Hair curl is determined by the shape of your hair follicle, the angle it grows from your scalp, and the chemical bonds holding each strand’s proteins together. Straight hair can become curly when any of these factors change, whether through genetics kicking in at a new life stage, hormonal shifts, chemical treatments, or even a humid day. Here’s what’s actually happening at each level.
Follicle Shape Sets the Foundation
Every strand of hair grows from a tiny tunnel in your skin called a follicle. If you could slice a straight hair follicle in cross-section, it would be perfectly round. Curly hair grows from an oval-shaped follicle, and the flatter that oval, the tighter the curl.
The angle of the follicle matters too. Straight hair follicles tunnel vertically down into the deeper layer of skin. When a follicle angles to one side instead, the hair curves as it emerges, producing a curl. This is why curl pattern is so consistent along the length of a strand: the follicle is acting like a mold, shaping every millimeter of hair that pushes through it. If something changes the follicle’s geometry (hormones, aging, or damage to the follicle itself), the hair it produces can shift from straight to wavy or curly.
The Bonds Inside Each Strand
Hair is mostly keratin, a tough protein. Long chains of keratin are held together by chemical cross-links between a building block called cysteine. These cysteine-to-cysteine connections, known as disulfide bonds, act like tiny bridges locking the protein chains into a fixed shape. They give hair its strength, elasticity, and resistance to water. The arrangement of these bridges determines whether a strand holds a straight line or a spiral.
On top of disulfide bonds, weaker connections called hydrogen bonds run between rows of protein molecules in the hair’s inner core (the cortex). Think of them as tiny magnets: positive charges on one protein chain attract negative charges on a neighboring chain, holding them in alignment. Disulfide bonds are permanent until chemically broken. Hydrogen bonds are temporary and can be disrupted by water or heat, which is why your hair can change shape on a humid day or after blow-drying.
Why Humidity Makes Hair Curl or Frizz
Water is exceptionally good at forming hydrogen bonds. When moisture from the air enters your hair, it breaks the existing hydrogen bonds between protein rows and forms new ones, rearranging the internal structure. A small amount of moisture can coax strands into gentle curls. When humidity is high, or when the hair’s outer protective layer (the cuticle) is damaged, much more water floods in. That’s when curling tips over into frizz: the proteins are being pulled in too many directions at once, swelling the strand unevenly.
This is also why a flat iron or curling iron works temporarily. Heat breaks hydrogen bonds, you reshape the strand around a barrel or plate, and as the hair cools, new hydrogen bonds lock in the new shape. The change lasts only until the next wash or humidity spike, because the underlying disulfide bonds haven’t been touched.
How Heat Tools Can Cause Lasting Damage
Keratin proteins in wet hair begin to break down permanently between about 120°C and 150°C (roughly 250°F to 300°F). In dry hair, that threshold rises to around 240°C (464°F), because water inside the strand accelerates protein damage. This is why styling damp hair at high heat is riskier than styling fully dry hair. Once keratin denatures, the internal structure of the strand is altered for good. That doesn’t give you a controlled curl; it weakens the hair, making it more porous, brittle, and unpredictable in texture.
Chemical Perms: Rewriting the Bonds
A permanent wave is the most direct way to make straight hair curly, and it works by breaking and rebuilding the disulfide bonds themselves. The process has two steps. First, a reducing solution (traditionally ammonium thioglycolate) is applied to hair that’s been wound around rods. This solution splits the disulfide bridges between cysteine groups, turning each broken bridge into two free-floating reactive ends. With the internal scaffolding loosened, the hair softens and conforms to the shape of the rod.
Then an oxidizing solution, typically containing a low concentration of hydrogen peroxide, is applied. This causes the free ends to reconnect, but because the strand is now in a curved position, each end bonds to a different partner than it had originally. The result is a new permanent molecular architecture inside the strand, one that holds a curl even after washing. The curl gradually grows out as new straight hair emerges from the follicle, because the follicle itself hasn’t changed.
Genetics and Ethnicity
Hair texture is a polygenic trait, meaning many genes contribute. Different genes appear to matter in different populations. In people of northern European ancestry, variants in a gene called TCHH (which produces a structural protein in the inner root sheath of the follicle) are linked to differences in texture. In East Asian populations, variants in EDAR and FGFR2 influence hair thickness. Dozens of additional genes related to keratin production, cell adhesion, and follicle development also play smaller roles.
Because so many genes are involved, hair texture doesn’t follow simple dominant-or-recessive inheritance. Two straight-haired parents can have a child with wavy hair, and siblings can have noticeably different curl patterns. The genetic blueprint mostly determines follicle shape and the distribution of different cell types within the strand, which in turn dictates curl.
Inside the Strand: Two Cell Types
The cortex of a hair fiber contains two distinct types of cells, called orthocortical and paracortical cells. In curly hair, these aren’t distributed evenly. Research on wool fibers (which share the same keratin biology as human hair) found that orthocortical cells on the outer edge of a curl are longer than paracortical cells on the inner edge. That length difference between cell types is what drives the strand to bend, much like a bimetallic strip curls because its two layers expand at different rates. Interestingly, it’s not the ratio of one cell type to another that matters, but the difference in their individual lengths.
Hormonal Shifts That Change Your Texture
Many people notice their hair texture change during puberty, pregnancy, or menopause. These shifts are driven by hormones acting directly on the hair follicle. Estrogen extends the active growth phase of hair and promotes the production of growth factors that keep follicle cells dividing. When estrogen drops sharply during menopause, hair often becomes thinner, loses volume, and changes texture.
Androgens play a role too. Testosterone is converted inside follicle cells into a more potent form called DHT by an enzyme called 5-alpha reductase. DHT can shrink follicles over time, altering both thickness and shape. Progesterone partially blocks this conversion, which is one reason hair texture can shift so noticeably during pregnancy (when progesterone is high) and then shift again after delivery. These hormonal changes can genuinely reshape the follicle, producing lasting texture changes rather than the temporary effects of humidity or heat.
Chemotherapy and “Chemo Curls”
About 65% of cancer patients who lose hair during cytotoxic chemotherapy find that it grows back with a different texture, often curlier, finer, or both. Chemotherapy targets rapidly dividing cells, and the hair bulb, where new hair cells are produced, is one of the fastest-dividing tissues in the body. The drugs damage the matrix cells responsible for building the strand and the pigment-producing cells alongside them.
When the follicle recovers and begins producing hair again, the rebuilt follicle may have a slightly different shape or internal cell distribution than it had before treatment. The resulting “chemo curls” are usually temporary, with hair gradually reverting toward its original texture over one to two years as the follicle fully regenerates. Targeted cancer therapies can also cause hair to grow in slower, finer, curlier, and more brittle.
Why Texture Can Change With Age
Even without a medical event, many people find their hair texture shifts over their lifetime. Children with ringlet curls sometimes develop straighter hair by their teens, while adults who had pin-straight hair in their twenties notice waves appearing in their forties. This happens because follicles are living structures that remodel over time. Hormonal changes, gradual shifts in follicle shape, and changes in the proportion and length of cortical cell types all contribute. The cumulative effect of aging on the follicle’s collagen scaffolding and blood supply can also subtly reshape the tunnel each strand grows through, altering the curl pattern that emerges.