The male pattern baldness (MPB) form of androgenetic alopecia (there is also
a female pattern baldness) accounts for more than 95% of hair loss in men.
By age 35, two-thirds of American men will have some degree of appreciable
hair loss and by age 50 approximately 85% of men have significantly thinning
hair. About 25% of men who suffer from male pattern baldness begin the
painful process before they reach 21. http://www.webmd.com/skin-problems-and-treatments/hair-loss/hai
大约四分之一的男人从21岁开始有秃头的趋势，35岁时大约三分之二的男人都有某种程
度的脱发，而到50岁时85%的男人的头发显著稀少。男人的脱发95%有明显特征，属于“
男性型脱发”：发际后退，头顶的头发掉得比周围的快。 http://blog.sina.com.cn/s/blog_474068790102dvet.html
