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Razib Khan
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Sex difference

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Sex ratio effects on reproductive strategies in humans:

Characterizations of coy females and ardent males are rooted in models of sexual selection that are increasingly outdated. Evolutionary feedbacks can strongly influence the sex roles and subsequent patterns of sex differentiated investment in mating effort, with a key component being the adult sex ratio (ASR). Using data from eight Makushi communities of southern Guyana, characterized by varying ASRs contingent on migration, we show that even within a single ethnic group, male mating effort varies in predictable ways with the ASR. At male-biased sex ratios, men’s and women’s investment in mating effort are indistinguishable; only when men are in the minority are they more inclined towards short-term, low investment relationships than women. Our results support the behavioural ecological tenet that reproductive strategies are predictable and contingent on varying situational factors.

The topline result is simple, and takes off on basic evolutionary logic. When there is a surplus of men then they compete to “lock in” women into long term relationships. In contrast, when there is a surplus of women the men tend to be much less inclined to invest in one specific woman. In human ethology and evolution there has long been a debate about “cads” vs. “dads.” Are humans by nature polygynous, or, are they are monogamous? More specifically, is high male investment our “environment of evolutionary adaptedness”? The evidence from cross-cultural comparisons seems to suggest that human social organization and individual strategy is facultative. Or, as an economist would say, humans respond to incentives.

This particular research occurred within one ethnic group in Guyana, but there are many other situations where one could test this thesis. For example, in the military (male surplus) or at historically women’s colleges where men are now admitted (e.g., Vassar). Personal experience suggests that when men are at a deficit caddish behavior is the norm. I have less knowledge of the converse, perhaps because there’s less communication about this between men (i.e., men are less likely to “brag” about locking down one particular woman in a long term committed relationship than they are to advertise the notches on their belt).

• Category: Science • Tags: Sex difference 
Razib Khan
About Razib Khan

"I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. If you want to know more, see the links at"