What Makes Hair Curly?

image credit: Wenceslaus Hollar
Why a strand of hair bends or falls the way it does may sound like a simple question, but the answer is rather convoluted. On one level, the texture of a person's hair derives from his or her genes. A 2009 study looked at the genetics of waves and curls and reported a heritability of between 85 and 95 percent.
How does this play out at the level of a single hair? Research shows that the curvature of a strand depends on the nature of its follicle. When a follicle is asymmetrical, the hair that it produces is oval in shape and tends to curl. When it's symmetrical, the strand that emerges grows round and straight.