of the drug of interest for the effects of other drugs. However, the complexity and expense of brain imaging studies usually precludes samples that have the statistical power to adjust for covariates. For example, Rivkin35 found an initial effect for prenatal cocaine exposure on brain volume but the cocaine effect was no longer statistically significant when other substances were controlled. Therefore, we cannot determine if the “true” effect in this study is a polydrug effect or if unique cocaine effects would be observed with a larger sample size.
