“Birds of a feather flock together” is something we frequently observe in romantic couples. Partners often resemble each other in many ways, including their personalities: if one is more outgoing, the other often is too; if one is curious about new ideas or likes to keep things orderly, the other often does as well. Research on the Big Five personality traits confirms that such resemblance in couples is more common than chance. 

But why is this the case? Do people already choose partners with similar personalities? Do partners’ personalities gradually become more alike as they live together? Or do people with dissimilar personalities more often split, so the couples we can still observe look more similar? 

A new study by DSS researcher Thomas Leopold tested these explanations by following 1,180 heterosexual couples in Germany from the first years of living together for up to 16 years, using data from the Socio-Economic Panel (SOEP). With repeated assessments of both partners’ personalities, the study examined whether partners were already similar at the start, whether similarity increased with time together, and whether being different raised the risk of separation.

The results clearly point to the first explanation: partner choice. Partners’ personalities were already more similar than chance when they moved in together – most strongly in openness to experience and conscientiousness, with smaller similarities in agreeableness and extraversion. Similarity did not grow over time, less similar couples – once living together – did not separate more often, and those who split did not grow more dissimilar in the years before. 

These results suggest that personality similarity in couples mainly reflects who people choose. There is one caveat to this conclusion: the data do not cover the early stages of dating, leaving the possibility that dissimilar couples sort themselves out before ever sharing a home. In short, much of the similarity we see is already there at the start, although very early breakup may still play a hidden role.