Your hair type is determined primarily by the shape of your hair follicles, which is set by your genetics. A round follicle produces straight hair, an oval follicle produces wavy or curly hair, and a highly flattened follicle produces tightly coiled hair. But genetics is only the starting point. Hormones, aging, and even the weather can shift your hair’s texture and behavior over the course of your life.
Follicle Shape Is the Physical Blueprint
Every strand of hair grows from a tiny pocket in your skin called a follicle. The cross-sectional shape of that follicle dictates the curl pattern of the hair that emerges from it. The rounder the follicle, the straighter the hair. The more oval or flattened it is, the curlier the hair will be.
It’s not just shape, though. The angle at which the follicle sits in your scalp matters too. Straight hair tends to grow from follicles that are embedded at roughly a right angle to the skin’s surface. Curly hair emerges at a sharper angle from follicles that curve back on themselves near the base, a feature researchers call “retrocurvature at the bulb.” The cells inside curly hair follicles are also distributed unevenly, creating an asymmetry that reinforces the curl as the strand grows out. Straight hair follicles, by contrast, have a more uniform, symmetrical cell arrangement.
Genetics Controls the Follicle
Your follicle shape is encoded in your DNA, and multiple genes are involved. Variations in two genes, EDAR and FGFR2, have been linked to differences in hair thickness in East Asian populations. A variation in a gene called TCHH appears to influence hair texture in people of northern European descent. Beyond these, researchers have identified a broader network of genes that affect the proteins holding hair cells together, the keratin fibers that give strands their strength, and the chemical signals that promote hair growth.
Because so many genes contribute, hair type doesn’t follow a simple inheritance pattern like a single dominant or recessive trait. Two curly-haired parents can have a wavy-haired child, and two straight-haired parents can occasionally produce a child with waves. The interplay of dozens of genetic variants, each nudging follicle shape and hair structure in small ways, creates the wide spectrum of textures people actually have.
What Happens Inside the Hair Strand
The internal structure of a hair fiber also plays a role in how it behaves. Hair is built from a tough protein called keratin, and all the layers of its structure are held together by chemical links called disulfide bonds. These are strong, permanent bonds that lock the hair’s shape in place. They’re the reason a straight hair stays straight and a curly hair springs back after being stretched.
Inside each strand, there are two types of structural cells. One type has tightly packed keratin fibers surrounded by a protein-rich material. The other has more widely spaced fibers in a sulfur-rich material. In straight hair, these two cell types are distributed evenly around the strand. In curly hair, they’re arranged asymmetrically, with one type concentrated on the inner curve and the other on the outer curve. This uneven internal architecture is what gives curly hair its spring.
How Humidity Temporarily Changes Your Hair
If your hair frizzes or curls more on a humid day, that’s a different chemical mechanism at work. In addition to the permanent disulfide bonds, neighboring keratin proteins in your hair can form temporary connections called hydrogen bonds. These bonds break every time your hair gets wet and reform when it dries.
On a humid day, the air is packed with water molecules. Each water molecule can act as a bridge between two keratin proteins, creating a new hydrogen bond. When thousands of these extra bonds form along a strand, they cause the hair to fold back on itself at the molecular level. The result, on a macro scale, is more curl and more frizz. This effect is temporary: once the humidity drops or you restyle your hair, those hydrogen bonds break and reform in a new configuration.
Hormones Can Reshape Your Hair Over Time
Many people notice their hair texture change at certain life stages, and hormones are usually the reason. During puberty, a surge in androgens (hormones like testosterone) can make hair thicker and alter its curl pattern. Some children with pin-straight hair develop waves or curls as teenagers.
The reverse often happens during menopause. As estrogen levels drop, hair tends to thin, lose volume, and change texture. Estrogen normally extends the active growth phase of the hair cycle and stimulates the cells that build each strand. When estrogen declines, the relative increase in androgens can lead to finer, slower-growing hair. Research on postmenopausal women has found that those experiencing significant hair changes tend to have lower estrogen and higher levels of testosterone and its more potent form, DHT, compared to women whose hair remained stable. Progesterone, another hormone that drops during menopause, normally helps keep DHT levels in check by slowing the enzyme that produces it.
Pregnancy, thyroid disorders, and certain medications can trigger similar texture shifts, all through their effects on the hormonal environment around the follicle.
How Hair Changes With Age
Even without dramatic hormonal shifts, hair changes as you get older. Research on women of European descent found that individual hair strands gradually increase in diameter from the twenties through the early to mid-forties, then begin to thin. At the same time, the total number of active follicles on the scalp slowly declines throughout adulthood. Until about the mid-thirties, the increasing thickness of each strand offsets the decreasing number of strands, so overall hair volume holds steady. After that, both diameter and density start dropping together, which is why thinning typically becomes noticeable in the mid-forties to late fifties.
Graying also affects texture. As hair loses its pigment, the internal structure becomes slightly different: gray and white hairs often feel coarser or wiry, even though they may actually be thinner in diameter than pigmented hairs from the same scalp.
Why Different Hair Types Evolved
The global variety in hair type isn’t random. Research from Penn State University found that tightly curled hair likely gave early humans a survival advantage in equatorial Africa, where the sun beats down on the top of the head for most of the day. The brain is highly sensitive to overheating, and as human brains grew larger, managing heat became critical. Tightly coiled hair creates a buffer of air between the scalp and the sun, reducing the amount of solar heat that reaches the skin. In testing, all hair types reduced heat gain, but tightly curled hair provided the best thermal protection while minimizing the need to sweat. That matters because sweating costs the body water and electrolytes, scarce resources in a hot climate.
As human populations migrated to cooler, less sunny regions, the selective pressure for tight curls relaxed, and other hair textures became more common. A large multinational study of over 19,000 people across nine countries found that roughly 58% had straight or wavy hair, 22% had curly hair, and 18% had tightly coiled hair. Those proportions reflect both evolutionary history and the geographic distribution of the populations sampled.
Hair Typing Systems and Their Limits
The most widely used classification is the Andre Walker system, which sorts hair into four broad types. Type 1 is straight, Type 2 is wavy (ranging from loose, close-to-the-head waves to wider, frizzier waves), Type 3 is curly (from loose spirals to tight corkscrews), and Type 4 is coily or kinky (from defined coils to extremely tight patterns with almost no visible curl definition unless viewed up close). Each type has subtypes labeled A, B, and C to capture further variation.
This system is useful as a starting point for choosing hair products and styling techniques, but it has real limitations. Most people have more than one texture on their head. The hair at your temples may behave differently from the hair at the crown. Porosity (how easily your hair absorbs moisture), strand thickness, and overall density all affect how your hair looks and responds to care, and none of those are captured by curl pattern alone. Your hair type is less of a fixed category and more of a snapshot that shifts with hormones, age, climate, and how you care for it.