How Aware Are We of the Competition for Mates?

We know from research that people do automatically assess the balance of men and women in a group, and it influences the decisions that they make. This is something that’s intuitive to the nightlife scene. Many places have “Ladies’ Night” promotions. These places know that if they can attract a lot of women, then the men will follow. We see this pattern in other species. The females follow the food and other resources, and the males follow the females. So these clubs are intuitively using something we know from across biological research: that females are viewed as a resource by males, and males will seek them out accordingly.

A lot of clubs also will restrict entrance by single men or groups of men. If you’re a couple, you’ll get in the door. And if you’re a single woman or a group of women, no problem. But the clubs will bar entrance to single guys or groups of guys unless they have enough women inside. And that’s something that they intentionally do to avoid a male-biased sex ratio because of the social consequences: They know by doing this they can have a happening scene.

This is a case in which people assess the sex ratio within a very short duration of time, and the behaviors they enact when interacting with each other might be influenced. Especially if there’s only one woman for every three guys, you might see a lot more competitiveness, and the women know they can be a lot more selective. But if there’s a shortage of guys around, these guys might be able to play the field a bit more.

Assessing the sex ratio doesn’t just happen in the very short term in these situational ways. It also happens in the long term because we can see it across populations. If you look at a metropolitan statistical area—which is the definition of a population that the U.S. Census uses—we can see that marital patterns and other phenomena are affected by the balance of the sexes in the population.

When there are more women than men, we actually see more female mating effort. We see things like raised skirt length—women’s fashions become much racier and sexier, and there’s more sexual promiscuity, too. Women have less selective power, and have to do more in order to entice or to keep partners. So the interval between the time a couple meets and the time they have sex is potentially shorter. They’re also more likely to have children out of wedlock, and we see higher divorce rates in female-biased populations because the guys know that they can play around to a greater extent.

On the other hand, when you have a male-biased population—when there’s more men than women—these guys will try to snap up the women before some other guy gets them. Women get married younger, as well as many of the men. Men are more willing and ready to commit so that they can secure that relationship and basically get the woman before somebody else does. So in cities that have more men than women, women marry several years earlier on average.

But, at the same time, women’s standards for these guys will go up because females know they have greater selective power and they can be choosier. So there are a lot of guys who will have to build up their social status and resources in order to be a viable candidate. Women are very interested in resource investment, and in male-biased environments, their standards for resource investment will go up. And some guys will not have what it takes. If you live in your parents’ basement and are riding around in a 1982 Ford Escort, that’s just not going to be attractive when the other guys are riding around in their BMWs and their Audis. Thus, we see greater variance in male marital age when there’s a surplus of males. The guys who are more able to get married are doing that younger, but some guys have to take longer to build up the resources to become marriageable.

And when there are more men around and the men have to put in more mating effort, that can lead to higher levels of risky behavior, and that leads to higher levels of accidents and violence. This is what I call “excess male mortality.” Excess male mortality is related to male mating competition because when guys have to compete more for women, they have riskier strategies. If you look internationally and across populations, you see that more mating effort and more mating competition among males lead to excess male mortality. Within the United States, we see that populations that are male-biased show greater excess male mortality.

So, on the one hand, you have promiscuity and relationships that aren’t that stable in female-biased populations. On the other hand, in a male-biased population, you’re going to see a lot more male violence and risky behavior. It’s a tricky call to say which one would be better. It’s a matter of perspective.

There are many who think that social norms for monogamy—a virtual balance of men and women in monogamous relationships—is something that has been encouraged by societies because of the social stability that it provides. When men have the ability to have more than one wife, then you’re going to have some guys who get a lot of women and some guys who don’t get any, and they’re going to fight each other to be in a position to get many partners. So monogamy might help facilitate social stability.

Dan Kruger is a research assistant professor at the Prevention Research Center of Michigan.