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What Makes Curly Hair Curly?

By J. Beam
Updated May 23, 2024
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A number of theories have been put forward to explain why some people have curly hair and others straight. 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 more quickly than the other, causing it to curve. Scientists currently think that the type is largely determined by the shape of the hair follicle, which seems to be controlled by a gene.

The Structure and Chemistry of Hair

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

Another difference, at the chemical level, is in the structure of the proteins of which hair is composed. It consists mainly of a protein called keratin — the same material that makes up the skin and nails — and molecules of this compound bundle together to form fibers. The protein molecule contains sulfur atoms that can sometimes bond with one another, forming what is called a disulfide bond. When these form between atoms that are quite far apart, it causes curving. The more disulfide bonds there are, the curlier the hair will be.

The Influence of Follicle Shape

Hairs grow out of tiny, tube-like cavities in the skin known as follicles. People with curly hair appear to have a hooked, rather than a straight, follicle shape. It is thought that this shape forces the parts of the keratin molecules containing sulfur atoms closer together, making it more likely that they will form disulfide bonds.

The Origins of Differing Types

Naturally curly hair is determined genetically. Some genes are said to be dominant over others; this means that, when an individual inherits two different genes for the same trait, one is more likely to express itself rather than the other. For example, the gene for long eyelashes is dominant over that for short eyelashes, so if someone inherits one of each from her parents, she will have long eyelashes. The non-dominant gene is said to be 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, so an individual inheriting one straight and one curly gene may have intermediate, wavy hair.

It is not clear why these different types evolved. It has been theorized that, as early humans spread out of Africa into cooler, cloudier regions, straight hair evolved to give greater protection from cold, at the expense of increased exposure to ultraviolet light. Some experimenters have claimed that curly hair gives better protection against ultraviolet, while straight 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 heat loss. There does not seem to be any consensus about this, however.

Altering Hair Type

Not all curly hair is naturally so. Many people choose to change their type by applying various treatments. One method is to wind the hair around rollers while it is wet. This causes hydrogen bonds to form between keratin molecules, giving a curved shape dependent of the size of the rollers. 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 more lasting effect, but new growth will be straight, so, again, the effect is eventually lost. Scientists are looking into ways of changing the follicle shape to achieve a permanent effect. This involves studying the way the curly hair gene works in detail, which is still an area of active research.


The biggest challenges most people with this type of hair face are dryness and unmanageability. Since many use chemical and thermal products, the dryness tends to become worse. Curly hair requires different care from straight in order for it to maintain its natural health. It does not require washing every day and will benefit from an occasional break from chemicals and heat. Each time it is washed, it should be deep conditioned, and if dry, styling products containing alcohol should be avoided. Proper attention to hair care will improve its overall health and manageability.

InfoBloom is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Discussion Comments
By anon281533 — On Jul 24, 2012

I'm happy with my curly hair. A lot of people like it.

By anon263755 — On Apr 25, 2012

When I was a baby I had super curly hair then I got a short hair cut. How can I get my naturally curly hair back? Help!

By anon244668 — On Feb 02, 2012

Don't try change your hair! I had dead straight hair in school, but it started turning curly in university. I did everything I could to style the new, messy curls. But after much experience I learned that washing less and using no styling products is the best solution.

I wash my hair once a week and it is more beautiful than it has ever been before! So go natural and see how well it turns out.

By anon158087 — On Mar 05, 2011

whoever is the first to come out with this pill stands to be a multi billionaire.

By anon148989 — On Feb 03, 2011

My daughter had curly hair, but it turned straight after a short hair cut. Will the natural curls come back after a chemical perm?

By C3Yaoihunter — On Jan 13, 2011

What types of curly perms should be used for caucasian hair? I'm a bi-racial guy with caucasian hair, and I was wondering what brands of perms caucasians used when they want to perm their hair?

By anon141681 — On Jan 11, 2011

my five year daughter had curly hair. they turned kind of wavy after her short hair cut. What to do to get her curls back?

By mrbluenun — On Jan 05, 2011

I know that some drugs cause hair that was dead straight to turn curly. A friend had just a little bit of curl when he let his hair grow, and he started to take some prescribed drugs, I think they were for Epilepsy or pain, not sure but the drugs made his hair go crazy curly and as he has to take these all the time he now has to find a new way of doing his hair, because it is so curly.

These curls are not tight, but more like a woman gets when she uses rollers, I guess?

A drug to induce or decrease curly hair specifically, I do not think so, but as a side effect, it definitely happens! My friend has moved and not told me his contact details so I can't ring and ask what the drug was he uses.

By anon124157 — On Nov 04, 2010

So you have curly hair, what a horror. I wonder how people who are born blind, deaf or with missing limbs feel about the horrors of dealing with curly hair.

By anon109310 — On Sep 07, 2010

I'm a male and I need something to my hair curly or wavy, I have very straight, spiky hair, and nothing seems to work, not even strong gel. So if there is anything that can make my hair curly or even slightly manageable.

By anon107194 — On Aug 29, 2010

This pill needs to come out already. What's the hold up Loreal? I'm a 29 year old white male with very curly hair that totally sucks. I want my straight hair back from when I was a little kid.

By anon99794 — On Jul 27, 2010

I lost my hair due to Systemic Lupus Erythematosus (about 95 percent of my hair fell out). When it grew back after the effluvium (resting period) about three weeks into my drug therapy, it started to grow back curly. It has been straight my entire life.

I'm rather enjoying the curls. The grass is always greener, I suppose. I now have to learn how to care for curly hair, but I have hair to care for.

Folks, if you have never involuntarily lost your hair you do not understand the impact it makes on a woman's life. I can straighten curly hair or curl straight hair, but I cannot grow hair at will. I'll take what I can get.

By anon96318 — On Jul 15, 2010

If your hair is determined by your genetic makeup, why would you take a pill to change that? Be smart and use a flat iron or embrace your characteristics that makes you different from the world

By anon89831 — On Jun 12, 2010

I recently shaved my hair off and now that it's growing back in it has changed to lots of waves. has anyone had this happen to them or know why?

By anon88869 — On Jun 07, 2010

To whoever thinks the drug to change your hair growth is just stupid, an go buy a straightening iron. That is easier said than done.

I have extremely curly hair; it takes me an hour and a half to blow dry and straight iron my hair. God forbid it rains. And to do that every day, yeah that's no easy task.

I am allergic to every hair care product there is and 99 percent of shampoos -- forget conditioner! You try having curly hair that looks like an afro and not being able to use products and the alternative is spending an hour and half (at best) every day fixing your hair.

If they had a pill that would give me straight hair I would take it in a heart beat!

By anon87243 — On May 29, 2010

I don't think drugs should be used for such a vain purpose. They could have serious side effects later in life. Drugs should be used for medical purposes, using them to change the shape of your hair is unnecessary. Just buy a straightener or curling iron.

By anon77642 — On Apr 15, 2010

It is true that it is a side effect of some medication - one of the drugs for epilepsy is particularly potent at making your hair curl.

I have no idea about a drug being made just to change the shape of your hair follicles though!

For me it was hormones that made my hair go from poker straight to having a serious curl in it - it was really straight and tended to not grow past my shoulders pre pregnancy, but after three term pregnancies I now have waist length hair, which just seems to get more curly as time passes.

By anon73618 — On Mar 28, 2010

I think this drug is a joke. 2005 was the last I have heard about this pill that changes your hair. It's a fraud.

By anon66164 — On Feb 18, 2010

When will these drugs be out? I really need to buy some. Will it be permanent?

By anon64471 — On Feb 07, 2010

I think hair changing drugs should be researched further.

By anon10807 — On Apr 03, 2008

how far along are hair changing drugs? and when is it likely that they will be available?

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