Why A Skewed Chinese Sex-Ratio Doesn’t Spell Disaster

Most of us are happy to accept that some people are more attractive than others. That is to say, that beauty is a real concept. Some people have it, and others don’t. But we’re just as willing to believe that we each have a ‘type’. That we are idiosyncratic in our preferences and find ourselves repeatedly drawn to those who possess particular traits: blonde hair, a good sense of humour, an ass that simply refuses to quit.

What I think we’re less comfortable with is the idea that what we want in a partner, our entire mating psychology, is malleable. That outside influences we’re not even aware of can change our priorities and make us seek relationships we would otherwise shun.

One aspect of our environment that might impact on our behaviour is the sex ratio of the population we live in, or the relative number of men and women we encounter on a daily basis. Left to its own devices, the sex ratio should be about equal. 50/50 men and women. But humans rarely leave anything to its own devices. The Second World War disproportionately depleted males, and China’s current one child policy has resulted in fewer female births than we would otherwise expect. If you were a man knocking about late 1940s Europe, or early 21st century China, how would the relative abundance or scarcity of potential female partners influence your behaviour?

Ryan Schacht of the University of Utah, and Monique Borgerhoff Mulder of the University of California at Davis decided to find out. But, perhaps because they couldn’t get their hands on either a DeLorean or two plane tickets to Beijing, they decided to head for Guyana. This small country on the Caribbean coast of South America is home to the Makushi, an indigenous ethnic group. The adult sex ratio (the proportion of males and females who reach adulthood) varies quite a lot across the homeland of the Makushi. Men are drawn away from their home villages to look for work farming cattle, mining, or logging. Women are attracted to urban areas in search of shop and domestic work. Schacht and Borgerhoff Mulder visited eight Makushi communities, whose adult sex ratios varied from 0.9 (slightly fewer males than females) to 1.4 (somewhat more males than females).

After counting the number of men and women in each community, the scientists had their research volunteers complete the Sociosexual Orientation Inventory, or SOI. This questionnaire tests whether a person is more interested in long-term or short-term relationships. A high score means an unrestricted sociosexuality, or that you’re more promiscuous. A low score means a restricted sociosexuality, or that you’re more marriage-orientated.

Schacht and Borgerhoff Mulder found that, in female-biased communities (places where men are outnumbered by women) men are more promiscuous than they are in male-biased communities. If men have lots of women to choose between, they are more interested in short-term casual sex. If women are more scarce, men are keener to settle down with one woman.

We might expect a sex difference in sociosexuality, with men being more drawn to short-term sex than women. And indeed, this is what the researchers found when they looked at communities where women outnumbered men. But the sex difference vanished when they looked at male-biased communities. The communities in which women were scarce were home to men and women whose sociosexuality scores were close to identical.

These results are interesting because they fly in the face of previous theories. Some evolutionary theorists have argued that when men are more numerous than women, they become more competitive, invest less in their relationships and children, and go sex mad. This kind of thinking has led to dire predictions for the future of countries like China, where the sex ratio is male biased and becoming more male biased every day. What will happen when the army of males that are the result of China’s one child policy reach adulthood and start looking for female partners? Many have predicted mayhem, with male competition over scarce females leading to rises in violence and sexual assault.

But Schacht and Borgerhoff Mulder’s results suggest the opposite could happen. As men’s choices are restricted, their sociosexuality scores may drop. In other words, they may attempt to make themselves more attractive to the dwindling pool of women by advertising their willingness to commit to a long-term relationship, and to invest in their offspring.

Schacht, R., & Borgerhoff Mulder, M. (2015). Sex ratio effects on reproductive strategies in humans. Royal Society Open Science, 2, 140402. Read summary

The content of this post first appeared in the 05 May 2015 episode of The Psychology of Attractiveness Podcast.