Spouses don't become more alike in their personalities as their marriage progresses, contrary to perceptions.

Rather, spouses often seem similar in character because they start out that way, a new study suggests. People tend to pick their partners based on shared personality traits, researchers say.

The findings are published in the November issue of the journal Personality and Individual Differences.

Surveying spouses

Researchers know spouses are more similar to each other than to random people, but there's debate about how this happens.

"This could reflect spouses' influence on each other over time, or this could be what attracted them to each other in the first place." said study author Mikhila Humbad, a doctoral candidate in clinical psychology at Michigan State University.

The current work analyzed the data of 1,296 married couples, one of the largest such studies to date, Humbad said. Participants had been married from two years to 19 years.

The participants answered 198 questions to assess their personality and behavior, including whether they were ambitious, sociable, easily upset, physically violent or someone who likes to plan ahead.

Opposites don't attract

There was no association between the length of the relationship and similarity of personalities within couples.

The conclusion: Spousal similarity is better explained by partner selection than by gradual convergence of personality.

The one exception to this pattern was aggression. "It makes sense if you think about it," Humbad said. "If one person is violent, the other person may respond in a similar fashion and thus become more aggressive over time."

The research could have implications for future spouses as well as their offspring. "Marrying someone who's similar to you may increase the likelihood that you'll pass those traits on to your children," Humbad said.