Given the focus in psychiatric genetics on adversity in the form of environmental risk factors and vulnerability in the form of genes associated with pathological conditions, it is not surprising that the possibility that the so-called vulnerability genes actually function more like plasticity genes could go unnoticed. It is almost as if, metaphorically speaking, sailors are so busy—and wisely—looking under the water line for extensions of icebergs that could sink their ship that they fail to appreciate that by climbing on top of the iceberg it might prove possible to chart a clear passage through the ice-laden sea. To the extent that it is appropriate to think in terms of plasticity rather than vulnerability, research will be required that extends the purview of the molecular-genetic study of behavior well beyond the investigation of dysfunction and environmental adversity. Once again Caspi et al.54 have served as groundbreakers, discovering that, with respect to intelligence, children carrying one variant of FADS2, a gene involved in the genetic control of fatty acid pathways, benefit from breastfeeding, whereas those carrying a different allele are not
