Curly hair is the result of three things working together: the shape of your hair follicle, the internal structure of each strand, and your genetics. The flatter and more oval-shaped your follicle, the curlier your hair grows. A perfectly round follicle produces straight hair. But follicle shape is only part of the story. What happens inside the hair strand, and the genes that orchestrate all of it, matter just as much.
Follicle Shape Sets the Curl Pattern
Every strand of hair grows from a tiny tunnel in your scalp called a follicle. The cross-sectional shape of that tunnel determines whether your hair comes out straight, wavy, or coiled. A circular follicle produces straight hair (Type 1 in the widely used Andre Walker classification system). An oval follicle creates waves (Type 2). The more elliptical and asymmetrical the follicle becomes, the tighter the curl, progressing through loose curls (Type 3) to tight coils (Type 4).
Think of it like squeezing toothpaste through different nozzle shapes. A round opening produces a smooth, straight ribbon. A flat, oblong opening forces the material to twist as it exits. Your hair does essentially the same thing. The follicle’s shape isn’t something you can change with products or styling. It’s built into your scalp from before birth.
Two Cell Types Create the Curve
Follicle shape gets the curl started, but the strand itself has a built-in asymmetry that sustains it. Hair fibers contain two distinct types of cells in the cortex, the thick middle layer of each strand. These are called ortho-cortical and para-cortical cells, and they behave differently.
In curly hair, these two cell types are arranged on opposite sides of the strand, like two halves of a cylinder. The para-cortical cells consistently sit on the inside (concave) edge of the curve, while the ortho-cortical cells dominate the outside (convex) edge. This bilateral arrangement is what gives each curl its spring. In straight hair, the two cell types are more evenly mixed throughout the strand, so there’s no lopsided pull in either direction.
This distribution pattern was long thought to work simply because one side had more cells than the other, physically pushing the strand into a curve. But research from the Journal of Experimental Biology found no correlation between the proportion of each cell type and the degree of curvature. Instead, the two cell types appear to grow at different rates, creating a differential tension that bends the fiber. It’s closer to how a bimetallic strip curls when heated, with two materials expanding at different speeds, than to one side simply being thicker.
Chemical Bonds Lock the Shape In
The protein that makes up your hair, keratin, is held together by a network of chemical bonds. The most important ones for curl are disulfide bonds, which are strong links that form between sulfur-containing building blocks in neighboring protein chains. Hair keratin contains an unusually high number of these bonds compared to the keratin in your skin.
When disulfide bonds form between protein chains that are slightly offset or angled, they pull the strand into a curve and hold it there permanently. The more of these cross-links that form at asymmetric positions, the tighter the curl. This is also why chemical straighteners and perms work: they break disulfide bonds with one chemical, physically reshape the hair while the bonds are broken, then use a second chemical to re-form the bonds in the new position.
Water also plays a role in day-to-day curl behavior. Hydrogen bonds, which are weaker than disulfide bonds, form between keratin proteins and break easily when hair gets wet. This is why curly hair can temporarily loosen or tighten depending on humidity. The disulfide bonds maintain the overall curl pattern, but hydrogen bonds fine-tune how the curl sits on any given day. The strand’s flexibility and its response to moisture come from the interplay between these two bond types.
Genetics Behind Your Curl Pattern
Hair texture is polygenic, meaning dozens of genes contribute rather than a single “curl gene.” But a few genes have outsized influence, and they differ depending on your ancestry.
In people of European descent, the most significant genetic player identified so far is the trichohyalin gene, known as TCHH. A specific variant in this gene accounts for roughly 6% of the variation in hair shape among Europeans. That may sound small, but for a single gene influencing a complex trait, it’s substantial. The protein this gene produces is active in the inner root sheath of the hair follicle, the sleeve that molds and guides the growing strand. Variants that change this protein alter follicle architecture, which changes the curl. The straight-hair version of this variant reaches its highest frequency in Northern Europeans and drops off sharply outside Europe and western-central Asia.
In East Asian populations, a different gene called EDAR plays the dominant role. A variant in EDAR produces thick, straight hair and shows one of the strongest signals of natural selection found anywhere in the East Asian genome. This means the variant spread rapidly through the population because it conferred some advantage, though researchers still debate exactly what that advantage was.
The fact that straight hair evolved independently in European and East Asian populations through entirely different genes is striking. It suggests that the ancestral human hair form was likely curly or coily, and that straightness is the derived trait, not the other way around.
Why Curly Hair May Have Evolved
Research published in the Proceedings of the National Academy of Sciences tested a long-standing hypothesis: that tightly curled hair protects against heat from the sun. The results were clear. Tightly curled hair provided the most effective protection against solar radiation reaching the scalp, while minimizing the amount of sweat needed to offset heat gain.
The mechanism is straightforward. Tightly curled hair doesn’t lie flat against the scalp. It creates an air gap between the hair’s surface and the skin, forming a layer of insulation that slows heat transfer. This is the same principle behind double-paned windows. The air pocket lets heat dissipate before it reaches the scalp, reducing the body’s need to cool itself through sweating. In hot, sun-exposed environments like equatorial Africa, where early humans evolved, this would have been a meaningful advantage for conserving water and preventing overheating.
Both dry and wet experimental conditions supported the same conclusion: curly hair reduces overall heat influx from solar radiation. Its advantage isn’t in trapping warmth or shedding rain. It’s specifically in blocking the sun’s heat while still allowing the body to release its own heat through the scalp.
Why Your Curl Pattern Can Change Over Time
Many people notice their hair texture shifting at certain points in life, and this isn’t imagined. Hormonal changes during puberty, pregnancy, and menopause can alter follicle shape and the chemical composition of the hair strand. A child with ringlet curls may develop looser waves as a teenager, or someone with straight hair through their twenties may find waves appearing in their thirties.
These shifts happen because hormones influence how keratin proteins are assembled and how the inner root sheath of the follicle develops. Changes in thyroid function, estrogen levels, or androgen levels can all subtly reshape the follicle’s cross-section over months or years. The genetic blueprint doesn’t change, but the way your body expresses those genes does. Medications, nutritional status, and aging itself can have similar effects, gradually altering the ratio of disulfide bond placement or the bilateral distribution of cortical cells within each strand.