When men outnumber women, females marry younger and the age gap between spouses grows, a University of Michigan study shows.
"Women don't stay on the market long because men are more motivated to commit," said Daniel Kruger, research assistant professor in the U-M School of Public Health. "They want to secure the relationship before some other guy gets her."
The study looked at the ratio of men to women in the 50 largest metropolitan statistical areas in the U.S with 2000 U.S Census data to determine how it impacted marital ages.
Male and female behavior differ because each gender has a somewhat different reproductive agenda, and the ratio of men to women impacts that behavior. Women want commitment from men, Kruger says, because having a reliable, stable provider greatly benefits their children. This means men may have to build up their social status and resources to be considered marriageable.
However, men can have more offspring with multiple sexual partners and thus are not as eager to settle down, said Kruger, who studies human behavior and motivation from an evolutionary perspective.
But the economics of supply and demand adds a twist, Kruger said.
"When women are scarce they have more bargaining power and can ask for more," Kruger said. "So the guys who are qualified will snap up the women and get hitched, whereas others need more time to build up their resources and social status."
As a result, the marrying age for men varies more widely when women are scarce, because younger men often have to wait until they're older to gather the resources to successfully compete for a mate, Kruger said.
"All things being equal, the guy with the better resources is going to win out," Kruger said.
The age gap between spouses also widens. The norm is for men to marry women three to four years younger but when women are scarce women can be five or six years younger on average, Kruger said.
For example in Las Vegas, where there are six men for every five women, the median marrying age for men and women is 28.3 and 24.5, respectively. Contrast that with Birmingham, Ala., where there are 11 women for every 10 men, and the median marrying age for men and women is 26.9 and 26.7, respectively.
When the ratio of men to women is greatly out of balance, it creates social tension and encourages markets for services like sex trafficking and prostitution, Kruger said. When there is a surplus of men, there are likely higher rates of accidents and violence as men engage in riskier behavior to compete for women and resources. When there is a surplus of women, sexually transmitted disease can increase as women become more promiscuous.
According to one study, by the year 2020 there will be 24 million more men than women in China, and if these men do not get married by age 40 they probably won't. This means they'll likely stay reliant on social security in old age.
Conversely, when women outnumber men, there is less incentive for competition among men for committed relationships and paternal investment, because male scarcity enhances their short- term mating success, Kruger said.
In short, men tend to play the field longer because there isn't the intense pressure to commit to one woman, he said. The paper, "Female scarcity reduces women's marital ages and increases variance in men's marital ages," appears in the journal Evolutionary Psychology.