Why some people don't choose equally good looking mates?

Partners who become romantically involved soon after meeting tend to be more similar in physical attractiveness than friends-first couples or partners who get together after knowing each other for a while, says a study.

For example, the pairing of an unattractive woman with an attractive man is 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, said the study published in the journal 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 co-author of the study Eli Finkel, professor of psychology at Northwestern University.

"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," Finkel said.

"If, in contrast, we know the person for a while before we start dating -- or if we are 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 added.

The researchers looked at data collected from 167 couples -- 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.

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.

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.