the crossover reflected true differential susceptibility or simply overfitting of the data across the environmental levels containing the majority of the observations, which contributed to a crossing over of the regression lines at one environmental extreme (Dick et al. 2011). Larger studies would have greater power to make these differentiations; however, there is the unfortunate paradox that the samples with the greatest depth of phenotypic information, allowing for more complex tests about risk associated with particular genes, usually have much smaller sample sizes due to the trade-off necessary to collect the rich phenotypic information. This is an important issue for gene-environment interaction studies in general: Most have been underpowered, and this raises concerns about the likelihood that detected effects are true positives. There are several freely available programs to estimate power (Gauderman 2002, Purcell et al. 2003), and it is critical that papers reporting gene-environment interaction effects (or a lack thereof) include information about the power of their sample in order to interpret the results.
