This is the most common cause for male hair loss and is mainly related to genetic factors and the abundance of DHT.
If family members on either or both the paternal and maternal side have experienced hair loss, a man will run higher chances of having male pattern baldness. Yet, the extent of hair loss and the age that one starts experiencing hair falling may vary. Thus, not having hair loss until a certain age is never an indication that baldness will not develop later in life.
Moreover, too much 5α-reductase accumulated in the scalp will generate DHT, which in turn lead to miniaturization of follicles and sparse hair growth. If the concentration of DHT remains high, hair loss will become more severe until hair ceases to grow. The severity of the problem can be measured by the Norwood-Hamilton Scale (see the figure below).