s almost entirely due to genetics. Your hair’s consistency and shape is probably similar to one of your parents’ in particular. Studies that have examined hair samples across multiple generations of families found a “heritability” factor between 85 and 95 percent. This means that the relative straightness or curliness of your hair has about a 9 in 10 chance of being the direct result of genetics.
The Nature of the Follicle
But there’s more to it than just that. How curvy (or how not curvy) the individual strand of hair may be is also related to the nature of the follicle from which it grows, of course. For example, a symmetrical, straight follicle will produce a straight hair that grows straight out and upward. An asymmetrical follicle, on the other hand, will lead to oval-shaped hairs…which tend to curl as they curve.
Another part of the equation involves how the follicle interacts with the scalp from which it sprouts. A straight hair follicle follows, naturally, a straight path, from the surface of the skin all the way back down into an inner layer of the scalp called the dermis. However, if that path is angled into the dermis in any way, well, then the follicle comes out at an angle, and so does the hair…which makes for a curly hair.
Major Hormonal Shifts
To answer the second part of the question: Yes. Major hormonal shifts can temporarily or permanently alter the way hair grows. Puberty, menopause, and pregnancy can all make curly hair go straight, or straight hair start to curl. When a woman is with child, levels of hormones like estrogen and progesterone increase. Because of those heightened levels, hair stays on the head longer—the hormone increase delays the rate at which the body normally sheds hair. That, in turn, leads to changes in how the hair (and its follicles) produce new hair.