Ideological affinity and educational level matter more than sharing psychological or physical traits when it comes to living with a partner, according to research from the University of Colorado at Boulder (USA) that has analyzed data from more than three million relations.

Research has studied the level of similarity between the two people in a couple taking into account more than 130 variables. According to the results presented in Nature Human Behavior , the greatest affinity has been registered for political values ​​and for religiosity. A high correlation has also been observed between members of couples for educational level and IQ.

For physical traits such as height or body mass index, the correlation is lower, although it is still positive. Therefore, the physical variables are related to the formation and/or maintenance of many couples, but less than the cultural variables.

Personality variables have even less influence. Neither a person’s agreeableness nor extraversion nor emotional stability are a good indicator that their partner will share these traits.

“People have all kinds of theories about whether extraverts like introverts, or whether they like other extroverts, but the reality is that it’s like flipping a coin. They are just as likely to mate with one as with another,” says psychologist Tanya Horwitz, first author of the research, in a statement from the University of Colorado.

For almost all the variables analyzed, a positive correlation has been observed between the members of the couple, although in some cases the correlation is low. “Our results show that people who are similar to each other are more likely to be together,” Horwitz says.

This resemblance has several possible causes, the researchers note. It may be a consequence of individual preferences when choosing a partner. Or the social stratification that leads to the formation of couples between people of the same origin. Or also from the very dynamics of life as a couple, which leads people to acquire more similar behaviors and opinions when they live together.

Thus, the high correlation observed for alcohol consumption may be partly due to the fact that people who drink are more likely to form partners with each other. And partly because people who are couples are more likely to drink together.

Of all the variables analyzed, there are only three in which couples tend to be different. The greatest discrepancy has been registered with sleep schedules. For reasons not clear from the research, people who go to bed and wake up late tend to be partners of people who go to bed and wake up early.

The other two variables in which opposites seem to attract each other are, also for unknown causes, the propensity to worry and hearing disorders.

The investigation has begun with a meta-analysis in which 199 studies have been reviewed with data from more than three million married, cohabiting or engaged couples. Subsequently, the team from the University of Colorado has carried out its own study of 79,074 couples using data from the UK Biobank project (or United Kingdom Biobank, in which half a million volunteers provide data on their lifestyle and health to conduct research).

The data from the UK Biobank have made it possible to analyze variables that had not been considered in previous studies. Thus, a high correlation has been identified in couples in terms of the age of first sexual intercourse or the number of sexual partners they have had throughout their lives: people who have had many partners tend to associate with people who they have also had many partners.

The 199 studies in the meta-analysis, carried out in different places and at different times, have in some cases reached different results for the same variables. Therefore, the researchers warn, the affinities identified in couples should not be interpreted as universal, but rather depend on each cultural context.

Only couples made up of a man and a woman have been studied, since they were the ones that had led the previous studies included in the meta-analysis. The University of Colorado team now has ongoing research on same-sex couples.