Human hair varies dramatically across populations because of genetics, and those genetic differences were shaped over tens of thousands of years by the climates where different groups evolved. The result is a wide spectrum of hair textures, thicknesses, growth rates, and structural properties that trace back to a handful of key genes and the environmental pressures that favored certain traits in certain parts of the world.
The Genes Behind Hair Texture and Thickness
Hair differences between populations aren’t controlled by a single gene. They’re the product of many genetic variants, each nudging hair shape, curl, or diameter in a particular direction. But a few genes stand out for having especially large effects.
One of the best-studied is EDAR, the ectodysplasin A receptor gene. A specific variant of this gene, called 370A, is found almost exclusively in East Asian populations. It’s strongly associated with the characteristically thick, straight hair shafts common in people of East Asian descent. This variant affects not just hair diameter but also the number of sweat glands and the shape of teeth, suggesting it was selected for broadly rather than for hair alone.
In European populations, a gene called trichohyalin (TCHH) plays a measurable role in whether hair grows straight or wavy. TCHH is active in the inner root sheath, the sleeve of tissue inside the follicle that guides the hair as it forms. Variants in this gene account for roughly 6% of the variation in hair shape among Europeans. People who carry more copies of a particular version of the gene are significantly more likely to have straight hair. Six percent may sound small, but for a single gene influencing a complex trait, it’s substantial. The remaining variation comes from dozens of other genes, each contributing a smaller share.
For people of African descent, the genetics of tightly coiled hair are less well mapped, partly because genomic studies have historically underrepresented African populations. What’s clear is that coiled hair involves an asymmetrical follicle: the opening is oval rather than round, which forces the growing hair shaft to curve as it emerges. The tighter the oval, the tighter the coil.
Why Climate Shaped Human Hair
Genetics explains the “how,” but the deeper question is why these hair types evolved in the first place. A 2023 study published in the Proceedings of the National Academy of Sciences tested a compelling answer: tightly curled hair is a cooling system for the brain.
The researchers measured how different hair types handled direct solar radiation and found that tightly curled hair provides the most effective protection against heat gain to the scalp, while requiring the least amount of sweat to compensate. The key is architecture. Tightly coiled hair doesn’t lie flat against the head. It creates an air gap between the hair surface and the scalp, acting like a built-in layer of insulation that blocks incoming heat before it ever reaches the skin.
This mattered enormously for early humans in equatorial Africa. As human brains grew larger over evolutionary time, keeping them cool became a serious biological challenge. The conditions were intense solar radiation and scarce drinking water, so evolution favored any trait that reduced heat absorption and conserved moisture. Tightly curled hair did both: it blocked solar energy more effectively than straight hair, and because it reduced the need for sweating, it helped the body hold onto water longer. That could extend how long a person could walk, run, or forage before needing to drink.
As human populations migrated into cooler, less sun-intense regions of Europe and Asia, the selective pressure for tightly coiled hair relaxed. Other genetic changes, driven by different environmental demands or simply by random genetic drift in smaller populations, gave rise to the wavy and straight hair types common in those regions today.
Structural Differences in the Hair Shaft
The visible differences between hair types reflect real physical and structural variation at a microscopic level. East Asian hair, for instance, has more cuticle layers and wider cuticle cells than European hair. The cuticle is the outermost protective shell of each strand, made of overlapping scale-like cells. In Asian hair, these scales stack more steeply and sit closer together, creating a smoother, harder surface. This partly explains why individual strands of East Asian hair tend to be physically stronger. A single strand of healthy European hair can support 50 to 100 grams of weight before snapping, while healthy East Asian hair can withstand even more.
African-textured hair, by contrast, has a more irregular structure along its length. Each twist in a coiled strand creates a point of mechanical stress where the hair is thinner and more vulnerable to breakage. This isn’t a weakness in any biological sense; it’s simply a trade-off that comes with a coiled architecture optimized for thermal protection rather than tensile strength.
Growth Rates and Density
Hair from different populations also grows at different speeds. East Asian hair grows the fastest, outpacing European hair slightly. African hair grows the slowest, with research showing a gap of roughly 5 centimeters per year between African and East Asian hair. That slower growth rate is linked to the smaller diameter of individual African hair fibers rather than any difference in the hair growth cycle itself. All three groups cycle through the same growth, rest, and shedding phases at similar intervals.
Density varies too. East Asian scalps tend to have fewer hairs per square centimeter than European scalps, but the individual strands are thicker, so the hair still appears full. European hair is typically the densest by follicle count. African hair falls between the two in density but compensates in volume because the coiled shape takes up far more space per strand.
How Curl Pattern Affects Moisture and Shine
One of the most practical consequences of hair shape is how oil moves along the strand. The scalp produces sebum, a natural oil that lubricates and protects hair. In straight and lightly wavy hair, sebum travels easily from root to tip because the shaft offers a smooth, direct path. This gives the hair a natural shine and keeps it moisturized along its full length.
In tightly coiled hair, the twists and bends of each strand act as physical barriers, preventing sebum from distributing evenly. The oil pools near the scalp while the mid-lengths and ends stay relatively dry. This is why coiled hair often appears matte or feels brittle even when the scalp itself is producing a normal amount of oil. It’s not that the hair produces less oil; the geometry of the strand simply makes it harder for the oil to travel. This also has a thermoregulatory side effect: because fluids don’t spread fully along the coiled strands, less moisture evaporates from the hair surface, further helping the body conserve water.
Understanding this mechanism explains why moisturizing routines for coiled hair focus on manually distributing oils and using heavier creams that cling to the hair shaft rather than relying on the scalp’s own sebum to do the job.
The Bigger Picture
Hair variation across populations is one of the most visible examples of how humans adapted to different environments over the course of roughly 100,000 years of migration and genetic change. Tightly coiled hair evolved as part of a cooling system for increasingly large-brained humans under equatorial sun. Straight, thick hair in East Asian populations reflects a different set of genetic changes, at least partly driven by a single powerful gene variant. European hair occupies a middle ground, influenced by its own set of genes fine-tuning texture along a spectrum from straight to curly.
None of these differences reflect any hierarchy of quality or health. They reflect geography, climate, and the specific evolutionary pressures that different populations faced over deep time. The genes involved are a tiny fraction of the human genome, and the variation they produce is entirely superficial, even as it remains one of the most immediately noticeable ways that human populations look different from one another.