When it comes to choosing a mate for life, a healthy state of mind is a quality not to be overlooked, according to a team of researchers from the Karolinska Institutet in Stockholm. Their study, published in JAMA Psychiatry, reveals that people who suffer from a mental disorder are more likely to mate with others with mental health disorders and produce children likely to have similar issues.

For the study, researchers collected medical information from roughly 700,000 people who were admitted to hospitals in Sweden between 1973 and 2009. Within the patient population, more than 70,000 people were diagnosed with schizophrenia, 10 with other major psychiatric disorders, and the rest had a chronic illness like diabetes or multiple sclerosis.

Next, they examined data from marital records to look at partnership patterns, and found a distinct and consistent mating sequence emerge. Those with mental disorders were more likely to marry and have children with people with either the same illness or a different psychiatric disorder. Researchers compared the marital patterns to those with chronic illnesses and physical disorders, and found that they did not gravitate toward one another in the same manner. This led researchers to believe that marriage to people who have mental disorders seems to be a contributing factor that’s keeping certain mental illnesses alive.

“Taken together, these results suggest that individuals with psychiatric diagnoses are mating — to a degree greater than would be expected by chance — with other diagnosed individuals,” said the study’s lead author Ashley E. Nordsletten, a psychologist and postdoctoral research fellow at the Karolinska Institutet. “Our results only raise questions to be answered, and answering them will represent the first real steps towards identifying and targeting any risk.”

Nordsletten and her colleagues came to the conclusion that people with severe psychiatric disorders tend to mate with each other, however, they don't know why. They believe it may be because those suffering from mental illness often have a difficult time forming and maintaining social relationships with people. It may also be plausible that people who aren’t suffering from mental illness are less willing to date and marry those with psychiatric disorders.

According to Dr. Matthew Lorber, a director of child and adolescent psychiatry at Lenox Hill Hospital, who was not involved in the study, said the results are as relevant to evaluating childhood psychiatric disorders as they are to investigating disorders among married adults.“[The findings are] very important to consider when doing future genetic research and when thinking about the higher incidence of psychiatric illness running in families.”

If two people both suffer from mental illness, the likelihood their child will also have a mental illness goes up. According to the National Institute of Mental Health, schizophrenia and bipolar disorder tend to run in families because there is a genetic root involved. While certain genes may increase the risk of developing a mental illness, sometimes it takes an environmental situation to trigger the onset, such as a traumatic event. However, geneticists haven’t been able to identify the exact marker that can predict whether or not a parent or a child will develop a mental illness.

The study’s authors conclude: “It means that the person closest to an individual with a psychiatric disorder is also likely to have psychiatric problems, which could exacerbate problems for both spouses and their offspring.”

Source: Plomin R, Krapohl E, and O’Reilly PF. Assortative Mating—A Missing Piece in the Jigsaw of Psychiatric Genetics. JAMA Psychiatry . 2016.