Why Curly Hair Starts With Hair Follicles

Fed up with pin-straight hair or impossible-to-manage curls? The reason is not primarily your washing and drying habits, nor the heat or humidity around you.

It starts beneath the surface of the skin, in the hundreds of thousands of minute follicles spread across your scalp.

In 2005, researchers dissected follicles from the human scalp and discovered that, for people with curly hair, the curl begins to take shape before each strand exits the skull and falls over the head.

They concluded that the follicle bulb itself determines whether hair grows curly or straight.

Since then, further research into hair follicles has brought added detail to our understanding of curly hair.

Here is how the process is currently believed to happen.

How hair follicles shape curly hair

Every hair on the scalp develops inside a follicle below the skin before slowly growing out through a pore.

When a follicle sits vertically, the hair emerging from it will be straight too.

However, when the follicle curves like a golf club, the newly produced section of hair bends as it develops. It works rather like a mould, although the process is more complicated than that.

Recent findings indicate that cells on one side of this 'mold' divide and mature differently from those on the other side.

As a result, keratin and other substances are not distributed evenly around the entire hair shaft.

This produces an asymmetrical strand, making it more prone to bending and curling in a particular direction.

Straight hair follicles, by comparison, are more symmetrical in form.

Although much about curly hair remains unknown, a 2019 review of the research concluded that, while the composition of an individual hair can influence its eventual thickness and texture, the follicle's shape is what decides whether it emerges straight, kinked or coiled.

Genetics, hair type and ancestry

After a strand leaves the scalp and hangs away from the head, it retains the form created at its point of origin.

Because hair follicle structure is inherited, people often have a hair type similar to that of their relatives.

Hair follicles are remarkably intricate structures containing stem cells, blood vessels and glands, all of which contribute to producing the fibrous strands that make up our hair.

Once hair has grown out of the skin, it is essentially a biological rope. Its cells no longer divide or mature, and little remains beyond a dead keratin fibre. Yet its development inside the follicle permanently determines what comes next.

Hair types are frequently classified by race, but scientific evidence does not support that approach.

Instead, the 2019 review found that all populations, whatever their ancestry, display a range of hair shapes, although certain curl patterns occur more often in some genetic lineages than in others.

Could scalp hair have evolved to keep us cool?

Why different hair follicles evolved in the first place is still not properly understood. However, emerging evidence supports the idea that scalp hair developed partly to help us remain cool.

In experiments published in 2023, researchers placed wigs made from human hair on a thermal manikin inside a climate-controlled chamber. The manikin absorbed less heat than it did when bald.

The team tested several wig types, including straight hair, loose curls and tight curls.

Every wig helped cool the head, but curly hair was most effective at shielding the manikin from the 'solar' radiation overhead.

It is a compelling possibility, yet identifying the evolutionary benefits of features such as hair can be exceptionally difficult.

Scientists are still working to untangle those roots and sift through the evidence.

This article was fact-checked by Michael Irving and edited by Rebecca Dyer. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.