ABSTRACT
In all known human societies, extant or historical, men and women have entered into formal reproductive alliances between individuals of opposite sex. In other words, they have married. Marriage is a special kind of relationship. By engaging in sexual reproduction and by the cooperative rearing of offspring, couples forge a powerful commonality of interest at the fundamental level of fitness. This shared interest is analogous to that existing between blood relatives, but it is more easily shaken or betrayed.
