Partners who become romantically involved soon after meeting tend to be more similar in physical attractiveness than partners who get together after knowing each other for a while, according to new findings published in Psychological Science, a journal of the Association for Psychological Science.

"This study shows that we make different sorts of decisions about whom to marry depending upon whether we knew the person before we started dating," said Eli Finkel, co-author of the study and professor of psychology at Northwestern University's Weinberg College of Arts and Sciences and professor of management and organizations at the Kellogg School of Management. "If we start dating soon after we meet, physical attractiveness appears to be a major factor in determining such decisions, and we end up with somebody who's about as attractive as we are.

"If, in contrast, we know the person for a while before we start dating -- or if we're friends first -- physical attractiveness appears to be much less important, and we are less likely to be similar to our spouse on the dimension of looks," Finkel said.

Finkel, along with Northwestern alumni Lucy Hunt, lead researcher of the study and now at the University of Texas at Austin, and Paul Eastwick, assistant professor in the department of human development and family sciences, also at UT-Austin, were interested in understanding why individuals tend to be paired with mates who have similar physical, behavioral and psychological characteristics -- a well-documented phenomenon psychological scientists refer to as "assortative mating."

One explanation for this pattern in pairing comes from a competition-based perspective. An individual's success in the mating "market" is limited by his or her own desirability. People who are physically attractive tend to be seen as very desirable and are, therefore, better able to win over highly desirable partners themselves.

The researchers hypothesized that the length of acquaintance between partners may shift the dynamics of this sexual competition. Their prior research showed that as people get to know each other more intimately and across various contexts, their opinions about the other person's desirability change, making objective physical attractiveness less relevant in determining whether the two individuals become a couple.

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"Having the time to interact with others in diverse settings affords more opportunities to form unique impressions that go beyond one's initial snap judgments," Hunt said. "Given that people initiate romantic relationships both with strangers and acquaintances in real life, we were interested in how time might affect how similarly attractive couple members are to one another."

The researchers hypothesized that partners who had known each other a short time before dating were likely to be similarly attractive, while partners who were well-acquainted before their romantic involvement might show a greater mismatch in physical attractiveness.

The researchers looked at data collected from 167 couples (from Evanston, Illinois, and the surrounding community) -- 67 dating and 100 married -- who were participating in a longitudinal study of romantic relationships. The couples had been together for as few as three months and as long as 53 years, with an average relationship length of eight years and eight months.

As part of the study, which was conducted in Finkel's lab at Northwestern, the couples were videotaped talking about how they had changed over the course of the relationship. Using these videos, independent, trained coders used rating scales to indicate the physical attractiveness of each partner; the ratings were strongly correlated among the coders, suggesting a high level of agreement on the physical desirability of each partner.

The results revealed that the longer the romantic partners had known each other before dating, the less likely they were to be matched on attractiveness, just as the researchers hypothesized. For example, the pairing of an unattractive woman with an attractive man was more likely to emerge if the partners had known one another for many months prior to dating.

Partners who began dating within a month of first meeting each other showed a strong correlation for physical attractiveness. But the correlation was much lower for partners who had known each other for a long time before dating. A similar pattern emerged when the researchers looked at whether pairs were friends before they started dating; friends-first couples were less likely to be matched on attractiveness than couples who were strangers before dating.

Interestingly, the level of match on attractiveness was not associated with relationship satisfaction for either men or women in the study. That is, both friends-first and stranger-first relationships seem approximately equally happy years later.

The researchers note that the research will need to be replicated across more diverse samples and contexts, but these findings suggest that length of acquaintance can influence whether we perceive someone as being a desirable partner.

"There may be more to the old saying than was previously thought," Hunt said. "Maybe it's the case that beauty is partially in the eye of the beholder, especially as time passes."