In places where young women outnumber young men, research shows the hemlines rise but the marriage rates don't because the young men feel less pressure to settle down as more women compete for their affections.

But when those men reach their 30s, the reverse is true and proportionately more older men are married in areas where women outnumber men.

Daniel Kruger, a University of Michigan researcher who studies evolution and how it relates to contemporary behavior, looked at the 50 largest metropolitan areas in the United States to test his hypothesis on how the balance between women and men affects marital patterns. Results showed that men aged 20-24 are more likely to cruise than to commit if they live in an area with more women than men.

One would think that rationally, fewer young men than women would naturally lead to proportionately more young men getting married, but that's not the case.

"Marriage patterns aren't rational because men and women have somewhat different reproductive strategies," Kruger said. "Men have a greater reproductive benefit than women from having a greater quantity of relationships. If they can leverage their scarcity into attracting multiple short-term partners, they will not have as much of an incentive to settle down."

There are about nine unmarried men for every 10 unmarried women in Birmingham, Memphis, New Orleans, and Richmond-Petersburg, Virginia, Kruger says. Philadelphia, Washington, DC, Baltimore, and New York metropolitan areas are tied for the next region where women are relatively most plentiful. In these areas, about 84 percent of the men aged 20-24 are unmarried. In Las Vegas, San Diego, Salt Lake City, Austin, and Phoenix, there are about nine unmarried women for every 10 unmarried men. In these areas, about 77 percent of the men aged 20-24 are unmarried.

Once those young men hit their 30s, they tend to shift from seeking short-term relationships to entering into committed relationships. That's because when women evaluate partners for short-term relationships they value physical features signaling the kind of genes that would be passed on to potential offspring, which may be the only legacy of men who don't stick around for child rearing. These physical features decline as men age, making it more difficult to lure women into uncommitted relationships.

"You see a complete reversal in the pattern," Kruger said, and thus, proportionately more older men are married when women outnumber men.

So, does this mean that middle aged women in these cities get a break? Not really, Kruger says. The higher marital rates for older men likely benefit women who are substantially younger than their husbands, because older men still prefer partners with higher reproductive potential.

The ratio of men to women has other aspects, as well. For instance, studies have shown that when women outnumber men, hemlines actually rise, overall, as women to do more to physically attract men. Also, the rates of divorce and out-of-wedlock births are higher, and interests in women's rights increases. Surpluses of men tend to be associated with more conservative social norms and restricted roles for women.