Why Hair Curls
The full spectrum of human hair — from pin-straight to tightly coiled — is determined by a combination of biological factors operating at multiple scales, from molecular chemistry to the geometry of the hair follicle itself. Understanding these mechanisms is not merely academic curiosity. It is foundational to CROWN’s diagnostic technology, which must classify curl patterns objectively across all ethnicities, and to our research programme, which correlates physical hair characteristics with discrimination experiences.
The Follicle: Architecture of Curl
The primary determinant of curl pattern is the shape and orientation of the hair follicle — the structure within the skin from which each hair strand grows.
Follicle shape. Straight hair grows from symmetrical, round follicles that are oriented perpendicular to the scalp surface. As follicle shape becomes more oval or flattened, the hair it produces becomes progressively curlier. Tightly coiled hair grows from highly asymmetric follicles with pronounced elliptical cross-sections.
Follicle curvature. The follicle itself is not always straight within the skin. Research has shown that curved follicles — where the follicle bends beneath the skin surface — produce hair with more curl. The degree of follicle curvature correlates with the tightness of the resulting curl pattern.
Follicle anchoring angle. The angle at which the follicle is anchored in the dermis affects how the hair emerges from the scalp. Follicles angled more acutely produce hair that lies flatter against the scalp initially, while follicles that emerge more vertically can create lift and volume.
These follicular characteristics are genetically determined, which is why curl pattern is heritable and closely associated with ethnic background. The genetics of hair texture are complex but increasingly well understood.
The Chemistry of Curl
Within the hair shaft, curl is maintained by chemical bonds between protein chains:
Disulfide bonds. The strongest bonds maintaining curl structure are disulfide (S-S) bonds between cysteine amino acids in adjacent keratin protein chains. These covalent bonds are permanent and determine the baseline curl pattern. Chemical straightening treatments (relaxers, perms) work by breaking and reforming disulfide bonds — permanently altering curl structure but also risking hair and health damage.
Hydrogen bonds. Weaker than disulfide bonds, hydrogen bonds form between water molecules and protein chains within the hair. These bonds are temporary — they break when hair is wet and reform when hair dries. This is why wet hair can be reshaped (straightened with a blow dryer, set in curls) but returns to its natural pattern after washing. Humidity disrupts hydrogen bonds, which is why curly hair may frizz in humid conditions.
Salt bonds (ionic bonds). Formed between positively and negatively charged amino acid side chains, salt bonds contribute to hair’s overall structure and are affected by pH. Hair care products with extreme pH can disrupt salt bonds.
The interplay of these bond types determines how hair behaves in different conditions — why it curls tighter in humidity, why heat styling is temporary, and why chemical treatments produce permanent changes.
The Physics of Fibre Geometry
Curl pattern is also a function of the physical geometry of the hair fibre:
Cross-section ellipticity. Straight hair fibres have nearly circular cross-sections, while curly and coily hair fibres have increasingly elliptical cross-sections. The degree of ellipticity (the ratio of the major to minor axis of the cross-section) correlates strongly with curl tightness. The CROWN Diagnostic measures cross-section ellipticity index as one of its core analytical dimensions.
Fibre torsion. Along its length, a curly hair fibre rotates — a property called torsion. The degree and regularity of this torsion contributes to whether hair forms smooth spirals, irregular waves, or tight zigzag patterns.
Mechanical asymmetry. In elliptical fibres, the thicker side of the fibre is structurally stronger than the thinner side. This asymmetry creates internal stresses that cause the fibre to curl — the structural basis of the curl itself. The more pronounced the ellipticity, the greater the internal stress, and the tighter the resulting curl.
Variation Across Populations
Research, including Washington State University’s Deep Hair Phenomics study (2024), has documented systematic variation in curl-related structural properties across populations:
European-type hair typically features round to slightly oval cross-sections, moderate disulfide bond density, and follicles that are generally straight within the skin.
Asian-type hair typically features round, large-diameter cross-sections with high disulfide bond density, producing the characteristically straight, strong, and glossy hair common in East Asian populations.
African-type hair typically features highly elliptical cross-sections, pronounced follicle curvature, and irregular torsion patterns along the fibre length. These structural properties produce the tight coils and zigzag patterns classified as Type 4 on the Walker scale.
These are population-level descriptions with significant individual variation. Mixed-heritage individuals may exhibit combinations of these structural properties, producing the wide range of curl patterns seen in diverse populations.
Why Objective Measurement Matters
The subjective assessment of curl pattern — looking at hair and assigning a type — captures only the final visual result of multiple underlying structural properties. Two individuals with similar-looking curls may have very different fibre geometries, bond chemistries, and follicle structures. Conversely, individuals with different-looking curls may share structural properties that affect their care needs.
The CROWN Hair DNA classification system captures curl pattern as one of 12+ measured dimensions — alongside fibre diameter, cross-section ellipticity, porosity, hydration, protein integrity, and chemical treatment history. This multi-dimensional approach provides the precision needed for research, diagnostics, and equitable hair assessment.
Understanding the science of curl patterns — the biology, chemistry, and physics that produce the full spectrum of human hair — grounds CROWN’s work in the rigour that our research mission demands. Hair is not merely an aesthetic characteristic. It is a complex biological structure whose diversity reflects the genetic heritage of our species. That diversity deserves to be understood, measured, and valued — not ranked in a hierarchy of desirability.