Spend enough time with someone and you start picking up their habits. You might steal their favorite slang, mirror their laugh, or even start looking weirdly alike in photos. According to a massive new study in Nature Human Behaviour, the overlap can go much deeper than quirks. Couples are significantly more likely to share psychiatric disorders than random chance would predict.
Researchers analyzed health data from over six million couples in Taiwan, Denmark, and Sweden. Across all three countries, partners were more likely to have conditions like depression, anxiety, ADHD, schizophrenia, OCD, bipolar disorder, anorexia, substance use issues, or autism. “We found that a majority of psychiatric disorders have consistent spousal correlations across nations and over generations,” the researchers wrote.
The pattern is known as spousal correlation, and it’s usually studied in contexts like education level, political beliefs, or religion. With psychiatric conditions, the causes are a bit murkier. Part of it may be “assortative mating”—choosing partners with traits similar to our own.
Another part is the simple fact of sharing an environment for years, which can blur the line between nature and nurture. And then there’s the reality that dating pools are limited. As the researchers put it, these three influences overlap, making it hard to tease apart the strongest driver.
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Interestingly, the similarities weren’t tied to cultural context. Despite differences in healthcare systems and social structures, the results were nearly identical across the three nations. “Spousal resemblance within and between psychiatric disorder pairs is consistent across countries and persistent through generations, indicating a universal phenomenon,” the team wrote.
There are caveats. The study didn’t account for whether people met before or after a diagnosis, and some conditions—like OCD, bipolar disorder, and anorexia—showed variation between countries. Still, the scale of the data makes the overall trend hard to ignore.
The findings also cause concern for genetics. Many studies assume that people mate randomly, which helps scientists separate environmental and genetic risk factors. But if couples with psychiatric conditions are more likely to pair up, then kids born into those relationships may face compounded risks.
“Given the ubiquitousness of spousal correlation, it is important to take non-random mating patterns into consideration when designing genetic studies of psychiatric disorders,” the researchers wrote.
Couples trade inside jokes and bad habits until they can’t tell whose is whose. According to this study, the same may go for psychiatric disorders.