Curly hair in people of European descent comes from a combination of genetic variants, follicle shape, and the internal structure of the hair fiber itself. About 12% of Europeans have curly hair, while another 43% have wavy hair, meaning more than half of all Europeans have some degree of curl. The trait is remarkably diverse in this population compared to most other ancestral groups, and researchers have now identified over a dozen genetic locations that influence it.
What Happens Inside the Follicle
Hair curl is determined before the strand ever breaks through the skin. The shape of the follicle, its angle in the scalp, and how cells organize during hair formation all set the final shape of the strand. Inside each hair fiber, two types of cortical cells do most of the structural work. In straight hair, these cell types are mixed evenly throughout the fiber’s cross-section. In curly hair, they separate into two distinct sides: one type clusters near the outer curve, and the other sits along the inner curve. This lopsided arrangement creates built-in mechanical tension that forces the fiber to bend as it grows.
Think of it like a bimetallic strip in a thermostat, where two metals with different expansion rates are bonded together and naturally curve. The two cortical cell types have different internal architectures. The cells on the convex side have closely spaced protein filaments embedded in a matrix rich in one type of structural protein, while cells on the concave side have more widely spaced filaments in a sulfur-rich matrix. These structural differences make each side respond differently to moisture and tension, reinforcing the curl pattern. Chemical bonds between sulfur-containing amino acids (the same bonds broken during a perm or straightening treatment) lock these structural differences into place.
The Genetics Behind European Curls
The single most studied gene linked to hair texture in Europeans is called TCHH, located on chromosome 1. It produces a protein called trichohyalin that’s active in the inner root sheath of the follicle, the sleeve of tissue that molds the hair as it forms. A genome-wide association study across three Australian cohorts of European descent found that variants in this gene account for about 6% of the variation in hair shape, a substantial effect for a single gene influencing a complex trait.
One variant in particular, rs11803731, swaps a single amino acid in the trichohyalin protein. Carrying more copies of the derived version of this variant shifts the odds toward straighter hair. The variant shows a striking geographic pattern: it reaches its highest frequency in Northern Europeans and tapers off outside Europe and western-central Asia, suggesting it arose somewhere in that broad region. Its presence essentially nudges the follicle’s inner root sheath toward producing a straighter fiber. People who don’t carry it, or carry fewer copies, are more likely to have wavy or curly hair.
But TCHH is far from the whole story. A large meta-analysis of European cohorts identified 12 genetic locations significantly associated with hair shape, including 8 that were previously unknown. These span genes involved in skin and hair development, keratin-associated proteins, and signaling pathways that guide how follicle cells grow and differentiate. Some of these genes influence the growth factors that shape the follicle during development, while others affect the structural proteins that make up the hair fiber. The sheer number of genes involved explains why European hair texture exists on such a wide spectrum, from pin-straight to tight ringlets, with every variation in between.
Why Europeans Have Such Diverse Hair Textures
One of the more interesting findings about European hair is just how varied it is. A multinational study of nearly 20,000 people found that among Europeans, about 31% had straight hair, 43% had wavy hair, 12% had curly hair, and roughly 11% had very tight curls. This level of diversity within a single population is unusual. Other ancestral groups tend to cluster more heavily toward one end of the spectrum.
The reason likely comes down to the particular genetic history of European populations. The TCHH variant that promotes straight hair is geographically specific to Europe and central Asia, but it’s not universal even there. Meanwhile, the many other genes that influence curl each have their own frequency patterns across different European subpopulations. The result is a wide mix of genetic combinations, producing the full range of textures. No single “curly hair gene” determines the outcome. Instead, it’s the cumulative effect of variants across many genes, each nudging the follicle’s shape, the distribution of cortical cells, and the protein composition of the fiber in slightly different directions.
How Curls Can Change Over a Lifetime
If your hair went from straight to curly (or the reverse) during puberty, hormones are the likely explanation. Androgens, the group of hormones that surge during puberty, don’t just trigger facial hair and body hair. They also reshape existing hair follicles by transforming small, fine follicles into larger, deeper ones over several hair growth cycles. This transformation can change the follicle’s geometry enough to alter curl pattern. Girls experience a milder version of this shift because their androgen levels are lower, but it’s still enough to change texture noticeably in some cases.
Pregnancy is another common trigger. Hormonal shifts during and after pregnancy can temporarily or permanently alter follicle behavior, and many women report their hair becoming curlier, wavier, or straighter during these periods. The mechanism isn’t fully mapped, but it follows the same basic principle: when hormones change the follicle’s size, depth, or angle in the skin, the curl pattern of the hair it produces changes too. These shifts can also happen during menopause or with thyroid changes, reinforcing that hair texture in Europeans isn’t a fixed trait. It’s the product of genetic potential being expressed through a follicle that responds to its hormonal environment throughout life.
How European Curls Differ From Other Hair Types
Curly hair in Europeans and tightly coiled hair in people of African descent share the same basic mechanism: an asymmetric distribution of cortical cell types within the fiber. But the degree of asymmetry, the cross-sectional shape of the strand, and the follicle geometry differ significantly. African-textured hair tends to have a more elliptical cross-section and a follicle that curves sharply beneath the skin, producing tighter coils. European curly hair typically has a rounder (though still somewhat oval) cross-section and a less extreme follicle curve, producing looser spirals and ringlets.
The protein composition differs too. The ratio of high-sulfur to low-sulfur matrix proteins varies between the two cortical cell types, and this ratio shifts across ethnic groups. These chemical differences affect how the hair responds to humidity, how easily it breaks, and how it behaves when stretched. European curly hair, for instance, tends to have different moisture absorption patterns than African-textured hair, which is why products and care routines that work well for one type don’t always translate to the other. The curls may look superficially similar at times, but the underlying architecture is distinct at nearly every level, from the gene variants involved to the protein chemistry of the finished strand.