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The Paradox of Monogamy

Page history last edited by Alex Backer, Ph.D. 15 years, 3 months ago

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 (Helen Fisher in Sternberg & Weis, p. 104). 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, for reasons that are explained in another essay, and that 90% of men marry by age 50 in most cultures (Sternberg & Weis, p. 104). Romantic love typically lasts for a relatively short time (Sternberg & Weis), and attachment lasts through the period of child-rearing (Sternberg & Weis, p. 104). 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 (Fisher, 1992, cited in Sternberg & Weis, p. 104). Data from a 2004 survey by the US census office show that the fraction of children living with both of their parents together declines from 73% for children less than 5 years old to 60% for children 5 to 14, to 52% for children 15 to 17. In other words, by the time children in the US reach age 15, half the couples that produced them have dissolved.


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, and 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 (Sternberg & Weis, p. 104). 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 (multiple wives) have an incentive for monogamy and adultery. Clearly, a sex ratio of 1 (equal number of males and females in a species) 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. 


Falling is love is stimulated by novelty and danger. Given that love is the outcome of a mate selection process, perhaps danger signals that the time to be picky is over and you'd better get her pregnant before that dagger cuts through your heart. As for novelty, that may be driven by the same selective drive as the women's attraction toward partners with an immune system different from the woman's (Wedekind et al., 1995, cited in Sternberg & Weis, p. 102).


If love does not precede sex, it can follow it. The Nepalese say "Naso pasyo, maya basyo": The penis entered and love arrived. 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 (Sternberg & Weis, p. 103).


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.


It is interesting to ponder why monogamy has been encouraged by those who promulgate laws, who, as those in power, are precisely those who can often afford multiple mates. In that regard, it is interesting to recall that wedding in the western world was for the longest time, and continues to be in many places, regulated by the clergy, that the Catholic clergy is not allowed to marry, and is thus unaffected by monogamy laws. 


So (serial) monogamy follows from the sex ratio: given the mathematics that allow the average man to be with only one woman at a time, monogamy ensures that the man next to any given woman is the one most likely to have fathered her children, which maximizes the incentive for paternal participation in child-rearing.


P.S. This is an article on the evolutionary biology of monogamy, not a self-help article on how to preserve your marriage.


Read more science articles.




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