Gender of the cohort member and parental social status when the cohort members were 1 year old were included as covariates. The 1-8 point parental social status index was based on information about breadwinner's occupation, breadwinner's education, type of income, and quality of housing (Zachau-Christiansen & Ross, 1975). Data on parental social status were missing for 1318 cohort members (18.4%): when parental social status was included in statistical models, the overall mean was substituted for missing values, and a dummy variable indicating missing values was included.
