It’s not news that when people find a partner to settle down with, they tend to end up with someone who is similar to them in key ways. There’s solid evidence that people “match” along the lines of social class, religion, education level, and other characteristics — though we’re talking averages here rather than set-in-stone rules, of course.
What about people with psychiatric disorders? Are they more likely to end up with partners who also have psychiatric disorders? If they do, can that help explain the heritability of certain disorders? These are the provocative questions posed in a JAMA Psychiatry article and accompanying editorial published Wednesday.
A team led by Dr. Ashley E. Nordsletten of Karolinska Institutet in Stockholm, Sweden, used data from 707,263 individuals in the Sweden National Patient Register, a giant data set that “includes diagnostic information on all individuals admitted to a Swedish hospital, with complete nationwide psychiatric records from 1973,” to try to determine whether and to what extent having one of 11 psychiatric disorders would raise the likelihood of someone being partnered with someone who also has one of those disorders (non-married cohabitation is big in Sweden, but the researchers partially got around this fact by looking not just at couples who were officially married, but also individuals who had produced offspring but weren’t married).
For each member of a pair, the researchers picked five other individuals at random who were matched on the basis of “age, sex, and county of residence,” and who didn’t have the diagnosis in question. In other words, we found a married guy with autism spectrum disorder, so let’s find five other guys without ASD who are the same age and living in the same place and look into their mating statuses to draw comparisons.
Overall, the researchers found a great deal of mating within and across various diagnoses. It varied a lot depending on the diagnosis, but to take a couple of examples (and these are heterosexual pairings): A man with ASD was 11 times more likely to be paired with a woman with ASD than other males of the same age in the same community, while a woman was 10 times more likely. Men and women with schizophrenia were both more than seven times more likely to be paired with a partner with schizophrenia, and men with ADHD were nine times more likely to be with a woman with social phobia.
As for the whys of all these connections, all we can do is speculate. I asked Nordsletten if the ASD data might be partially accounted for by the fact that having that disorder can bring certain social difficulties that only someone else with the disorder can truly understand. “Certainly, there is work that would support your speculation — [Simon] Baron-Cohen’s writing on his assortative mating theory of autism would fit with this kind of conception, for instance,” she replied via email. “However, this is really a question that needs to be followed up outside of the registers, which — while a wonderful tool for illustrating the scale and substance of these mating patterns — offers very little in the way of this kind of mechanistic insight.” She also pointed out that folks with autism were also far less likely to be paired up at all than folks with any other diagnoses — only about 5 percent of them were — so it is not a particularly large sample size to draw inferences from anyway.
In the accompanying editorial, a team led by Dr. Robert Plomin of King’s College London offered up some speculation of their own. “Is it possible that assortative mating is induced environmentally by convergence of spouses rather than genetically by assortment?” they ask. “For cognitive abilities—the other behavioral domain long known to show substantial assortative mating—assortative mating is due to initial selection of a mate (assortment) rather than by couples becoming more similar to each other after living together (convergence). Nordsletten and colleagues cite literature suggesting a similar conclusion for psychiatric disorders.” This ties into the idea that people with certain disorders might be attracted to others who share certain experiences or tendencies connected to those disorders.
As for the next step in this sort of research, Nordsletten wrote in her email:
The immediate implications of this study are most pronounced for psychiatric genetics research, in particular genetic models that are usually conducted under the assumption of random mating. Our findings suggest that this is not an accurate assumption in many cases, and thus models should allow for the correlation of spouses to avoid biases in heritability estimates (for instance, in twin studies, where neglect of these spousal correlations may result in an underestimation of disease heritability).
There is a whole lot more to figure out here, but this is an interesting, important study.