People often choose, deliberately or unconsciously, romantic partners similar to themselves. And according to a new study, that extends to people with mental disorders, including schizophrenia and autism. That finding has important implications for the children of these pairings.

Previous studies have indicated that might be the case, but those “have largely focused on resemblance for psychiatric features rather than clinical diagnoses,” said lead researcher Ashley Nordsletten, a professor of clinical neuroscience at Karolinska Institute in Stockholm. She added that previous studies also depended on voluntary self-reporting from very small groups or second-hand accounts from spouses.

For the current study, published in JAMA Psychiatry, the researchers analyzed the records of 700,000 individuals who had been diagnosed with attention-deficit hyperactivity disorder, autism spectrum disorder, schizophrenia, bipolar disorder, depression, anxiety, agoraphobia, social phobia, obsessive-compulsive disorder, anorexia, or substance abuse. They evaluated how often these diagnoses correlated with that person’s spouse or partner having either the same psychiatric condition or a different psychiatric condition.

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They found that, overall, people with mental disorders were two to three times more likely than the general population to have a romantic partner with any mental disorder. Some disorders showed a greater likelihood of both partners having the same diagnosis. People with ADHD or schizophrenia, for instance, were seven times more likely to partner with someone else with their condition than the general populace. For autism, it was over 10 times.

That has important implications if these couples decide to have children. Many psychiatric conditions are shaped to some extent by genes, so these couples would be at an increased risk of passing along their condition. “Although few, existing population-based examinations suggest that up to 67.5 percent of offspring from dual-schizophrenia couples and 44.2 percent from dual–bipolar disorder couples may develop these disorders,” the authors note in their paper.

Nordsletten said these findings could change the way scientists study heredity in mental illness.

“Genetic models are usually conducted under the assumption of random mating,” she said. “Our findings suggest that this is not an accurate assumption in many cases. For instance, in twin studies, neglect of these spousal correlations may result in an underestimation of disease heritability.”

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