Genes have pleiotropic biologic effects and can be expected to have diverse effects on the development and function of the brain. The role that genes play in increasing risk for a psychiatric disorder presumably reflects a particular effect of that gene on neural systems that impact on the biology of susceptibility. A crucial point in identifying the mechanisms through which genes confer risk for psychiatric disorders is to define whether the brain phenotype that the risk gene modulates is a biological mechanism implicated in the psychiatric disorder and in risk for the psychiatric disorder. Finding an association between a gene and a brain function does not mean that that association is related to the mechanism of risk for the clinical illness. The analogous conundrum in clinical medicine would be to find that a newly identified risk gene for diabetes also affects BMI, without having evidence that BMI itself is a risk factor for diabetes (which it clearly is).
