Men and women in relationships need not be similar in personality in order to have a successful long-lasting marriage, a new study suggests.

The study, which included couples who had been married for at least 40 years, found that neither personality similarities nor differences appeared to affect how happy the couples were.

The findings suggest the personality matching carried out by dating websites may make little difference in a relationship's ultimate success, the researchers say.

The results also run counter to what we might intuitively expect, said study researcher Frederick Coolidge, a psychologist at the University of Colorado at Colorado Springs. But Coolidge suspects the evolutionary need to bond with someone may trump incongruencies in personality.

"One of my very speculative suspicions is that this need for a relationship is so strong that it overcomes differences," Coolidge told LiveScience.

The study sample was quite small, and more research is needed to find out exactly what makes a relationship last, the researchers said.

The study was presented Aug. 4 at the American Psychological Association's annual meeting in Washington, D.C. The work has not been published in a peer-reviewed journal.

Happy marriages

Much of the research on relationship stratification has focused on younger and middle-age couples rather than older adults.

Coolidge and colleagues had 32 couples take surveys to assess their personality and how satisfied they were in their marriages. Men and women were asked to evaluate their own personality as well as their perceptions of their partners' personalities.

The couples completed the surveys in separate rooms and were not allowed to see their partners' responses.

"We didn’t want to create any divorces," Coolidge said.

Both men and women were pretty happy in their marriages, but on average, the women reported being slightly happier than the men. Neither the length of the marriage nor personality traits (self-identified and those perceived by the spouse) were associated with the couples' level of marital satisfaction.

Coolidge said his results agree with those of previous studies on marital satisfaction in younger people.

What makes a marriage last?

So far, the key to a lasting marriage remains elusive, Coolidge said.

"It looks like, what leads to marital satisfaction, it's almost as if it escapes detection, at least by standard psychological tests and personality measures," Coolidge said.

Other research suggests differences in personality may be more beneficial than similarities in terms of maintaining a relationship over the long term. In a 2007 study of middle-age and older couples (some of whom had been together 55 years), Robert Levenson, of the University of California, Berkeley, found personality similarities were associated with decreased marital satisfaction over a 12-year period. In some areas, personality differences were linked with greater marital success, Levenson said.

This may be because, over the long haul, "different personalities may provide couples with complementary resources for dealing with life's challenges," Levenson told LiveScience.

For instance, take a couple in which one partner is more social and the other more detail-oriented. "On a given evening, if someone needs to pay the bills and balance the checkbook, and someone needs to call other parents to arrange a carpool, the 'complementary' couple will presumably argue less about who does what than the 'similar' couple," Levenson said. And late in life, when couples spend more time together, "too-similar spouses may find themselves becoming bored with each other," he said.

This article was reprinted with permission from LiveScience.

Related on LiveScience: