Our sample consists of individuals nested within families, which creates a correlation that violates the assumption of independent residuals in regression analyses. We used Rapid Feasible Generalized Least Squares (RFGLS; Li, Basu, Miller, Iacono, & McGue, 2011), a computationally efficient form of generalized least squares, to account for this source of dependency. Correlations are estimated separately for MZ and DZ twin families; the 65 stepparents in the present sample (61 of them male) were treated as independent observations. Additive SNP effects were modeled, with each SNP represented as a count of the number of minor alleles. The conventional threshold for genome-wide significance of 5 × 10−8 was used to evaluate the significance of each SNP.
