Couples often share more common traits than we might think

Finding lasting love can be really difficult. We’ve all heard the annoying adages like “there’s plenty of fish in the sea,” not to mention the old “opposites attract” chestnut. However, many people tend to end up being quite similar to their partners, according to the results of a study published August 31 in the journal Nature Human Behaviour.

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The new research included numerous studies dating back more than a century. The team examined 130 traits from millions of couples, ranging from political leanings to age of first sexual intercourse to substance use habits. For between 82 and 89 percent of traits analyzed, partners were more likely than not to be similar. In only one part of the analysis, and for only three percent of studied traits, did individuals tend to be coupled with someone who is demonstrates an opposing trait.

In addition to shedding light on some of those unseen forces that may shape human relationships, this research could have some important implications for the field of genetic research.

“A lot of models in genetics assume that human mating is random. This study shows this assumption is probably wrong,” study co-author and University of Colorado at Boulder psychologist and neuroscientists Matt Keller, said in a statement. Keller noted that a tendency called assortative mating—when individuals with similar traits couple up—can actually skew findings of genetic studies.

To find their results, the team conducted both a meta-analysis of previous research and their own original data analysis. In the meta-analysis, they examined 22 traits across 199 studies of millions of male-female co-parents, engaged pairs, married pairs, or cohabitating pairs. The oldest study in this analysis was conducted back in 1903. They also used a dataset called the UK Biobank to analyze 133 traits across almost 80,000 opposite-sex pairs in the United Kingdom.

Same sex couples were not included in the research because the patterns in these types of partnerships may differ significantly. The authors are now pursuing those relationships in a separate study.

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Traits such as political and religious attitudes, level of education, and certain measures of IQ showed particularly high correlations. For example, on a scale of 0 meaning no correlation and 1 meaning couples always share a trait, the correlation for political values was .58. Traits surrounding substance use also showed high correlations, with heavy drinkers, smokers, and teetotalers tending to strongly pair with those who share similar traits. Traits like height and weight, medical conditions, and personality showed much lower but still positive correlations. For example, the correlation for neuroticism was .11.

Interestingly, some traits, such as extroversion, did not have much of a correlation.

“People have all these theories that extroverts like introverts or extroverts like other extroverts, but the fact of the matter is that it’s about like flipping a coin: Extroverts are similarly likely to end up with extroverts as with introverts,” study co-author and University of Colorado at Boulder PhD student Tanya Horwitz said in a statement. 

The meta-analysis found “no compelling evidence” that on any trait that opposites attract. However, in the sample from the UK Biobank, the team did find a handful of traits in which there seemed to be a small negative correlation, including hearing difficulty, tendency to worry, and whether someone is more of a morning person or night person (called chronotype). Additional studies will be needed to understand those findings, according to the team. 

Some of the less-frequently studied traits including number of sexual partners and whether an individual had been breastfed as a child also showed some correlation.

“These findings suggest that even in situations where we feel like we have a choice about our relationships, there may be mechanisms happening behind the scenes of which we aren’t fully aware,” said Horwitz.

According to the authors, couples could share traits for a variety of reasons, including growing up in a similar area. Some people are simply attracted to those who are similar based on the traits studied, and some couples grow more similar the longer they stay in the relationship. 

These pairings could lead to some downstream genetic consequences. For example, if short people are more likely to produce offspring with a similar height and vice versa, there could be more people at the height extremes in the next generation. This same thing apply for medical, psychiatric, and other traits according to Horowitz. 

Some of the social implications include those with similar educational backgrounds continuing to pair up, which could widen socioeconomic divides.

The team cautions that the correlations found were fairly modest and should not be overstated or misused to promote an agenda. Assortative mating has historically been dangerously co-opted by the eugenics movement, which gained traction during the early 20th century.