Preliminary analyses were performed using SPSS (version 15.0 for Windows, 2006). The effects of sex, age, age2, sex*age, and sex*age2 were examined using stepwise linear regression. The effects of significant covariates (p <. 05) were regressed out before linkage analyses. For the younger dataset, sex, age, age2 and their interactions (sex*age & sex*age2) were significant in both the full and truncated samples (all p-values < 0.001). In the older dataset, sex, age2 and sex*age were significant in the full sample (p = 0.001, 0.001, 0.009 respectively) while only sex*age (p < 0.001) was significant in the older dataset with the truncation. Note that age and year of birth correlated >0.90, and consequently, age was a good surrogate covariate to account for the effects of possible secular trends.
