Straight hair comes down to the shape of your hair follicle and how it sits in your skin. If you sliced a straight hair fiber in cross-section, you’d find it is perfectly round, and the follicle that produced it tunnels vertically from the skin’s surface into the deeper layers beneath. Curly hair, by contrast, grows from an oval or flat follicle set at an angle. This geometry is mostly determined by your genes, though chemistry and heat can temporarily or permanently alter the shape of a hair fiber after it leaves the follicle.
Follicle Shape and Angle
Every strand of hair begins inside a tiny tube in your skin called a follicle. The cross-sectional shape of that tube dictates the shape of the fiber it pushes out. Round follicles produce round fibers that emerge straight. Oval or elliptical follicles produce flatter fibers that naturally bend and coil. The more elliptical the cross-section, the tighter the curl.
Angle matters too. Straight hair follicles point straight down into the skin, so the fiber exits without bending. Curly hair follicles are tilted or even hooked, forcing the fiber to curve as it grows out. You can’t change follicle shape with products or styling. It’s set during development and stays largely the same throughout your life.
How Cells Are Arranged Inside the Fiber
The outer shape of the follicle is only part of the story. Inside each hair fiber, the bulk of the strand is made up of a region called the cortex, which contains several types of cells packed together. In straight hair, these cell types are arranged evenly in a ring-like pattern throughout the cortex. That uniform distribution means no side of the fiber contracts or expands more than the other, so the strand holds a straight shape.
In curly hair, two major cell types cluster on opposite sides of the fiber in a lopsided pattern. One type is denser and stiffer, the other more flexible. This bilateral arrangement creates uneven internal tension, pulling the fiber into a curve the same way a bimetallic strip bends when heated. The more uneven the distribution, the tighter the curl.
The Genes Behind Straight Hair
Hair texture is a polygenic trait, meaning many genes contribute. But two genetic variants stand out for their strong, well-documented effects on straightness, and they operate in different populations.
TCHH in Europeans
A gene called TCHH (trichohyalin) plays a key role in building the inner root sheath of the follicle, the structure that molds hair as it forms. A specific variant in this gene, involving a single amino acid swap at one position in the protein, is strongly linked to straight hair in people of European descent. Carrying two copies of the associated allele noticeably increases the likelihood of straight hair compared to carrying zero. This variant accounts for roughly 6% of the total variation in hair shape among Europeans, which is a large effect for a single genetic change in a complex trait. It reaches its highest frequency in Northern Europe and is rare outside Europe and western-central Asia.
EDAR in East Asians
In East Asian populations, a different gene variant does much of the work. A mutation in the EDAR gene, known as V370A, has risen to very high frequency in East Asian and Native American populations while remaining almost absent in Europeans and Africans. This variant is one of the strongest signals of natural selection ever detected in the human genome, suggesting it conferred a significant advantage at some point in human history (though researchers still debate what that advantage was).
Studies across Han Chinese, Tibetan, and Mongolian populations found a strong, consistent association between this variant and straight hair. The effect is additive: each copy of the variant roughly doubles the odds of having straight hair. The same variant also influences hair thickness, sweat gland density, and tooth shape, so its effects extend well beyond texture.
These two variants illustrate an important point. Straight hair evolved independently in different parts of the world through entirely different genetic pathways. There is no single “straight hair gene.” Instead, multiple genes influence follicle shape, internal cell arrangement, and protein structure, and the specific combination you inherit determines your texture.
How Hydrogen Bonds Keep Hair Straight
Even after a hair fiber has left the follicle, its moment-to-moment shape depends on weak chemical links called hydrogen bonds. These bonds form between protein chains inside the fiber and act like tiny clips holding the strand in whatever configuration it dried into. In naturally straight hair, hydrogen bonds hold the fiber in a linear arrangement.
Water breaks these bonds. That’s why your hair changes texture when it gets wet and resets when it dries. It’s also the principle behind blowouts and flat ironing. When you wet your hair, the hydrogen bonds release and the internal structure temporarily loosens. Using a dryer and flat iron while pulling the hair taut dehydrates the strand in a straight configuration, locking new hydrogen bonds into place. The effect lasts only until the next time the hair gets wet, because water will break those bonds all over again.
This is also why humidity causes frizz. Moisture in the air penetrates the outer layer of the hair fiber and disrupts the hydrogen bonds holding it straight. The strand absorbs water unevenly, and parts of it swell or shift, producing that puffy, undefined look.
Chemical Straightening and Permanent Changes
Hydrogen bonds are weak and temporary, but hair also contains much stronger links called disulfide bonds. These are permanent connections between sulfur atoms in neighboring protein chains, and they’re a major reason curly hair springs back to its original shape even after heat styling. To permanently straighten hair, you have to break these bonds and reform them in a new position.
Chemical relaxers do exactly that. Lye-based relaxers use sodium or potassium hydroxide, a strongly alkaline substance that cleaves disulfide bonds throughout the fiber. The hair is physically smoothed into a straight shape while the chemical works, and then the bonds reform in the new configuration. The result is permanent for the treated section of hair, though new growth from the follicle will still come in with your natural texture.
No-lye relaxers use milder compounds like calcium hydroxide or guanidine hydroxide. They work through the same bond-breaking mechanism but cause less scalp irritation, which makes them a common alternative. Both types carry a risk of damage, because the same bonds that create curl also give hair its strength. Over-processing can leave hair brittle and prone to breakage.
Why Your Hair Texture Can Change Over Time
Many people notice their hair getting straighter or curlier at certain points in life, even without chemical treatment. Hormonal shifts during puberty, pregnancy, and menopause can alter follicle shape slightly, changing the cross-section of the fiber it produces. Medications, thyroid changes, and aging can have similar effects. These shifts are usually gradual and sometimes reverse on their own.
Nutritional status matters too. Protein deficiency can weaken the structural proteins in hair, sometimes altering its texture. And mechanical damage from years of heat styling or chemical treatment can wear down the outer protective layer of the fiber, changing how it holds moisture and how it behaves in different environments. If your hair texture has shifted noticeably and recently, it often reflects something happening internally rather than anything you’re doing to the hair itself.