Why do so many people need their teeth straightened?  Why do some need to have permanent teeth extracted in addition to their wisdom teeth because their arches can not house a natural amount of teeth?  Why do teeth become crooked again after having them straightened?  If these occurrences were an exception to the rule, not asking the questions would be understandable.  Unfortunately, this is not an exception.  

Studies of preindustrial skulls have shown us that historical societies have virtually no crowded teeth.  They comfortably house all 32 adult teeth within them (that includes the wisdom teeth).  This is no longer the norm today.  Studies show that the change from naturally large, straight arches to smaller crowded ones can occur within as little as a generation.  This data suggests that jaw development and crowding is highly driven by environment and to a lesser degree by genetics.