"The selection process has been powerful enough to produce one indisputable outcome: the family is a universal human institution. . . . In virtually every society into which historians or anthropologists have inquired, one finds people living together on the basis of kinship ties and having responsibility for raising children. . . . Even in societies where men and women have relatively unrestricted sexual access to one another beginning at an early age, marriage is still the basis for family formation. It is desired by the partners and expected by society."