Two common adages when it comes to human mating are "Birds of a Feather Flock Together" and "Opposites Attract." Which is operative and which one yields a higher level of marital satisfaction? Anecdotally, one can find endless examples of successful couples that belong to each of the two camps. However, the accumulated scientific evidence seems to support the similarity hypothesis, at least when it comes to relationship satisfaction. In general, couples who share similar beliefs, personality types, and values, and who otherwise possess similar mating values are much more likely to be successful in their romantic unions. Both Luo and Klohnen (2005) and Gaunt (2006) found that similarity along personality traits was a predictor of marital quality/satisfaction. There is one dimension along which couples choose others who are maximally dissimilar to them. Specifically, scientists have found that humans tend to prefer the smells of prospective suitors who are maximally dissimilar to them along the Major Histocompatibility Complex, a set of genes that marks an individual's immunogenetic profile (see the recent review by Havlicek and Robert, 2009). Hence, whereas on many traits we prefer to engage in positive assortative mating (i.e., choosing similar mates), there are some instances when negative assortative mating (i.e., choosing dissimilar mates) is operative.