This study also had limitations. It included a relatively homogenous population of Caucasian college students, and it is not known whether these findings would apply to more heterogeneous populations including participants of different ages or races/ethnicities. We only analyzed behavioral and mood outcome measures in our study: It would be interesting to determine whether these DRD2 variations modulate brain activity corresponding to impulsive behavior, both in a drug-free condition as well as in response to d-amphetamine. One other limitation concerns the interpretation of the interaction between drug and genotype on stop RT. It is possible that the differential effect of amphetamine in the A/G+/G/G and A/A groups was driven by differences in baseline (placebo) stop RTs. In addition, the multiple adjustment procedure for haplotype analyses included only haplotypes in Block 2 and not all haplotypes evaluated in the study.
