The answer is it’s all down to genes and when you’re developing as an embryo your body develops as a series of segments. Written into those segments is a genetic pattern that tells that bit of the body where it is in the body and anything that develops on that segment inherits that genetic pattern which dictates to it how it should grow and develop.
If you look at how hairs work - hairs have three phases to their life cycle. They have what’s called an anagen phase and this is where they grow. The hair follicle has a number of stem cells that are very, very active and they pump out keratin which is the hair chemical. Keratin forms a big polymer which is a filament for hair which you see.
After the anagen phase, which can last anything from days – in the case of an eyelash that’s about 2-3 weeks, to a head hair which can be three or four years. That determines how long the hair grows for its ultimate length. Then the hair goes into what’s called a catagen phase. That’s where the follicle switches off and the hair falls out.
Then there’s a third phase which is called a telogen phase when the follicle rests. It then resets the system and the whole thing starts again.
So the hair length is down to how long the hair grows for, the anagen phase, and that is determined by your genes. Basically the genes that are programmed into the bit of the body that’s got the hair in it.
http://www.thenakedscientists.com/HTML/content/questions/question/2035/
