One source of heterogeneity within an individual study is the source of stress. A subset of our participating groups minimized this particular source of heterogeneity by ascertaining subjects that had all been exposed to a uniform stressor (e.g. pregnancy, military conscription, medical residency, coronary heart disease). Because the primary hypothesis supposes that the effect of genotype can be seen most strongly (or only) in subjects exposed to stress, analyses including only these datasets may have particular power to detect the hypothesized effect.
