In the original report, no main effect of genotype was found. If the genotype exerts an effect only on those exposed to the stressor, i.e. a diathesis-stress model, the lack of main effect may be due to insufficient power [16]. Alternatively, the genotypic effect may be one of differential environmental susceptibility [17], in which the L-carriers are indifferent to the environment, whereas the S allele confers environmental susceptibility, allowing S carriers to benefit more from positive experiences as well as being more sensitive to stress, resulting in no net genotype-depression association irrespective of sample size [18]. Studying a large sample may distinguish these possibilities.
