Male Pattern Baldness
As boys reach puberty, their levels of testosterone increase and this includes a testosterone derivative called dihydrotestosterone (DHT). Some men are more sensitive to DHT and this results in the growth phase (anagen) of the hair lifecycle to shorten and the resting phase (telogen) to become longer. The hair follicles shrink which causes the hair to become thinner and eventually unable to grow at all. This process is known as follicular miniaturisation.