The Paradox of Monogamy

Monogamy seems maldaptive for a male from an evolutionary point of view: a male who simultaneously rears several children with multiple wives will have more descendants than one who must wait 1-2 years in between conceptions of his progeny. So why did monogamy evolve?

Polygyny (multiple wives at the same time) is allowed in 84% of human societies. Only 10% of men practice it, but this is due to the mathematical fact that the number of women is approximately equal to the number of men, and that 90% of men marry by age 50 in most cultures. Romantic love typically lasts for a relatively short time, and attachment lasts through the period of child-rearing. Worldwide divorce rates peak 4 years after marriage, approximately the inter-birth period, suggesting divorces exhibit the remains of an ancestral strategy for couples to remain together for the rearing of one child.

Clearly, females, who can only give birth once every nine months or more, have an incentive for monogamy to maximize parental involvement in the rearing of her children. A female whose male is philandering and thus whose male she has to share, with the consequent reduction of parental child-rearing attention, may be better off interrupting a pregnancy and getting pregnant by a male whose full attention she believes she can procure.

Given a female's preference for (seemingly) monogamous males, a male has two incentives to be monogamous:

1. to keep his mate with him and keep her from mating with others, to maximize probability of their common progeny's survival
2. if he cannot get multiple females to cohabitate, he has an incentive to maximize his time next to his children and his parental involvement.

Adultery is common in every society in which it has been studied. A high-status or high-attractiveness male who can mate with multiple females during any child-rearing period has an incentive to commit adultery: namely to be perceived monogamous by his wife while fathering extra-marital children with other females who are not already pregnant with children from other males.

A low-status and low-attractiveness male who cannot get a second female pregnant during the period of rearing that child has an even greater incentive to remain next to the mother of his children to maximize his children's probability of success through his involvement and minimize the probability of his mate straying.

Thus, males who cannot get multiple females to cohabitate have an incentive for monogamy and adultery. Clearly, a sex ratio of 1 dictates that most males cannot get multiple wives. Thus, monogamy follows as the behavior of the mode of the population.

Love evolved to facilitate mate selection, mating and monogamy. If love does not precede sex, it can follow it. Sex is prone to make people, and particularly women, fall in love: seminal fluid contains dopamine and tyrosine, a precursor of dopamine. Sex can also stimulate attachment via orgasm, which releases oxytocin and vasopressin.

It is interesting to note that the increasing complexity of society, which lengthens education and may lengthen parental influence, may tend to extend the period of monogamy.