Psychiatric disorders are highly polygenic and show patterns of partner resemblance. Partner resemblance has direct population-level genetic implications if it is caused by assortative mating, but not if it is caused by convergence or social homogamy. Using genetics may help distinguish these different mechanisms. Here, we investigated whether partner resemblance for schizophrenia and bipolar disorder is influenced by assortative mating using polygenic risk scores (PRSs).

PRSs from The Danish High-Risk and Resilience Study—VIA 7 were compared between parents in three subsamples: population-based control parent pairs (N=198), parent pairs where at least one parent had schizophrenia (N=193), and parent pairs where at least one parent had bipolar disorder (N=115).

The PRS for schizophrenia was predictive of schizophrenia in the full sample and showed a significant correlation between parent pairs (r=0.121, p=0.0440), indicative of assortative mating. The PRS for bipolar disorder was also correlated between parent pairs (r=0.162, p=0.0067), but it was not predictive of bipolar disorder in the full sample, limiting the interpretation.

Our study provides genetic evidence for assortative mating for schizophrenia, with important implications for our understanding of the genetics of schizophrenia.

Sex differences in brain structure and neurodevelopment occur in non-clinical populations. We investigated whether sex had a similar effect on developmental domains amongst boys and girls with a familial risk of schizophrenia (FHR-SZ), bipolar disorder (FHR-BP), and controls.

Through Danish registries, we identified 522 7-year-old children (242 girls) with FHR-SZ, FHR-BP, and controls. We assessed their performance within the domains of neurocognition, motor function, language, social cognition, social behavior, psychopathology, and home environment.

FHR-SZ boys compared with FHR-SZ girls had a higher proportion of disruptive behavior and attention-deficit hyperactivity disorder (ADHD) and exhibited lower performance in manual dexterity, balance, and emotion recognition. No sex differences were found between boys and girls within FHR-BP group. Compared with controls, both FHR-SZ boys and FHR-SZ girls showed impaired processing speed and working memory, had lower levels of global functioning, and were more likely to live in an inadequate home environment. Compared with control boys, FHR-SZ boys showed impaired manual dexterity, social behavior, and social responsiveness, and had a higher proportion of ADHD and disruptive behavior disorder diagnoses. Stress and adjustment disorders were more common in FHR-BP boys compared with control boys. We found no differences between FHR-BP girls and control girls.

Impairment within neurodevelopmental domains associated within FHR-SZ boys v. FHR-SZ girls was most evident among boys, whereas no sex differences were found within the FHR-BP group (FHR-BP boys v. FHR-BP girls). FHR-SZ boys exhibited the highest proportion of early developmental impairments.