Sept. 1, 2010 — -- If you live with your mate long enough, eventually you will look like each other, think like each other, and become so similar your friends will have trouble telling you apart, right?
Not really, according to new research that challenges the long-held belief that many years of cohabitation causes spouses to grow more alike as the years roll by.
The research, by psychologists at Michigan State University and the University of Minnesota, is based on a relatively huge data base of 1,296 couples who have been married for an average of 19.8 years. It was published in the current issue of the journal Personality and Individual Differences.
What they found is that couples who had been married for a long time -- up to 39 years -- were no more alike in fundamental personality traits than newlyweds, leading the researchers to conclude that personalities do not grow more similar as the years pass. More likely, the couples were looking for specific traits during the courtship and they ended up with someone who was very much like themselves.
"They may not have been conscious of that at the time," Mikhila Humbad, a doctoral candidate in clinical psychology at Michigan State and lead author of the study, said in a telephone interview.
So does that forever put to rest the thought that opposites attract?
Not necessarily, because the study focused on personality traits, like extroversion vs. introversion, a happy disposition vs. a sourpuss, a social butterfly vs. a hermit. Those traits are likely to remain constant throughout an adult's lifetime, Humbad said, unlike hobbies, interests and life-changing events that can also define us.
So this is a fairly narrow window, but it flies in the face of conventional wisdom. After all, we all know someone who even ended up looking like the family dog, so surely our personalities converge as we blend our lives together. Maybe not.
When Humbad and her colleagues began studying the large data base collected by the Minnesota Center for Twin and Family Research they were struck by the fact that married couples seemed so similar to each other, at least in terms of definitive personality traits.
Psychologists Debate: Why Are Married Couples Similar?
"We wondered why that similarity was there," she said.
Research by others has led to a mild debate among psychologists over whether similarities are a product of convergence (spouses grow more similar over time) or selection (similarity is what the spouses were looking for in the beginning.)
"That could be what attracted them to each other in the first place," she added, a suggestion that could be very helpful to the companies that now try to link potential mates together based on similarities.
The data base used in her research does not include multiple interviews and personality profiles over the lifespan of the marriages. Instead, the conclusions are based on a single interview, but the marriages differed in length, so the researchers could compare newer and older marriages.
They found that nearly all the spouses had many personality traits that they shared with their mate, regardless of how long they had been together. And the similarities did not grow or diminish when the older marriages were compared to the newlyweds -- with one notable exception, aggression.
The data shows that if one spouse is physically aggressive, the other is likely to react in a similar manner.
"It is possible that individuals might reinforce each other's aggressive tendencies due to hostile interpersonal exchanges, thereby promoting greater convergence over time," the study concludes.
"It makes sense if you think about it," Humbad added. "If one person is violent, the other person may respond in a similar fashion and thus become more aggressive over time."
This study is particularly significant in that it is based on a very large sample, 1,296 couples who answered a personality questionnaire encompassing 198 items. It is limited by the fact that the conclusions were based on one interview, instead of multiple interviews over the course of each marriage.
And these were relatively successful marriages, based on the fact that they had survived an average of nearly two decades at the time the questionnaires were completed. It would be interesting to know if a survey of failed marriages would show more dissimilarities than similarities.