Asked by: Rebecca Walker
There are three contributing factors, all governed by your genetics. Firstly, the sac from which the hair grows (the ‘follicle’) is oval in shape, compared to straight hair’s more circular follicle.
Next, curly hair exits the skin’s surface at more of an angle than straight hair, causing it to curve as it grows.
Lastly, the shape of curly hair tends to result in more chemical bonds forming between protein molecules in the hair strand, which adds to the curliness.
Subscribe to BBC Science Focus Magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun facts.