It's no secret that the old adage "opposites attract" has little scientific basis; you're far more likely to settle down with someone who shares your love for the Sharknado series than someone who doesn't understand the value of terrible CGI. That being said, this trend goes beyond a shared love for scifi. According to a recent study, people with certain mental illnesses tend to pair off with each other — in other words, people with certain psychiatric disorders are more likely to end up in a long-term romantic relationship with people who have the same disorder.
Researchers in Sweden looked at hospital records dating back to the 1970s and followed up on married or cohabitating patients who had at least one of 11 major psychiatric disorders, like autism spectrum disorder (ASD) or major depression, as well as patients with physical diseases. Researchers then noted whether their significant other had the same mental illness. At the same time, they looked at the relationship statuses of five other people with the same gender, age, and location as each half of the couple in question, but without the same psychiatric or physical disorder. (As is the case with most scientific studies, the relationships in question were strictly heterosexual.)
Here's where it gets interesting. Researchers found a substantial likelihood of relationships among people with mental illnesses, both within and across diagnoses, although people with some disorders were more likely to pair off than others. Disorders that tend to show up at an early age, like autism spectrum disorder (ASD), or express severe symptoms, like schizophrenia, had particularly strong spousal correlations. In fact, people with ASD, schizophrenia, or attention-deficit/hyperactivity disorder (ADHD) were the most likely to marry someone with the same disorder, quickly followed by substance abuse. Affective disorders like major depression had a weaker correlation, but the link was still notable.
Keep in mind that the study is correlational, so researchers can't draw any conclusions about why this happens. That being said, they do have some ideas; lead researcher Dr. Ashley E. Nordsletten told New York Magazine that "there is work that would support" speculation that people with mental illness are drawn to others who have firsthand experience with what they're going through. However, she went on to state that more research is needed to establish any causes.
Others not affiliated with the study have pointed out that the results have implications for a field you might not expect: Genetics. For decades now, research has indicated that mental illness is highly heritable, although some are more so than others. Schizophrenia and bipolar disorder are particularly likely to be inherited; estimates put the rate of schizophrenia inheritance at 80 percent and bipolar disorder between 60 to 85 percent. Researchers generally assume this is due to complex genetic interactions, but it certainly makes sense that some mental illnesses are likely to be inherited if both parents have disorders. Again, though, more research needs to be done before anyone can start drawing conclusions.
Surprisingly, there's little research focusing specifically on mental illness and romantic relationships, although a 2011 study indicated that people with certain disorders were slightly less likely to get married. However, there's plenty of research showing the importance of support from family and friends, as well as the various negative effects of the stigma surrounding mental illness. Whether you share a disorder with someone or not, it's still possible to understand and support each other — after all, people with mental illness are people, first and foremost.