When choosing romantic partners, we’re all familiar with the phrase “like attracts like”. Ironically, however, this phenomenon is all too often overlooked in modern genetics studies.

In a recent study by Nodsletten et al. (2016), a team of researchers reports on exactly that: nonrandom mating in relation to neurodevelopmental and psychiatric conditions. The group studied ADHD, autism, schizophrenia, bipolar disorder, major depression, generalized anxiety disorder, obsessive-compulsive disorder, social phobia, agoraphobia, anorexia nervosa, tic disorder, and substance abuse. And they asked the question: Do people with these conditions tend to marry within themselves at a higher rate than one might expect?

First off, as is perhaps unsurprising, the researchers found that the odds of mating (as measured either by marriage or the presence of biological offspring) was significantly lower in the neurodevelopmental/psychiatric group than controls. As such, one assumes that there is negative selection against these traits given the lower rates of reproduction.

However, as common wisdom might predict, the researchers also found that people with these neurodevelopmental or psychiatric conditions were 2-3 times as likely to partner with somebody with the same or a similar condition (e.g., ADHD x ADHD, autism x ADHD, etc.). This finding was consistent across both sexes. Interestingly, the more comorbidities a person had, the more likely their spouse would have the same or similar neurodevelopmental or psychiatric condition. These findings likewise suggest that there are positive factors for selection of these traits– gene variants that may in fact be additive to one another, increasing risk of occurrence in the offspring.

The research team also investigated mating patterns in other nonpsychiatric conditions for comparison, such as Crohn’s disease, diabetes, rheumatoid arthritis, and multiple sclerosis. Mating rates were unremarkable compared to controls and most showed little to no relationship in mating patterns across these medical conditions. There was, however, a single exception: multiple sclerosis (MS). Though the researchers didn’t postulate why individuals with MS may be attracted to one another, the answer may lie in the high preponderance of psychiatric symptoms associated with the condition that may also be present prior to onset of the illness [1, 2]. Once again, this suggests that when it comes to cognition, like tends to attract like.