When it comes to relationships, you might have heard the saying that “opposites attract.” However, a recent study from the University of Colorado Boulder suggests otherwise. In a comprehensive analysis of data spanning more than a century and involving millions of couples, researchers found that, in most cases, people tend to be attracted to those who are similar to them.

Birds of a Feather

The study focused on more than 130 different traits that people may value when prospecting for a potential mate, ranging from political leanings and religion to the age of first intercourse and substance use habits.

The researchers found that for a significant majority of traits analyzed (between 82% and 89%), partners were more likely to be similar. In contrast, only 3% of traits showed a tendency for individuals to partner with those who were different in values and world views.

“Our findings demonstrate that birds of a feather are indeed more likely to flock together,” said first author Tanya Horwitz, a doctoral candidate in the Department of Psychology and Neuroscience and the Institute for Behavioral Genetics (IBG) at Boulder.

This research may have important implications for genetic studies. Many genetic models assume that human mating is random, which is pretty wild when you think about how people choose partners in the real world. Such research should be more mindful of “assortative mating,” where individuals with similar traits couple up.

A Century of Data

To arrive at these conclusions, the researchers conducted both a review, or meta-analysis, of previous research and their own original data analysis. They examined 22 traits across 199 studies, some dating back to 1903. Additionally, they analyzed data from the UK Biobank, studying 133 traits across almost 80,000 opposite-sex pairs in the United Kingdom. Note that same-sex couples were not part of this study, and their patterns for coupling may differ significantly.

Certain traits showed particularly high correlations among partners. For example, political and religious beliefs, level of education, and measures of IQ exhibited strong correlations. Traits related to substance use, such as smoking and drinking habits, also showed significant correlations. On the other hand, traits like height, weight, medical conditions, and personality had lower but still positive correlations.

One of the more surprising findings was that there was essentially zero correlation between extroverted and introverted personality traits.

“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,” said Horwitz.

The researchers found “no compelling evidence” that opposites attract. In the UK Biobank sample, they did find a few traits with small negative correlations, such as chronotype (whether someone is a morning person or night owl). However, these were exceptions rather than the rule.

Even when we feel we have a choice in our relationships, there may be underlying mechanisms at play that we are unaware of. Whether it’s growing up in the same area, being attracted to similar individuals, or becoming more alike over time, these factors influence our choice of partners — sometimes in unconscious ways.

However, nothing about human attraction is set in stone. The strength of correlations among traits varied across populations and may change over time. The researchers caution against overinterpreting these findings or using them to promote a particular agenda. Instead, they hope their study will encourage more research in various fields to better understand the complexities of human relationships.

The findings appeared in the journal Nature Human Behaviour.