Breasts evolved through sexual selection , meaning they don ' t serve any functional value other than that our male ancestors found them attractive . They are not functional in the sense that large and small breasts don ' t have any differences in milk production during lactation . Different cultures and populations over the last 100 thousands of years had different preferences and the constant mixing of these genes causes the great variety of breast size and shape that we see today . So the simple answer is genetics , and the why is sexual selection . This goes both ways too . Flaccid penis size and facial hair were sexually selected for as well . These features were selected for by our female ancestors . The interplay of these various genetic factors creates the variety that we see today .