“Assortative mating increases the contribution of additive genetic variance (narrow heritability) for any trait on which it acts. This boost to heritability from assortative mating could help to explain why psychiatric disorders have such high heritability despite reduced fecundity,” wrote Dr Plomin and colleagues in the editorial.
The editorial authors also explore why some psychiatric disorders are more heritable than others. Because ADHD, ASD, and schizophrenia are more heritable in twin studies than other disorders, Dr Plomin and colleagues explain that this research might mean that these disorders show twice as much assortative mating as other disorders—but point out that the explanation might not be as simple as that.
“Twin studies suggest that nonadditive genetic influence is greater for these same 3 disorders than for other disorders in that dizygotic twins are much less than half as similar asmonozygotic twins,” the authors wrote. “Nonetheless, it is possible that these 3 disorders are so highly heritable because they include injections of both additive genetic variance from assortative mating and unusually high nonadditive genetic variance. Without assortative mating, these disorders might show little additive genetic variance.”
For the third question of why there is so much genetic comorbidity across psychiatric disorders, the editorial authors noted that “genome-wide estimation of genetic correlations across psychiatric disorders supports the conclusion that many genetic effects are general across disorders … Assortative mating across psychiatric disorders could help to drive this genetic comorbidity.”
Dr Nordsletten and colleagues noted that these findings may hold important implications for understanding familial transmission of these disorders.
These results also challenge how DNA-based heritability estimates are currently conducted — Dr Plomin and colleagues noted that genome-wide association studies typically only search for additive genetic effects, which, according to previous research, may not provide accurate heritability estimates for various psychiatric disorders. This suggests that more attention is warranted for this issue.
1. Nordsletten AE, Larsson H, Crowley JJ, Almqvist C, Lichtenstein P, Mataix-Cols D. Patterns of Nonrandom Mating Within and Across 11 Major Psychiatric Disorders. JAMA Psychiatry. 2016; doi:10.1001/jamapsychiatry.2015.3192.
2. Plomin R, Krapohi E, O’Reilly PF. Assortative Mating—A Missing Piece in the Jigsaw of Psychiatric Genetics. JAMA Psychiatry. 2016; doi:10.1001/jamapsychiatry.2015.3204.