Twins present many biological complexities and myths, but few people know the realities. A wide range of topics is covered in this chapter. They include the third type of twin (polar body twinning), twins with different fathers (superfecundation), twins conceived weeks apart (superfetation), and the surprising question of whether male–female twins can be identical. Birth order is another aspect of twinship that warrants attention. Birth order is meaningful when it comes to twins, but not in the way that most people think it is. It is also known that identical twins are excellent partners for organ donation, but how ideal are they really? And can one twin really substitute for another when it comes to taking responsibility for a crime he or she did not do? A final topic concerns the applicability or generalizability of twin research findings to the general public.
