WASHINGTON POST - May 3 - In general, couples tend to be more similar than two randomly selected people. Scientists call this "assortative mating". In a study published last year in the journal Psychological Science, researchers offered an explanation for when and where couples of mixed attractiveness are most likely to arise. The study found that heterosexual couples who were friends before they dated were more likely to be rated at different attractiveness levels. Those who met each other and started dating within a month had relatively similar levels of attractiveness.