Like nearly all of a person's physical traits, how hair grows depends on genetics. People with curly hair inherit it from their parents. Even if neither of their parents have curly hair themselves, they can still carry the genetic markers for curly hair, passing the trait down to their children.
There are several biological causes of curly hair. One of these causes is the shape of a person's hair follicle. Hair follicles are the tiny sacs in the skin of people's scalps. These sacs hold the hair bulb from which hair grows. As the hair grows, it emerges from the top of the hair follicle. People with curly hair have hair follicles that are shaped like an elongated oval.
Direction and Twists
The direction that hair grows out from the scalp also determines if it will be curly. Within the follicle, the hair bulb may lie at a certain angle. This angle will determine in which direction hair will grow when it emerges from the scalp. Curly hair grows out of the scalp at a sharp angle because their bulbs sit to one side within their hair follicles. Hair also twists as it grows out of the scalp. Even straight hair twists, but only a little, so there aren't as many twists in straight hair. The more twists there are in hair as it grows, the curlier the hair will be.
Hair contains atoms of sulfur. When these sulfur atoms join, they form disulfide bonds. Sometimes, sulfur atoms are located far apart along a strand of hair, and the hair will bend so the sulfur atoms can form their disulfide bonds. The more disulfide bonds that are created between two sulfur atoms that aren't close together, the curlier hair can be. Shampoos and other hair treatments that straighten hair usually work by chemically breaking these disulfide bonds.
Oil and Water
All scalps secrete oil. This oil cannot travel down curly hair as easily as it can travel down straight hair, and curly hair can become dry. Dryness contributes to hair becoming even curlier or frizzier. Wetness, such as humidity, also makes hair more curly or frizzy. When hair absorbs water, it will take on its natural shape. If that natural shape is curly, then hair can become curlier.