Naturally curly hair is often viewed as a singular trait, but it exists along a wide spectrum of textures, ranging from gentle waves to tight coils. Understanding the prevalence of this trait requires looking beyond a simple straight-or-curly dichotomy, as hair texture is a complex characteristic determined by physical, biological, and genetic factors. The degree of curl can vary significantly even within a single individual, requiring a nuanced approach to determine how common this hair type truly is across the global population.
Biological Basis of Curl
The fundamental factor determining hair curl is the shape of the hair follicle, the structure beneath the skin that produces the hair shaft. Straight hair grows from a nearly round follicle, causing the hair fiber to emerge symmetrically. Conversely, curly hair is generated by a curved and asymmetrical follicle, often described as elliptical or “S-shaped” beneath the scalp.
This curved follicle dictates the shape of the growing hair shaft itself, which takes on an oval or flattened cross-section. The curl is further established by an uneven distribution of structural proteins within the hair shaft, a process known as cellular or keratin asymmetry. Specifically, the cells on the inner curve of the hair shaft differentiate differently from those on the outer curve, causing the fiber to twist as it hardens, forming the characteristic spiral or coil.
The Genetics of Hair Texture
The instruction set for creating curved follicles and asymmetrical hair shafts is encoded in DNA, making hair texture a complex, polygenic trait. This means that multiple genes, rather than a single gene, contribute to the final curl pattern, explaining the wide variety of textures observed. The inheritance pattern for curl is not a simple dominant or recessive model but often involves incomplete dominance or additive effects, where gene variants determine the degree of wave or curl.
Researchers have identified several genes that play a role in hair texture across different populations. For example, a variant in the TCHH (Trichohyalin) gene is associated with hair curliness in people of European ancestry, accounting for approximately 6% of the variation in hair shape. Another gene, EDAR (Ectodysplasin A Receptor), is linked to the thicker, straighter hair common in East Asian populations. The interplay of these and other genes determines whether a person will have straight, wavy, or tightly coiled hair.
Global Prevalence and Distribution
Determining how rare naturally curly hair is depends heavily on how “curly” is defined and which population is being studied, making precise global figures difficult to obtain. Some studies suggest that less than 20% of the world’s population has distinctly curly or coily hair. However, if loose waves are included, the percentage of individuals with textured hair is much higher, potentially reaching over 65%. The most significant variations are seen geographically, reflecting ancient genetic adaptations.
The highest prevalence of tightly coiled and coily hair textures is found in populations indigenous to Sub-Saharan Africa, where studies indicate a high percentage of individuals possess these patterns. In contrast, straight hair is the most common texture in East Asian populations, with curly hair being relatively rare. Wavy and looser curl patterns are more common in people of European and South Asian descent. This distribution highlights that fully coiled hair (often classified as Type 4) may be less common globally than straight hair.
Categorizing Curl Patterns
To better understand the spectrum of textured hair, various classification systems have been developed. The Andre Walker Hair Typing System is one of the most widely adopted, dividing hair into four main types: Type 1 (Straight), Type 2 (Wavy), Type 3 (Curly), and Type 4 (Coily/Kinky). These types have further subdivisions (A, B, and C) based on the tightness of the curl or coil, allowing for a precise description of hair texture.
Type 2 hair is characterized by an “S” shape and ranges from loose waves (2A) to more defined waves (2C). The term “curly” is typically reserved for Type 3, which features distinct spirals that look like corkscrews and range from loose (3A) to tight and springy (3C). The tightest textures fall into Type 4, which includes tightly coiled hair with an “S” or “Z” pattern (4A/4B) and the most compact, undefined coils (4C).