Study confirms it: Opposites don't actually attract

That’s the takeaway from a sweeping analysis of more than 130 traits and including millions of couples over more than a century.

What the researchers say: “Our findings demonstrate that birds of a feather are indeed more likely to flock together,” said the lead author.

The study, published in the journal Nature Human Behavior, confirms what individual studies have hinted at for decades, defying the age-old adage that “opposites attract.” It found that for between 82% and 89% of traits analyzed—ranging from political leanings to age of first intercourse to substance use habits—partners were more likely than not to be similar.

For only 3% of traits, and only in one part of their analysis, did individuals tend to partner with those who were different than them.

Aside from shedding light on unseen forces that may shape human relationships, the research has 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,” the researchers explained, noting that what is known as “assortative mating”—when individuals with similar traits couple up—can skew findings of genetic studies.

For the new paper, the authors conducted both a review, or meta-analysis, of previous research and their own original data analysis.

For the meta-analysis, they looked at 22 traits across 199 studies including millions of male-female co-parents, engaged pairs, married pairs or cohabitating pairs. The oldest study was conducted in 1903.

In addition, they used a dataset called the UK Biobank to study 133 traits, including many that are seldom studied, across almost 80,000 opposite-sex pairs in the United Kingdom.

Same sex couples were not included in the research because the patterns there may differ significantly, the authors are now exploring those separately.

Across both analyses, traits like political and religious attitudes, level of education, and certain measures of IQ showed particularly high correlations.  For instance, on a scale in which zero means there is no correlation and 1 means couples always share the trait, the correlation for political values was .58.

Traits around substance use also showed high correlations, with heavy smokers, heavy drinkers and teetotalers tending strongly to partner up with those with similar habits.

Meanwhile, traits like height and weight, medical conditions and personality traits showed far lower but still positive correlations.

In the meta-analysis, the researchers found “no compelling evidence” on any trait that opposites attract. In the UK Biobank sample, they did find a handful of traits in which there seemed to be a negative correlation, albeit small.

Those included: chronotype (whether someone is a “morning lark” or “night owl”), tendency to worry and hearing difficulty.

The trait for which couples were most likely to be similar was, not surprisingly, birth year.

But even seldom-studied traits, like how many sexual partners a person had had or whether they had been breastfed as a child, 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 the lead author.

The findings could have implications for future generations, the researchers explained. For example, if short people are more likely to produce offspring with short people and tall people with tall people, there could be more people at the height extremes in the next generation. The same goes for psychiatric, medical or other traits.

There could also be social implications.

For instance, some small previous studies have suggested that people in the U.S. are growing more likely to couple up with people with similar educational backgrounds—a trend that, some theorize, could widen the socioeconomic divide.

Notably, the new study also showed that the strength of correlations for traits differed across populations. They likely also change over time, the authors suspect.

So, what? This study has important implications far beyond couples. Other recent research has shown that employers tend to hire people similar to themselves, we collaborate more readily with those that we have more in common with. Commonality in trait, genes or behavior is one of the five basics of trust.

An interesting study published a few years ago showed that the genetics of Britain’s elite in the 16th century were similar to the UK’s ruling class today. We not only trust those that we have a lot in common with, we protect them as well perpetuating a ruling class while at the same time establishing classes—or a races—of underprivileged people.

The elite’s offspring get better education, better nourishment, more opportunities, better health care, are taller than average—and are therefore more able to influence—and see themselves as superior and justified in their privilege.

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