What makes curly hair curly? (with photos)

Man with curly hair.

Various theories have been put forward to explain why some people have curly hair and others have straight hair. For many years it was believed that this was determined by the shape of the hair shafts: those with a round cross section were straight, those with an oval shape were wavy, and those with a flat profile were curly. Another theory was that in curly hair, one side grows faster than the other, causing it to curl. Scientists believe that the type is largely determined by the shape of the hair follicle, which appears to be controlled by a gene.

The Structure and Chemistry of Hair

Woman with curly hair.

It is well known that curly hair differs from straight hair in several ways besides its shape. It is extremely rare to find someone with frizzy, oily hair. Biologically, this makes sense, because the oils secreted by the sebaceous glands in the follicle can travel more easily down a straight shaft. Anyone with this type of hair knows that it can be dry, unmanageable, and often frizzy. The good news is that split ends are less common.

Curly hair can be difficult to style on humid days.

Another difference, on a chemical level, is in the structure of the proteins that make up the hair. It consists mainly of a protein called keratin – the same material that makes up skin and nails – and the molecules of this compound bond together to form fibers. The protein molecule contains sulfur atoms that can sometimes bond together, forming what is called a disulfide bond. When these atoms form between atoms that are far apart, it causes curvature. The more disulfide bonds there are, the curlier the hair.

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The influence of follicle shape

Wrapping your hair around the rollers when it’s wet can make it curly.

Hair grows from tiny tube-shaped cavities in the skin known as follicles. People with curly hair appear to have a hook-shaped follicle shape rather than a straight one. This shape is thought to force the parts of the keratin molecules that contain sulfur atoms closer together, making them more likely to form disulfide bonds.

The origins of different types

Naturally curly hair is genetically determined. Some genes are considered dominant over others; this means that when an individual inherits two different genes for the same trait, one is more likely to be expressed than the other. For example, the gene for long eyelashes is dominant over the gene for short eyelashes; therefore, if someone inherits one from each of her parents, she will have long eyelashes. The non-dominant gene is considered recessive and will only be seen if it is inherited from both parents. The gene for curly hair is said to have incomplete dominance over that for straight hair, so an individual who inherits one gene for straight hair and one for curly hair may have intermediate wavy hair.

It is unclear why these different types evolved. It has been theorized that as early humans spread from Africa to cooler, cloudier regions, straight hair evolved to give greater protection from the cold, at the expense of greater exposure to ultraviolet light. Some researchers claim that curly hair offers better protection from ultraviolet rays, while straight hair is better at trapping air close to the scalp, providing an insulating layer for a part of the body that would otherwise be very prone to hair loss. heat. However, there does not seem to be a consensus on this.

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Changing hair type

Not all curly hair is naturally like this. Many people choose to change their type by applying various treatments. One method is to wrap your hair in rollers while it is wet. This causes hydrogen bonds to form between the keratin molecules, giving a curved shape dependent on the size of the rollers. The hydrogen bonds, however, are much weaker than the permanent disulfide bonds that cause natural curls, and the effect is lost when the hair gets wet again.

Chemical methods can be used to create disulfide bonds in straight hair, giving a longer lasting effect, but the new growth will be straight, so again, the effect is eventually lost. Scientists are looking for ways to change the shape of the follicle for a permanent effect. This involves studying in detail how the curly hair gene works, which is still an active area of ​​research.


The biggest challenges for most people with this type of hair are dryness and lack of control. As many use chemical and thermal products, dryness tends to get worse. Curly hair requires different care than straight hair to maintain its natural health. It requires no washing every day and will benefit from an occasional break from chemicals and heat. Each time it is washed, it should be deeply conditioned and, if it is dry, styling products that contain alcohol should be avoided. Proper attention to hair care will improve your overall health and manageability.

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