As any family photo will attest, genes are mostly to thank (or blame) for the nature of our hair. If you need a more data-driven argument, a 2009 study showed heritability of hair curliness is between 85 and 95 percent.
That answers the question of why, but what’s physically happening with our locks to cause such a wide variation of strands? It all comes down to the shape of the follicle—the part of the epidermis from which a strand of hair springs forth. The follicle is what determines both the shape of the hair and the angle at which is grows. An asymmetrical or flat follicle causes curvature, resulting in an oval shaped strand that grows at a sharper angle than its pin-straight counterparts.
Another factor might have to do with the protein structure in a stand of hair. Using electron microscopy, a research team from Clichy, France compared differing fibers and found that curly ones had a lopsided distribution of the protein keratin, mostly gathered on the inside edge under the curve.
As for the frizz often associated with curly hair? It’s because natural oils have a harder time traveling down the shaft of a curly hair strand.
Other temporary factors like humidity, hormones, medication taken, or a change in the hair’s proteins via heat or water can alter the state of your ‘do. For those who wish their straight hair would go curly, or curly hair would go straight, there’s hope: the quest to understand why hair naturally changes over time is fueling research projects that might mean that someday hair manipulation will be as easy as taking a pill.