Curl pattern

Key takeaways

• Curl is made in the follicle, not by the hair after it leaves the scalp. Curly hair grows from a curved follicle whose bulb is bent like a golf club, with cells dividing and maturing unevenly on its two sides. Curly follicles keep producing curly hair even when grown in a dish Thibaut 2005.
• Inside the fiber, what matters is how cortex cells are arranged, not how many of each type there are. In curly fibers the cell types sit on opposite sides; in straight fibers they are mixed evenly Bryson 2009, Thibaut 2007. The overall proportions are similar across hair types Wortmann 2018, so "curly hair has more paracortex" is an oversimplification.
• An oval cross-section goes with curl but may not cause it. In a mixed African-European sample of 140 people, the link looked like a by-product of ancestry Lasisi 2021.
• Curl is strongly genetic and involves many genes. The trichohyalin gene explains about 6% of curl variation in Europeans Medland 2009, and a 28,964-person meta-analysis found 12 genome regions Liu 2018. DNA predicts straight versus not straight only modestly (AUC 0.66 in Europeans, 0.79 in non-Europeans) Pospiech 2018.
• One head holds a range of curl types. Nobody's 10 hairs from a 0.56 cm2 patch landed in a single class; each person spread across 2-3 classes Mkentane 2017.
• Curl can change during life. Reported triggers include androgens after puberty (rare), aging (more irregular curvature, less shine) and some drugs (usually reversible). Pregnancy measurably thickens individual hairs Nissimov 2003.
• Curl changes how hair behaves. Bends and twists are weak points where cracks and knots form, so grooming breakage is more common in very curly hair even though the chemistry is similar Camacho-Bragado 2015, Daniels 2023. Curls also scatter light in many directions, which lowers visible shine McMullen 2004.
• Tight coils have a real biological upside: in thermal-manikin tests they blocked the most solar heat from reaching the scalp Lasisi 2023.
• Curl type was the only hair attribute linked to goals and routines in a 506-person survey of women with curly hair Daniels and Heitmayer 2024.

Where curl comes from

• Curved follicle, bent bulb. In scalp biopsies, follicles from African volunteers were curved under the skin with a backward hook at the bulb. Caucasian follicles were straight, and the bulb itself was bent. The sheaths around the follicle were thicker on one side. Follicles kept their curvature in culture and produced curved shafts, which is why curl is said to be programmed from the bulb Thibaut 2005, Bernard 2003.
• The two sides grow differently. Dividing cells were more numerous on the convex side of curly follicles, and the inner root sheath and shaft matured later there. The backward hook appeared in curly follicles regardless of ethnic origin Thibaut 2005, Thibaut 2007.
• The root sheath matters. A 2014 letter reported that natural curvature correlates with bending of the follicle and bulb, caused by a structural imbalance in the root sheath Yoshida 2014. Several curl genes (TCHH, PRSS53, KRT74) are active in the inner root sheath, the sleeve that moulds the growing hair Medland 2009, Adhikari 2016, Westgate 2017.

Inside the fiber

• In straight Japanese hair, cortex cell types were arranged in even rings. In curved hairs they were split: one dominant type near the outer side of the bend and another near the inner side, with different packing of keratin filaments Bryson 2009.
• One hair keratin built up on the inner side of curly fibers but was spread evenly in straight ones. The authors concluded that curl comes from asymmetric maturation of the pre-cortex Thibaut 2007.
• A heat-analysis study found similar fractions of the two cell types across hair types, with no consistent link to curliness Wortmann 2018. The distribution around the fiber is what matters.
• Proteomics of very straight versus very curly hair (50 people per group, pooled) found 13 proteins that differed. The authors say it is unknown whether these cause curl or simply reflect the populations sampled Maes 2021.

Cross-section shape

Reviews report that straighter fibers tend to be round with a larger cross-section, and curlier fibers elliptical with a smaller one Daniels 2023. The mixed-ancestry data suggest this link may be an artefact of ancestry affecting both traits Lasisi 2021. So "oval follicle equals curly hair" is a fair mnemonic, not an established mechanism. Hairs that thin with age in women over 50 become rounder as they shrink Baltenneck 2022, so cross-section also tracks hair size and age.

Genetics

In a twin study, trained observers' ratings of curl and machine-measured curvature both pointed to the same TCHH signal, so careful observation of curl is a valid measurement Ho 2020. Because common variants explain only a small share, curl cannot be read reliably from ancestry or a DNA test.

Why curl changes during life

• Puberty and androgens: a case series of six people described "acquired progressive kinking", with coarse kinky hairs appearing at or after puberty at the front, temples and crown, thought to be androgen-dependent Mortimer 1985. It is rare.
• Aging: in 230 Japanese women aged 10-70, perceived luster fell with age as hair became irregularly curved Nagase 2009 (industry). A lab study found age-related curved hair developed disulfide-bond breakage under repeated stretching; straight hair did not Fuse 2025 (in vitro, industry). Thin hairs in women over 50 had fewer cuticle layers and stiffer mechanics Baltenneck 2022.
• Pregnancy: individual hairs got thicker (+0.58% per cm toward the scalp, versus -0.66% per cm in non-pregnant women; 12 vs 13 women) Nissimov 2003. Curl change in pregnancy is widely reported but has no controlled data.
• Drugs: a systematic review of 31 papers (2,594 patients) found texture changes with chemotherapy drugs, antiepileptics, retinoids, immunomodulators and antiretrovirals. New or stronger curling was most common, starting 4.5-17 months into treatment, and usually reversible Phong 2022. Examples are case reports with isotretinoin Bunker 1990, valproate Jeavons 1977, antiretrovirals Colebunders 2000 and nivolumab Dasanu 2017. See drug-induced hair changes.
• Chemical straightening or perming reshapes treated lengths by breaking and reforming disulfide bonds; new growth comes in with the natural pattern. Wetting breaks hydrogen bonds, so blow-dry and flat-iron shapes are temporary and revert with water or humidity Gavazzoni Dias 2015. Bleach, UV and flat ironing damage both straight and curly hair Bloch 2019.

What curl does to hair behavior

• Breakage: in a 2005 industry survey cited by a later paper, 96% of African-American women reported breakage. Lab work on one volunteer's virgin hair showed cracks starting at the cuticle-cortex junction and growing under grooming stress, with no compositional difference from other groups Camacho-Bragado 2015. A 2023 review concludes that fragility comes mainly from knots and cracks, not different chemistry Daniels 2023.
• Mechanics: curly fibers show an initial region where the curl uncoils before the material stretches, and standard test preparation may underestimate their strength (a small study) Cloete 2020.
• Shine: straight, aligned fibers give one continuous highlight band, while curls create many small, scattered reflection points, so curly hair looks less glossy even when healthy McMullen 2004.
• Heat protection: tightly curled wigs let in the least solar heat Lasisi 2023 (a co-author sits on an industry advisory board).

What it means for your hair

The groups map roughly onto the measured 8-type scale Callender 2026, but no study has formally cross-validated 1A-4C against it.

Myths and facts

• "Your follicle opening is oval, so your hair is curly." Curl tracks the curved follicle, the bent bulb and asymmetric cell maturation Thibaut 2005. Oval cross-sections go with curl but may not cause it.
• "Curly hair has more paracortex." The cell types are arranged on opposite sides in curly fibers Bryson 2009, but their overall fractions are similar Wortmann 2018.
• "Your ancestry tells you your curl type." In measured samples, "African" hair ranged over groups V-VIII and "Caucasian" hair over I-VI Mkentane 2017, De la Mettrie 2007.
• "You have one curl type." Hairs from one small patch fall into 2-3 classes, and the pattern often differs between nape, crown and front Mkentane 2017.
• "Straightening or brushing changes your real type." Heat and brushing change shape only temporarily. Chemical relaxers permanently change only the treated lengths; the follicle keeps its program.
• "Curly hair breaks because it is chemically weaker." Composition is largely similar; breakage comes from bends, knots and grooming forces Camacho-Bragado 2015.

Practical points

1. Judge curl on wash day: clean, conditioner rinsed out, no leave-in or gel, air-dried without touching. Product, wetness, brushing and weight change the visible pattern; see taking a good hair photo.
2. Check at least three areas (front hairline, crown, nape) and note the main pattern and the range, for example "mostly 3A, 2C at the crown".
3. Reassess after puberty, pregnancy, menopause, starting or stopping retinoids, chemotherapy or antiepileptics, or a big cut that removes damaged lengths.
4. A new frizzy, kinky, wiry patch at the temples or crown in a young man can be acquired progressive kinking, which preceded pattern balding in a small case series Mortimer 1985. See hair loss scales and talk to a dermatologist if shedding or recession is also present.
5. Let curl set your priorities. Straighter hair: oiliness, cleansing and shine. Curlier hair: detangling, friction reduction and length retention. See bond builders for repair claims.

Bottom line

Curl is programmed by follicle shape and many genes, varies across one head and shifts a little over a lifetime. It is the most useful single trait for choosing a routine, but ancestry and DNA tests predict it poorly. Judge it on clean, air-dried hair and record a range rather than one letter.