Format: Long-form editorial | Topic: Hair biology and curl science
Curly, coily, and kinky hair types have specific biological and structural characteristics that explain their behavior — the shrinkage, the dryness, the tendency to tangle, the way they respond differently to products than straight hair does. Understanding this science is not merely academic; it provides the foundation for making genuinely informed decisions about products, techniques, and routines. Most natural hair care advice makes much more sense when you understand the biology behind the recommendations.
The Shape of the Follicle
The curliness of hair is determined primarily by the shape of the hair follicle from which it grows. Straight hair grows from round, symmetrical follicles. Wavy hair grows from slightly elliptical follicles. Curly and coily hair grows from highly asymmetric, oval, or flattened follicles. The asymmetric follicle shape causes the hair shaft to grow in an asymmetric manner — proteins on one side of the strand grow at a different rate and in a different configuration than those on the other side, causing the strand to naturally curve as it grows. The more pronounced the asymmetry of the follicle, the tighter the curl the resulting strand produces.
The Internal Architecture of the Hair Shaft
The hair shaft consists of three layers: the medulla at the core, the cortex surrounding it, and the cuticle on the outside. The cortex makes up approximately eighty percent of the hair shaft’s mass and is where the hair’s structural proteins — primarily alpha-keratin — are organized. In curly and coily hair, the distribution of the two main types of cortical cells — paracortex and orthocortex — is asymmetric within the shaft, with more paracortex on the inside of the curl and more orthocortex on the outside. This asymmetric distribution is directly responsible for the curl — the paracortex is less crystalline and more hydrophilic than the orthocortex, and this difference in water absorption creates a differential in the fiber that causes the strand to curve.
Why Coily Hair Is Inherently Drier
The cuticle of the hair shaft is made up of flat, overlapping scales arranged like roof tiles along the length of the strand. In coily hair, the tightly curved shape of the strand means that these cuticle scales are more raised on the outer edge of each curve than on the inner edge, creating a more open, less smooth cuticle surface overall. This structural characteristic has two important consequences: the raised cuticle allows moisture to escape from the hair shaft more rapidly, making coily hair inherently more prone to dryness; and the uneven cuticle surface creates more friction between adjacent strands, increasing tangling. Additionally, the sebum produced by the scalp can only travel down the hair shaft by following its surface. On a tightly coiled strand with multiple bends per inch, this journey is significantly more interrupted than on a straight strand, meaning coily hair receives far less natural conditioning from scalp oils than straight hair does.
The Role of Disulfide Bonds
The curl pattern of hair is maintained structurally through disulfide bonds — strong chemical bonds between sulfur atoms in adjacent protein chains within the hair shaft. These bonds are responsible for the hair’s natural form and are what relaxers and perms alter to change the curl pattern permanently. Disulfide bonds are stable and resistant to breaking under normal conditions — water, humidity, and mechanical manipulation do not break them. What changes the appearance of curly hair temporarily — stretching it, straightening it with heat — does so by affecting hydrogen bonds, which are much weaker and reform when the hair is re-wetted or cools down. This is why natural hair reverts when wet or when heat is removed — the hydrogen bonds that were temporarily reconfigured return to their natural state while the disulfide bonds that define the curl pattern have been undisturbed throughout.
The Cuticle, Porosity, and Product Performance
The raised cuticle characteristic of coily hair is directly related to the hair porosity concept that is so central to natural hair care. High porosity hair — with its open, raised, or damaged cuticles — allows products and moisture to enter the shaft quickly but also allows them to escape equally quickly. Low porosity hair — with tight, flat cuticles — resists product penetration but, once moisturized, retains that moisture well. The structural biology of coily hair predisposes it toward higher porosity, which is why coily hair types are often described as struggling with moisture retention. Understanding that this is a structural property rather than a personal failure of one’s hair care routine is both clarifying and genuinely empowering.
Implications for Practice
This science has direct practical implications. The inherent dryness of coily hair is not a sign of poor hair care — it is a structural feature that demands a specifically moisture-focused approach. The tangling tendency of coily hair is a predictable consequence of its cuticle structure and is best addressed with adequate slip from conditioner rather than by forcing a comb through dry strands. The way coily hair responds to water — swelling dramatically as the cortex absorbs moisture unevenly between the inner and outer curves — explains the shrinkage phenomenon. And the way protein treatments work — temporarily filling cuticle gaps and reinforcing the protein architecture of the shaft — makes perfect sense in the context of the shaft’s structural role. Science and practical hair care are not separate domains. They are, when understood together, a remarkably coherent and actionable body of knowledge.