Differences in sample characteristics between COGA and Add Health could have introduced additional noise into our estimates. While we tried to make the phenotypes across sample match as closely as possible, they were not identical. There were important differences between samples, as COGA was enriched for families with a high density of alcohol dependence, while S4S is college based. These differences can influence the performance of polygenic scores (Savage et al., 2018). There was also greater variation in the age of COGA participants, whereas young adults in the S4S sample may have yet to reach the highest risk period for the onset of these externalizing problems.
