Curly hair grows from a follicle that is curved or angled beneath the skin, producing a strand that twists as it emerges. Straight hair grows from round, symmetrical follicles that point straight up, while curly hair follicles are oval or asymmetrical and sit at an angle. The curl pattern you see on the outside starts with the shape and biology happening underneath.
What Happens Inside the Follicle
Every strand of hair is built from a protein called keratin, assembled inside the hair follicle before the strand pushes through the skin’s surface. In straight hair, the follicle is roughly symmetrical, so keratin cells are distributed evenly around the strand. In curly hair, the follicle has a curved or hook-like shape, which forces the strand to bend as it grows upward.
This bend isn’t just mechanical. The cells inside a curly hair strand are arranged unevenly. Research on curved hair fibers has shown that two different types of cortical cells (the cells making up the bulk of the strand) sit on opposite sides. One type clusters along the outer curve of the strand, and the other lines the inner curve. The cells on the outer, convex side contain protein fibers arranged in a helical pattern, while the cells on the inner, concave side have denser, more parallel protein arrangements. This built-in asymmetry is what locks the curl into the strand’s structure as it forms, not just as it dries or gets styled.
How Protein Bonds Create the Curl Shape
Keratin proteins in hair are held together by chemical bridges called disulfide bonds. These bonds form between sulfur-containing building blocks in the protein chain, and their placement determines whether your hair curls or lies flat. When disulfide bonds form in an asymmetrical pattern, they pull the strand into a curved shape. The more unevenly these bonds are distributed across the strand, the tighter the curl.
This is the same chemistry behind perms and relaxers. A perming solution breaks existing disulfide bonds, allowing the strand to be reshaped around a rod. A neutralizing solution then reforms the bonds in the new curved position. Relaxers do the reverse for curly hair, breaking bonds and resetting them in a straighter configuration. The fact that both processes work by rearranging the same type of bond shows how central disulfide bridges are to curl formation.
The Genetics Behind Curl Pattern
Follicle shape is largely inherited. Multiple genes influence whether your follicles are round, oval, or flat, and therefore whether your hair grows straight, wavy, or coily. One well-studied gene, TCHH, provides instructions for making a protein called trichohyalin. This protein creates organized cross-links with keratin inside the hair shaft, giving the strand its cylindrical shape. When trichohyalin doesn’t function properly, those cross-links break down and the strand’s cross-section becomes triangular, heart-shaped, or flat instead of round. This is what causes conditions like uncombable hair syndrome, where hair is extremely frizzy and won’t lie flat.
But curl pattern isn’t controlled by a single gene. Dozens of genetic variants contribute, each nudging follicle shape, protein distribution, or bond placement in small ways. That’s why curl patterns exist on a wide spectrum rather than as a simple curly-or-straight binary, and why siblings with the same parents can have noticeably different textures.
Why Curly Hair Looks Shorter Than It Is
Curly hair grows at roughly the same rate as straight hair, typically around half an inch per month. But because each strand coils back on itself, it appears much shorter than its actual length when stretched out. This is called shrinkage, and it’s one of the most misunderstood aspects of curly hair growth.
Shrinkage varies dramatically by curl pattern. Looser waves might shrink 20% from their stretched length, while tight coils can shrink 75% or more. Very kinky, coily textures sometimes show up to 90% shrinkage, meaning a strand that measures 10 inches when pulled straight might sit at just one inch from the scalp in its natural state. This doesn’t mean the hair isn’t growing. It means the curl pattern is compressing a lot of length into a small space. If you’ve ever wondered why your curly hair seems to “stop growing” at a certain point, shrinkage is almost always the explanation.
Why Your Curl Pattern Can Change Over Time
Hair texture isn’t always permanent. Hormonal shifts at puberty, during pregnancy, or after menopause can change the way your follicles function by activating or deactivating genes that influence curl. Someone with wavy hair might find their curly hair gene becomes more active during puberty, shifting their texture from wavy to distinctly curly. These changes happen at the follicle level, so they affect new growth rather than existing strands.
After menopause, lower estrogen levels combined with relatively higher androgens can cause follicles to shrink. This doesn’t always change curl pattern directly, but it can alter strand thickness and density, which affects how curls behave and hold their shape. Pregnancy often causes temporary changes in both directions, with some people experiencing curlier hair and others finding their curls loosen, then revert months after delivery as hormone levels stabilize.
How Curly Hair Grows Differently Day to Day
Because curly strands twist and coil, they face friction challenges that straight hair doesn’t. Each bend in the strand is a point where the outer protein layer (the cuticle) can lift or chip, which is why curly hair tends to be drier and more prone to breakage at mid-lengths and ends. The natural oils produced at the scalp have a harder time traveling down a coiled strand than a straight one, so moisture doesn’t distribute evenly.
This means that while curly hair grows from the scalp at the same speed as any other hair type, retaining that length is the real challenge. Breakage from dryness, manipulation, or friction can make it seem like growth has stalled when the issue is actually happening at the ends, not the roots. Keeping curly hair hydrated and minimizing mechanical stress on those vulnerable bend points is what allows length to accumulate visibly over time.