Our sample is ethnically homogenous, but composed of distinctly different age groups comprising middle-aged parents and age-17 offspring. Although we adjusted for age, generation, and birth year to account for possible age-related effects, as Baker noted (2014, this issue), it is possible that the genes contributing to variation in our endophenotypes are different at various points in development, a factor that could weaken our ability to detect an effect. Multivariate, longitudinal approaches can assess genetic contributions to “innovations,” or new sources of variance in a longitudinal context, as well as genetic contributions that are relatively constant. They can also be sensitive to gene-environment interactions (Kaprio, 2012). Indeed, in other work in the MTFS sample, we have shown that genetic variants associated with height and smoking in adulthood show smaller effects for those respective measures during adolescence, and we look forward to future work evaluating similar hypotheses for endophenotypes. For the moment, however, we caution that we are not aware of studies that show developmentally heterogeneous genetic effects for our chosen endophenotypes at the ages we examined. For the one endophenotype
