Tightly curled hair was the most effective type at reducing solar radiation reaching the head in a controlled mannequin experiment. The finding suggests that hair shape may be linked not only to appearance, but also to protection from heat exposure.
The 2023 experiment tested several human-hair wigs on thermal mannequins inside a climate-controlled room. Every wig type reduced heat absorbed by the mannequin head compared with the bald condition.
| Wig hair type | Effect on heat absorption | Key observation |
|---|---|---|
| Straight hair | Reduced absorbed heat | Helped cool the head |
| Loosely curled hair | Reduced absorbed heat | Helped cool the head |
| Tightly curled hair | Reduced absorbed heat | Most effective against overhead radiation |
The study, titled Human scalp hair as a thermoregulatory adaptation, does not settle every question about why humans evolved different hair forms. It does, however, provide evidence that scalp hair can contribute to managing heat from above.
The result does not mean one hair type is universally superior in every environment. It shows that, under the conditions tested, tightly curled hair offered the strongest barrier to incoming overhead radiation.
The Curve Is Set Before Hair Reaches the Surface
The basic shape of a hair strand is established beneath the skin, inside the hair follicles that produce it. A straighter and more symmetrical follicle tends to produce straighter hair, while a curved follicle can guide a new strand into a bend from the start.
A 2005 examination of human scalp follicles directly observed this pattern. In people with curly hair, bends in the strand were already visible while the hair remained inside the follicle.
This means curls are not simply created after exposure to humidity, combing, or hair-care products. Those factors may affect how hair behaves or looks, but the underlying shape is formed during growth beneath the scalp.
Follicles can be compared with highly complex moulds, although the biology involves far more than a fixed physical shape. They contain stem cells, blood vessels, and glands that work together as the fibre develops.
Uneven Fibre Formation Creates a Curl
More recent evidence indicates that cells on one side of a follicle can divide and mature differently from cells on the other side. This uneven process affects how hair keratin and other fibre-forming materials are distributed around the hair shaft.
An imbalanced structure produces an asymmetrical strand that is more likely to curve in one direction as it lengthens. Straighter hair, by contrast, generally develops through a more symmetrical follicle and fibre structure.
Once a strand emerges from the scalp, it no longer contains living cells that divide or mature. It becomes a dead keratin fibre, but it usually retains the foundational form established during its growth phase.
A 2019 literature review identified follicle shape as a major factor in whether hair grows straight, wavy, or curly. Follicle shape is also genetic, helping explain why relatives often share similar hair patterns.
CNN Indonesia, citing ScienceAlert, noted that hair type cannot be mapped simply by race. The 2019 review found that every population includes a broad spectrum of hair forms, including straight, wavy, curled, and tightly curled hair.