Couple holding hands. LEHTIKUVA


A new study conducted by the University of Colorado Boulder challenges the age-old notion that "opposites attract." The comprehensive analysis, which examined data from millions of couples over more than a century and across more than 130 traits, found that similarity often draws people together in relationships.

The study, published on August 31 in the journal Nature Human Behaviour, not only debunks the popular belief that dissimilar individuals are more likely to form romantic bonds but also sheds light on the underlying forces shaping human relationships.

Tanya Horwitz, a doctoral candidate in the Department of Psychology and Neuroscience and the Institute for Behavioral Genetics (IBG), who served as the study's first author, explained, "Our findings demonstrate that birds of a feather are indeed more likely to flock together."

The research has implications beyond the realm of relationships; it also challenges assumptions made in genetic research. Matt Keller, senior author and director of IBG, noted, "A lot of models in genetics assume that human mating is random. This study shows this assumption is probably wrong."

The research combined a meta-analysis of previous studies with original data analysis. In the meta-analysis, the researchers examined 22 traits across 199 studies, including millions of male-female couples, engaged pairs, married pairs, or cohabitating pairs, with the oldest study dating back to 1903.

Additionally, the researchers utilized the UK Biobank dataset to analyze 133 traits, including seldom-studied ones, in nearly 80,000 opposite-sex pairs in the United Kingdom.

The study excluded same-sex couples, as their patterns of assortative mating may differ significantly, and the authors are exploring those separately.

Across both analyses, traits such as political and religious attitudes, educational levels, and certain measures of IQ exhibited high correlations among couples. For example, the correlation for political values was .58 on a scale where 0 indicates no correlation and 1 means couples always share the trait.

Traits related to substance use also displayed high correlations, with heavy smokers, heavy drinkers, and teetotalers tending to partner with individuals who had similar habits.

However, traits like height, weight, medical conditions, and personality traits showed lower but still positive correlations. For instance, the correlation for neuroticism was .11.

The meta-analysis revealed "no compelling evidence" supporting the idea that opposites attract. In the UK Biobank sample, a few traits appeared to have a negative correlation, although it was small. These included chronotype (whether someone is a morning person or night owl), the tendency to worry, and hearing difficulty.

The trait for which couples were most likely to be similar was their birth year.

The study suggests that even in situations where people believe they have a choice in their relationships, various mechanisms may be at play behind the scenes. Couples share traits due to growing up in the same area, being attracted to similar individuals, or growing more alike over time.

For instance, if short individuals tend to have offspring with short partners and tall individuals with tall partners, there could be more people at the height extremes in the next generation. A similar pattern may apply to psychiatric, medical, or other traits.

The study also revealed that the strength of correlations for traits varied across populations and likely changed over time. The authors hope their research will encourage further interdisciplinary studies in fields like economics, sociology, anthropology, and psychology.

Horwitz emphasized, "We're hoping people can use this data to do their own analyses and learn more about how and why people end up in the relationships they do."