Having hair that’s curly in some spots and straight in others is extremely common, and it comes down to a mix of genetics, hormones, damage, and environment. Your hair follicles aren’t all identical. Each one has its own shape, angle, and internal structure, which means different parts of your head can genuinely produce different curl patterns.
How Follicle Structure Creates Different Curls
Whether a strand grows curly or straight is determined inside the follicle before the hair ever reaches the surface. The inner structure of each hair strand contains two types of cell groups arranged in different patterns: one type runs in parallel lines, the other in roughly spiral arrangements. The degree to which these two cell types are separated from each other along the strand is the primary driver of curl. When they’re heavily segregated to opposite sides, the strand curls tightly. When they’re more evenly distributed, the hair grows straighter.
Cross-section shape matters too, but less than most people think. A flatter, more oval cross-section contributes to curling, but research published in the Journal of the Mechanical Behavior of Biomedical Materials describes it as “synergistic but not determining.” In other words, a round hair shaft can still curl if the internal cell structure is asymmetric. Since each follicle on your head is slightly different in its internal architecture, it’s entirely normal to have a mix of textures growing from the same scalp.
Hormones Reshape Your Follicles Over Time
If your hair texture has changed, hormones are one of the most likely explanations. Androgens, the group of hormones that includes testosterone, directly alter hair follicle size and behavior. During puberty, rising androgen levels transform many tiny, fine follicles into larger, deeper ones that produce thicker hair. This transformation doesn’t happen uniformly across your scalp, which can shift your curl pattern in unpredictable ways.
Women experience similar follicle changes during pregnancy, postpartum recovery, and menopause, all periods when hormone levels swing dramatically. A person who had uniformly curly hair as a child may find that some follicles now produce straighter strands after puberty, or vice versa. These changes can take several hair growth cycles to fully appear, so the shift often feels gradual and confusing. The follicles aren’t damaged; they’ve simply been reprogrammed by your endocrine system.
Heat and Chemical Damage Break the Bonds That Hold Curls
Your hair’s curl pattern is physically held in place by protein bonds, especially disulfide bridges between keratin chains. These are strong, permanent bonds that give each strand its shape. Heat styling tools, chemical straighteners, bleach, and relaxers all break these bonds to varying degrees.
Chemical straighteners work by deliberately destroying disulfide bridges and reforming them in a straighter configuration. Research examining both lye-based and no-lye straighteners found that both caused significant loss of cystine, the amino acid that forms these critical bonds, with damage visible under microscopy as cuticle irregularities and detachment. Even a single treatment changes the molecular structure of the strand. Repeated treatments compound the damage.
This is why many people notice their hair is curlier at the roots and straighter or limper at the ends. The new growth emerging from the follicle has intact bonds and its natural pattern. The older hair toward the ends has accumulated months or years of heat, chemical exposure, and mechanical stress. The result is two visibly different textures on the same head.
Humidity Affects Strands Unevenly
If your hair seems to shift between curly and straight depending on the weather, porosity is likely involved. Porosity refers to how open or closed the outer cuticle layer of each strand is, and it determines how easily moisture moves in and out of the hair shaft.
Low-porosity hair has tightly sealed cuticles that resist absorbing moisture. High-porosity hair has an open cuticle structure that absorbs water quickly but also loses it fast. Here’s the key: you can have different porosity levels on the same head. Your roots, which are newer and less damaged, tend to be lower porosity. Your ends, especially if they’ve been colored, bleached, or heat-styled, tend to be higher porosity. On a humid day, the high-porosity sections absorb atmospheric moisture and swell, which can either enhance or disrupt curl pattern depending on your hair type. The lower-porosity sections barely react. The result is an uneven mix of textures that changes with the weather.
Medications Can Alter Hair Texture
Certain medications are documented to change hair from curly to straight or straight to curly. A 2022 review of reported cases found texture changes associated with several drug classes: cancer treatments were the most common culprit (97 reported cases), followed by anti-seizure medications (56 cases), retinoids used for skin conditions (15 cases), and immune-modulating drugs (3 cases). The changes typically happen because these medications affect rapidly dividing cells, and hair follicle cells are among the fastest-dividing cells in the body. In many cases, the original texture returns after the medication is stopped, though it can take a full growth cycle of six months to a year.
Why It Varies Across Your Head
Even without any external factors, having multiple textures is built into your biology. The follicles on different parts of your scalp sit at different angles and depths. The crown, temples, nape, and hairline each have their own follicle density and orientation. Hair at the nape of the neck, for example, is often curlier or wavier than hair at the top of the head. The temples frequently produce finer, straighter strands. This variation is genetic and completely normal.
Gravity and weight also play a role, especially for wavy or loosely curly hair. Longer hair is heavier, and that weight pulls curls straighter. If you cut your hair shorter and notice more curl, you haven’t changed your follicles. You’ve just removed the weight that was stretching them out. Similarly, layered cuts can reveal curl that was hidden when all the hair was one length, because shorter layers have less weight pulling them down.
If your mixed texture is new, think about what’s changed recently: a hormonal shift (puberty, pregnancy, birth control, menopause), a new styling routine, a medication, or even just growing your hair longer. If it’s always been this way, your follicles simply aren’t uniform, and that’s the most common explanation of all.