These analyses are conducted in three main steps: First, a quantitative phenotype value is obtained for a population of MZ twin pairs. Second, the absolute phenotypic difference score is calculated for each MZ pair. This score reflects phenotypic differences due to environmental effects, as the environment makes genetically identical twins diverge phenotypically. The absolute phenotypic difference is corrected for age, sex, ten genetic principal components and any study-specific covariates. The residuals are standardized and inverse rank transformed. Third, a GWAS of the MZ differences score is conducted, using the phenotypic differences score for one twin from each pair and their genotype data; therefore, the sample comprises unrelated individuals.
