Researchers from Karolinska Institutet in Sweden and the University of Carolina at Chapel Hill relied on population registers from Sweden to determine the nature and extent of nonrandom mating within and across psychiatric conditions, including attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorder, schizophrenia, bipolar disorder, major depression, generalized anxiety disorder, substance abuse, and more.
For the purposes of comparison, cases of select nonpsychiatric conditions of similar incidence and age at onset were also identified, including Crohn’s disease, type 1 and type 2 diabetes mellitus, multiple sclerosis, and rheumatoid arthritis. Mating relationships were identified through marriage records or records of individuals being the biological parent of a child in other registers.
On average, an individual’s psychiatric disorder was associated with a twofold to threefold increase in the likelihood that his or her mate would also have a psychiatric disorder, with slightly higher odds that it would be the same disorder. These risks were elevated in autism, ADHD, and schizophrenia. This trend was not seen in the nonpsychiatric disorders, with the exception of multiple sclerosis (but even this was a moderate effect compared to many psychiatric similarities).
“This phenomenon may hold important implications for how we understand the familial transmission of these conditions and the ubiquity of comorbidity and complex symptoms in clinical populations,” the study authors wrote. “Furthermore, the results challenge a fundamental assumption of current genetic research methods, suggesting that more attention to this issue is warranted.”
In a related editorial, Robert Plomin, Ph.D., of King’s College London and colleagues wrote, “Beyond genetics and genomics, assortative mating matters because 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.”
For more about the importance and implications of the heritability of psychiatric disorders, see the Psychiatric News article “Familial Mental Illness Risk Cuts Across Disorders.