Population-Level Implications of Parental Similarity for the Prevalence and Heritability of Psychiatric Disorders.

Importance: Partner similarity for psychiatric traits, often attributed to assortative mating, has been repeatedly documented, but its theoretical consequences for the prevalence and heritability of psychiatric disorders have been examined only under strong and often unrealistic assumptions and have never been empirically estimated using population-based data. Objective: To quantify the extent of partner similarity for psychiatric and somatic disorders and to estimate its empirical intergenerational implications for the prevalence and heritability of psychiatric disorders and their corresponding somatic comorbidities. Design, Settings, and Participants: This population-based cohort study used linked nationwide Danish registry data on residents born between 1969 and 2017. Parental pairs and offspring were identified through the Danish Civil Registration System, and psychiatric and somatic diagnoses were obtained from national health registers. Data were analyzed from 2021 to 2026. Main Outcomes and Measures: Parental correlations for psychiatric and somatic disorders were calculated. Prevalence and heritability in the observed data were then compared with corresponding estimates under a random mating scenario. Results: Among 4 244 585 Danish residents born between 1969 and 2017 (2 200 795 male [51.9%]; mean [SD] age, 26.3 [13.5] years for males and 25.9 [13.5] years for females), we identified 549 541 parental couples and 822 209 offspring. Parental correlations were substantial for psychiatric disorders overall (r = 0.28; SE, 0.003), with the highest within-disorder correlation observed for schizophrenia (r = 0.38; SE, 0.015) and the lowest for anorexia (r = 0.11; SE, 0.029). Cross-disorder correlations ranged from 0.01 to 0.26. Parental correlations were also observed for somatic illnesses but were weaker (r = 0.03-0.14) and were almost entirely accounted for by comorbid psychiatric disorders. Simulations further showed that this pattern of nonrandom mating inflated the prevalence and family-based heritability of any psychiatric disorder by 1.8% and 12.5%, respectively. Conclusions and Relevance: These findings suggest that partner similarity for psychiatric disorders is substantial and includes both within- and cross-disorder patterns, suggesting assortative mating alongside other mechanisms that may generate similarity between coparents. Regardless of its underlying mechanisms, this structure contributes meaningfully to observed family-based heritability and prevalence and leads to clustering of familial disease risk with important societal and public health implications.