Modeling assortative mating and genetic similarities between partners, siblings, and in-laws

Assortative mating on heritable traits can have implications for the genetic resemblance between siblings and in-laws in succeeding generations. We studied polygenic scores and phenotypic data from pairs of partners (n = 26,681), siblings (n = 2,170), siblings-in-law (n = 3,905), and co-siblings-in-law (n = 1,763) in the Norwegian Mother, Father and Child Cohort Study. Using structural equation models, we estimated associations between measurement error-free latent genetic and phenotypic variables. We found evidence of genetic similarity between partners for educational attainment (r_g = 0.37), height (r_g = 0.13), and depression (r_g = 0.08). Common genetic variants associated with educational attainment correlated between siblings above 0.50 (r_g = 0.68) and between siblings-in-law (r_g = 0.25) and co-siblings-in-law (r_g = 0.09). Indirect assortment on secondary traits accounted for partner similarity in education and depression, but not in height. Comparisons between the genetic similarities of partners and siblings indicated that genetic variances were in intergenerational equilibrium. This study shows genetic similarities between extended family members and that assortative mating has taken place for several generations.