Men tend to behave better when they're married - both because marriage likely helps improve their behavior, and nicer men are more likely to be married in the first place, a new study reports.

The researchers found that men with fewer nasty qualities were more likely to eventually end up married. But among men who did marry, some showed signs indicating that their bad behavior decreased after the union.

These findings address a long-standing debate among researchers, concerning why married men display fewer qualities associated with antisocial personality disorder, such as criminal behavior, lying, aggression, and lack of remorse. Is it because marriage reforms them, or because men with more of these nasty traits are less likely to get married in the first place?

The answer: A little bit of both, study author Dr. S. Alexandra Burt at Michigan State University told Reuters Health. Married men "are just not as antisocial to begin with," she said. "And when they get married, they get even less antisocial. So both things are going on."

Burt and her colleagues adopted a "novel approach" to investigate the link between marriage and antisocial personality disorder, said Dr. Ryan King at the University at Albany, SUNY, who was not involved in the study.

Specifically, they followed 289 pairs of male twins for 12 years, between the ages of 17 and 29. More than half of the twins were identical, meaning they shared all of their genes - and, largely, their childhood environment, as well, since both were raised in the same household.

The authors found that men who eventually married during the study period - about 60% of them -- showed less antisocial behavior at ages 17 and 20, suggesting that men with more of these traits are less likely to get married in the first place. Specifically, they found that by the age of 29, unmarried men had an average of 1.3 antisocial behaviors, compared with 0.8 among married men.

However, among identical twins in which one was married and one wasn't, the married twin had fewer antisocial behaviors after the union than the unmarried twin. Given that identical twins, with their similar genetics and childhood environments, are likely to have the same antisocial tendencies, these findings indicate that marriage helped weed out those bad behaviors.

"Not everyone is equally likely to enter the institution of marriage," King said. "But those that do enter into it get some benefit from it."

It's not clear why men's behaviors might improve after marriage, he noted. Married men may spend more time with their spouses than their friends, King said, and bad behaviors such as delinquency and binge drinking tend to be group activities. In addition, married men "have more to lose" if they're caught doing illegal activities, and may care what their spouses think.

It's also not clear why men with more antisocial behaviors may not marry in the first place, Burt said. They are probably not the most eligible bachelors, she noted. "You may not be looking to settle down with someone who's prone to aggression, theft, and other things." And for men with these tendencies, marriage may not be so appealing, she added.

Whether the same trend is true in women is also not clear, Burt noted, since women are less likely to have antisocial behaviors in the first place.

The results, presented in the Archives of General Psychiatry, help explain the consistent findings from other studies that men who are married commit fewer crimes. One recent study, for example, showed marriage was associated with a 35% reduction in crime.

Historically, studies have also found that married people as a group tend to be healthier than singles -- though recent research suggests the health advantage of marriage may be fading. Still, people with spouses tend to live longer, be less depressed, and suffer less from heart disease and stroke.