Determining gene moderation of environmental risk factors for a mental disorder: a "perfect storm" of methodological problems.

For most of the twentieth century, the focus was on "nature" versus "nurture", i.e. genetic versus environmental effects on disorders. Now it is increasingly recognized that a disorder may reflect genes and environments "working together". A gene may moderate an environmental risk factor, it may be mediated by an environmental risk factor. The environmental risk factor may be proxy to the gene, and the two may be independent risk factors. Which of these situations pertain influences both subsequent research and clinical and policy decision-making. However, recent meta-analyses attempting to confirm the Caspi et al. (Science, 301, 386-389, 2003) hypothesis indicate that the methodological issues relating to establishing specifically a moderating effect of a gene on an environmental factor are not well understood. The discussion here concerns the definition of "moderator", how it is distinct from other ways in which gene and environment can "work together", the methods needed to establish such a moderator, and the public health significance of such efforts.