While there is less literature on gene x development interactions (GxD), they are conceptually similar to GxE interactions and may suffer many of the same pitfalls described above. However, GxD also presents unique challenges. First, as in GxE, there is the need to filter genome-wide arrays for SNPs or other variants that have promise to show a developmental trend (e.g., use SNPs with demonstrated main effect). Second, longitudinal studies are uncommon (relative to cross-sectional studies), expensive, and often span no more than a few assessments over a few years. Obtaining or combining longitudinal sample sizes large enough to detect novel GxD interactions against the background of a million or more common SNPs is probably not achievable at this time. Third and related, using a cross-sectional approach (e.g., two samples that are of different ages) introduces cohort effects that confound any GxD test but, more importantly, they do not allow for the measure of individual change. That is, a cross-sectional approach with two samples of different ages would allow between-group tests but not change at the individual level (Curran & Bauer,
