Do bald people not grow hair, or does their hair simply fall out immediately?
I'm no expert on the whole matter, but I know that at least when it comes to male pattern baldness, it is caused by a hormone called dihydrotestosterone. As those men with the right (or wrong genes, however you choose to look at it) age, this hormone causes the hairs to progressively lighten and thin up until the hair stops growing altogether. The hair follicle dies out basically.