A new comprehensive analysis that includes data from millions of couples shows that people prefer those who are similar to them
Opposites don’t attract. This is the conclusion of a comprehensive analysis by the University of Colorado in Boulder, USA, of more than 130 traits and millions of pairs spanning more than a century.
“Our results show that birds with the same plumage are more likely to congregate,” says first author Tanya Horwitz, a PhD student in the Department of Psychology and Neuroscience and at the Institute of Behavioral Genetics (IBG). The study, published Aug. 31 in the journal Nature Human Behavior, confirms what individual studies have pointed out for decades and challenges the old adage “opposites attract.”
According to the publication, between 82% and 89% of characteristics analyzed – from political leanings to age at first sexual intercourse to substance use habits – were more similar than dissimilar. In only 3% of the traits and only part of the analysis did individuals tend to mate with others.
Not only does it shed light on unseen forces that can shape human relationships, it also has important implications for the field of genetic research. “Many genetic models assume that human mating occurs randomly. “This study shows that assumption is probably wrong,” says Matt Keller, lead author and director of the IBG, noting that so-called “assortative mating” — when individuals with similar traits mate — can skew the results of genetic studies .
A look back over a century
For the new work, the authors performed both a review or meta-analysis of previous research and their own analysis of the original data. In total, they examined 22 characteristics in 199 surveys involving millions of co-parents, dating partners, married couples or civil partners. The oldest test was conducted in 1903.
In addition, the researchers used a dataset called the UK Biobank to examine 133 characteristics of nearly 80,000 opposite-sex couples in the UK, including many that have not been adequately studied.
Same-sex couples were not included in the study. Because the patterns can differ significantly, the authors now examine them separately.
In both analyses, characteristics such as political and religious attitudes, level of education and certain IQ measurements showed particularly high correlations. For example, on a scale where 0 means no correlation and 1 means couples always share the trait, the agreement for political values was 0.58.
Those associated with substance use also showed high correlations: heavy smokers, habitual drinkers and teetotalers They tend to associate with people who have similar habits.
Meanwhile, traits such as height and weight, medical conditions, and personality traits showed significantly lower, albeit positive, correlations. For example, neuroticism was 0.11. For others, such as extroversion, there was not much agreement.
“People have all these theories that extroverts are like introverts or extroverts are like other extroverts, but the reality is that it’s like flipping a coin: extroverts are just as likely to become extroverts as introverts are,” says Horwitz.
In rare cases, opposites attract.
In the meta-analysis, the researchers found “no compelling evidence” that opposites attract. In fact, in the UK Biobank sample, he found a handful of traits that appeared to have a negative, albeit low, correlation.
These factors include chronotype (whether someone is a “morning lark” or a “night owl”), a tendency to worry, and hearing difficulties. Still, the researchers noted that more studies need to be done to find out these results.
The characteristic in which the couples were most likely to be similar was, unsurprisingly, year of birth. But even little-studied conditions, such as how many sexual partners a person had or whether they were breastfed as a child, showed some association.
“These results suggest that even in situations where we feel we have choices about our relationships, there may be behind-the-scenes mechanisms that we are not fully aware of,” says Horwitz.
Impact on the next generation
The authors note that couples share characteristics for a variety of reasons: some grow up in the same area, others are attracted to people who are similar to them, and some become more alike the more time they spend together.
Depending on the cause, further consequences can occur. For example, Horwitz explains that if short people are more likely to have offspring with short people, and tall people are more likely to have offspring with tall people, there could be more people with extreme height in the next generation. The same applies to psychiatric, medical or other characteristics.
There might even be social implications. For example, some previous smaller studies suggest that people in the United States are becoming more likely to mate with people with a similar educational level, a trend that some theories suggest could widen the socioeconomic gap.
Notably, the new study also showed that the strength of trait correlations differed between populations. The authors suspect that they may also change over time.
The researchers caution that the correlations discovered are modest and should not be overstated or misused to further an agenda (Horwitz points out that assortative mating research has been tragically co-opted by the eugenics movement).
They hope the study will stimulate more research in a variety of disciplines, from economics to sociology to anthropology and psychology. “We hope people can use this data to do their own analysis and learn more about how and why people end up in the relationships they end up with,” he said.
Evidence of correlations between human partners based on systematic reviews and meta-analyses of 22 traits and a UK Biobank analysis of 133 traits