Curls Week – Why does curly hair curl?

Curls Week - Hair Romance

It’s Curls Week on Hair Romance and we’re celebrating all things curly.

Today the question is why does curly hair curl?

Hair Romance - Curls Week - why does curly hair curl

What makes curls happen and what is the difference between straight hair and curly hair?

The chemistry behind your curls

Hair Romance - why does curly hair curl

It all comes down to chemistry.

Hair is made up of proteins, mainly keratin. Your hair shaft grows from the follicle and that’s why a protein-rich diet is important for growing hair.

The protein molecules contain sulphur atoms. These sulphur atoms pair up and form a disulphide bond.

The distance between the sulphur atoms cause your hair to curl. The further apart the sulphur atoms, the more the protein molecules bend, and so the more your hair curls.

Disulphide bonds are permanent bonds, or covalent bonds. Unlike frizz caused by humidity which are hydrogen bonds. These bonds form and break more easily.

Styling your hair changes the hydrogen bonds, which is why your hair will go back to its natural state over time. Permanent chemical hair straightening treatments change the disulphide bonds, but as new hair grows the natural hair structure will come back.


Curly hair is genetic so you can thank your parents for your gorgeous curls.

The gene for curly hair is not completely dominant so if you have a mix of straight hair and curly hair genes you end up with mixed, wavy hair.

The shape of your hair follicle is determined by your genes and this is also a factor in how curly your hair is. Curly hair follicles have a hook shape which affects the shape of your hair shaft.

If you could see a cross section of a hair shaft, you would see that straight hair is round, wavy hair is oval and curly hair is flatter.

This flatter shape affects the distance between the protein molecules and the disulphide bonds, hence making your hair curly.

Does that make sense? Hope this helps to explain why your hair is curly.

What other curly hair questions would you like answered?

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  1. Caitlin says

    I had dead straight hair until I was about 18. Over the past 8 years it started to get wavy in the back section and today at 27 I have wavy hair around my face and full on curls at the back. Hairdressers find my hair mysterious!

    • Hair Romance says

      Hi Caitlin, my hair totally changed too! So weird. It’s usually hormones that create this change. We know hormones (especially androgen and thyroid hormones) change your hair but it’s not exactly clear how the shape of the hair changes. One theory is that these hormones also change your muscle tone. This includes your scalp and this change surrounding the hair follicles could cause them to change shape (become rounder or flatter) and hence make your hair curly or straight. It’s not proven but it’s the theory that makes the most sense from all that I’ve read. Hope this helps! Christina x

  2. Isa says

    I had the most beutiful and defined curls ever!! and 3 years ago my hair began to change… i am 25 now… i miss my curls… now i have wavy hair with some undefined curles here and there…. i would love to have my natural curls back :'(

  3. Annette says

    Hi Isa, exact same thing has happened to me. I’m a lot older than you (!) but basically after some perms as a teenager, my hair became naturally curly! and has gone through various degrees of curliness since my teens. My hair became super curly from about aged 28 onwards and stayed like that until approximately 3-and-a-half years ago (I’m 40 now). Like you, I have wavy hair with undefined curls now and I miss my telephone-wire curls so so much and sadly nothing I have tried has brought them back. I moved house about 3-and-a-half years ago and so I thought that the loss of curl had something to do with different water! but it doesn’t seem so as I still have access to the water supply where I used to live (at my mum’s) & have tried washing it there to see if it would help & it hasn’t :( I can only assume my hormones have changed or something!, or maybe it’s due to years of over-processing as I do colour my hair, though, during it’s curliest periods I was continuously colouring my hair too and so I don’t know why all of a sudden one day my curls just died! If you come up with any miracle curl revival, please let me know, as I will you! Thanks!

  4. alice says

    Hello, i have curly hair and a couple of months ago my hair was very nice and curly it used to just bounce up! But recently my hair has been going frizzy and the curls look undefined, i have bery thick hair and no layers so i wanderring if maybe my hair was weighing my curls down as they are lacking in bounce and volume. Should i get layers??* Please help me as i would love to regain my beautiful curls. Also is there a good way to dry curly hair to prevent frizz?! :)

    • Hair Romance says

      Hi Alice, layers may help but look at your hair routine and the weather, has anything else changed? Maybe you just need to add a hydrating treatment once a week or change your shampoo/conditioner. As the change has only been for a couple of months, I doubt your haircut is the reason things have changed. Curls are very sensitive to humidity and weather, so that could be a factor. Good luck and I’m sure your curls will settle back to their normal shape soon x

  5. SHERLY says

    hi, when I was little my hair was completely straight and then I went out of town with my families, and I had a haircut there, I don’t know with what they were washing my hair with. When I came back home, next thing I know my hair started to get curly up until now, and it is so hard to style them as it curls and was so puffy sort of, and I don’t really like it, so I started to have my hair straightened since I was in 4th grade and now I’m in the university almost graduating. In a year I could have rebonded my hair twice. It makes my hair dry a lot. I’m really stressed out about the conditions of my hair. My parents didn’t have curly hair and so does my sister, I’m the only one with the curls. I’m soo upset cause it makes it harder for me to style my hair and wanting to have that very sleek look. I don’t mind if my hair is wavy, but the curls is just too much for me.

    • Hair Romance says

      Hi Sherly, you just need more friends with curly hair! If you’re really after the sleek look, you can get a straightening treatment. Talk it through with your hairdresser and make sure you do weekly conditioning treatments to keep your hair feeling good x

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