Why do some men thin their hair in the back and sides of their heads?

Diffuse thinning impacts the hair around the side and back of the head in males in a condition of diffuse thinning we call DUPA (Diffuse Unpatterned Alopecia) which impacts all of the hair on the head even the area normally spared by the genetic balding process.  Women have this condition most commonly, but it is not common in men. About 50% of men respond to the drug finasteride to reverse this condition.  I almost always examine the back hair on men in my office with a hand microscope looking for this diagnosis because men who have this condition should never consider a hair transplant because the transplanted hairs will thin just like they do in the donor area.