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Yes, the game is changing. Dating is on the ropes. But more, the whole mating repertoire is shifting. Coyness is, well, not quite a (thank you, Andrew Marvell), but not a fashionable feint either, and certainly not an inherent attribute of females. And no, this has nothing to do with the Internet. It has to do with numbers. It turns out that they have changed a whole lot more than you might suspect, and probably without your awareness.

The way people match up varies according to how many partners they sense are available—no surprise there. And the number of men, and especially quality men, is declining relative to the number of women, especially for those of college age, when people are primed for pairing off, in the moment and as they march through post-college life. You’ve seen the headlines: Men are falling behind women by almost every measure there is—school completion rates, number of graduate degrees, labor force participation. It especially matters at college, where men and women are housed together in a more or less closed community. For every 60 women now at college, there are 40 men—that's 50 percent more women than men.

The ratio tends to be closer to 50-50 at highly selective schools—with an excess of applicants they can draw deeper into the pool to balance their classes. The gender mismatch at such schools is not one of numbers but, at some point, of quality. Complaints of college women that men “act like they’re in high school” or “don’t have it together” are not sour grapes or losers’ laments; they reflect a true and growing reality.

But what isn’t at all obvious is that the very desire to pair off—and especially on what terms—changes radically as one's number of possible mates changes, and in ways that are extremely counterintuitive. The surprise is that men are more apt to be single when men are rare than when men are abundant. They’re surrounded by potential partners but have little interest in committing to or marrying them.

When males are abundant (China, take note), on the other hand, they are especially eager to put considerable effort into finding a mate and settling down; rates go up and crime rates go down. Conventional would have us believe all that on the loose would lead to mayhem, including lots of promiscuity and maybe rampant violence—you know, rape, pillage, and plunder.

Not so.

It’s when males are in short supply that men turn promiscuous and spawn babies out of wedlock, and male violence rages: Male-male homicide rates go up; so do rates. Men muster little energy for finding a mate and, preferring casual , happily engage in multiple relationships, University of Utah anthropologist Ryan Schacht reports in a recent issue of Royal Society Open Science.

Schacht studied a homogenous population in southwestern Guyana whose members, in search of work, moved to several geographically dispersed communities that consequently varied naturally in gender composition. There, he observed, the sex roles and mating efforts of men and women were not tethered to biology but instead varied significantly depending on numbers of the opposite sex. Mating dynamics are mating dynamics, and the same dynamics, Schacht says, likely apply on college campuses—and in other populations where males are in short supply, such as older adult communities.

The findings, Schacht reports, “reject simplistic labeling of reproductive roles by gender based on inherent sex differences.” In fact, there’s evidence from many studies that testosterone levels—which prime males for mating , or something—vary tremendously based on situational factors. They dip significantly, for example, among new fathers.

Women are not destined to be coy or passive in the mating game. When they are the more numerous sex, they engage in sometimes fierce competition among themselves for the few available mates. Exhibit A might be Schacht’s home base, Utah, dominated by the Mormon Church. Such is the nature of Mormonism that men are apt to leave it while women are more likely to convert to it, and stay Mormon. That gives the community a strongly female-heavy sex ratio. It also gives Utah a wildly disproportionate rate of plastic surgery procedures—with higher rates of breast implants, nose jobs, all kinds of facial surgery, and Botox injections, even among college students.

It’s not that Utahans are vain, as Forbes magazine has claimed: They’re . And they want a romantic partner. So do women outside Utah—say, in colleges across America—and to have one, they may feel compelled to behave in ways they are not entirely comfortable with, such as hooking up.

This is what I found when I interviewed young women for a recent post, Crisis U, on the declining mental health of college students. Relationships were the leading cause of distress, and women were especially distressed over breakups, typically due to “commitment issues” among men. Further, some of what is being reported as rape may stem from regret over drink-fueled encounters devoid of emotional connection.

The distress also makes its presence felt in my email inbox. As Psychology Today’s advice columnist, I increasingly hear from young women who are dissatisfied with relationships that don’t even qualify as relationships. They meet a guy, he calls every couple of months, they spend a night or two together each time, and then they’re miserable because they’re emotionally attached to him and want more from him—for which he might even call them "needy" or "greedy," should they summon the courage to ask.

It was in trying to understand this growing phenomenon that I turned to the literature for insight and discovered Schacht’s work. One of the novelties of his approach is to factor into research on mating what economists have known all along—that mating functions like a market in some ways, in response to the supply and demand of partners. Once he did, he found that those variations also change psychology.

“Relationship preferences and sexual behavior of individuals are responsive to context,” Schacht says. “Men want fundamentally different things from relationships when males are rare than when they are abundant. The rare-male male is the stereotypical fling-seeking cad we expect him to be. However, the abundant-male male is the committed, devoted male from the age of Camelot.”

For the generation now coming of age—and for successive ones as long as males continue to fall behind—the dearth of good men at an important life juncture has repercussions for mental health. To name just one, success at dating and mating affects .

With our egocentric , we think that everything in our life happens because of us individually, due to one characteristic or another, and, when we don’t get what we want, a characteristic that is in some way flawed. We probably seriously underestimate how much factors having nothing to do with us, but everything to do with our environment, including demographics, affect the shape of our lives.