Couples tend to be more similar in many respects — including their genetics, physical attractiveness and cultural characteristics such as religion, politics and socioeconomic status — than two randomly selected people would be. Scientists call this “assortative mating,” and it’s a principle that has been studied for over a century: As early as 1903, scientists reported similarities in the height and arm length of couples.
There are various theories for why people sort themselves out into similar pairs. One is competition: As individuals contend for the most desirable mates, they end up being constrained by their own characteristics. People just prefer or are more likely to meet others who are more similar to them.
Inital pysical attraction is what brings most couples together. However, when the people get to know each other first, perhaps by being friends of friends, or studying at the same school, they learn unique information about other characteristics that may influence someone’s desirability — and thus form an opinion of their partner that is different from the pack.
“In contexts that allow people to develop divergent perceptions about each other’s positive and negative idiosyncrasies, the traditional trapping of market forces falls away, permitting individuals to seek mates on a more level playing field,”