New research says men change their spending habits when they outnumber women.

The cities of Macon and Columbus, in Georgia, have a lot in common. They're located within a hundred miles of one another and have similar histories and economies. So at a glance it's hard to explain why the people of Columbus have, on average, consumer debt that's roughly $3,500 more per capita than the people of Macon.

A group of psychologists thinks the disparities may have something to do with the fact that these two cities have widely diverging sex ratios. Macon has considerably fewer men than women (.78 to 1) and Columbus has considerably more (1.18 to 1). The research group, led by Vladas Griskevicius of the University of Minnesota, suggests that when men outnumber women in a population, they spend more and save less.

Previous studies have suggested that sex ratio may indeed have an impact on mating behavior. For instance, populations with an abundance of men have shown increased paternal investments — perhaps because women in these populations can be more selective with their spouse. In a paper published earlier this year in the Journal of Personality and Social Psychology, Griskevicius and company extended this evolutionary logic to financial decisions. They write:

Findings show that male-biased sex ratios (an abundance of men) lead men to discount the future and desire immediate rewards. Male-biased sex ratios decreased men's desire to save for the future and increased their willingness to incur debt for immediate expenditures. Sex ratio appears to influence behavior by increasing the intensity of same-sex competition for mates. Accordingly, a scarcity of women led people to expect men to spend more money during courtship, such as by paying more for engagement rings.

The researchers arrive at their conclusion through a series of tests that examine the behavioral effects of sex ratios. One was a simple statistical analysis of spending habits in 120 cities. Griskevicius and colleagues found a significant link between sex ratio and credit card debt. In places like Columbus, Georgia, where men outnumbered women, people had both more credit cards and higher debt.

The researchers then conducted several controlled experiments to see if that trend was truly caused by the sex ratio. In the first test, the suggestion of a male-heavy sex ratio led male participants to opt for more immediate rewards in a task designed to measure short-term and long-term payment preferences. A second experiment found that this same suggestion led men to spend 42 percent more and save 84 percent less than the suggestion of a female-heavy sex ratio.

In the final test, they gathered 147 participants (both men and women) and asked them to read a phony news piece about a local campus population. Some read an article suggesting the sex ratio was skewed toward women, and others read one suggesting the opposite (sample headline: "Fewer Women for Every Man for Today’s Student").

Griskevicius and company then asked the test participants to indicate how much they would spend on three romantic gestures: a Valentine's Day gift, a dinner date, and the purchase of an engagement ring. When test participants believed men outnumbered women in the population, they expected men to spend more money on the items. This was true of both male and female participants — suggesting that when men have more competition for mates, women become choosier and men attempt to out-spend any rivals.

The research has its limitations. For starters, financial and mating decisions are complicated processes whose outcomes are hard to reduce to any single factor. Beyond that, it seems reasonable to wonder whether most people even know the sex ratio of their cities — and you'd have to know that ratio to act on it. Timing also seems important here: sex ratios might cause men to spend more during courtship, but what about once they start a family? There's a lot more to consider.

Still the theory is incredibly intriguing. In the latest issue of Current Biology, British researchers Aron Szekely of Oxford and Tamas Szekely of the University of Bath call the study "ground-breaking." The two Szekelys suggest that behavioral and population researchers develop models to assess how demographic shifts like sex ratio might influence consumer behavior. The theory might have a particularly ripe testing ground in China, which will soon have a male surplus of 40 million, for an overall sex ratio of 1.2 men to every woman. Break out your yuan, gentlemen.

Top image: Nikola Bilic/

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