Most of the common brain phenotypes and disorders that lack gross neuropathological underpinnings have been shown to represent “complex” disorders (or phenotypes) from a genetic perspective [228]. Twin studies support heritability of at least half of total vulnerability to many such disorders or phenotypes. Major psychiatric disorders such as bipolar disorder, major traits such as general cognitive abilities and major brain phenotypes such as the volume of frontal and temporal cerebral lobes each display at least 50% heritabilities in a number of well-performed twin studies (Table III). Linkage studies, which have been good at identifying individual genes whose variants exert substantial effects on phenotypes or diseases, have established a lack of genes of major effect for most of these phenotypes or disorders. Genome-wide association approaches, however, are now revealing more and more of the gene loci that contain variants that contribute to such disorders, and thus more and more of the classes into which genes that contribute to these disorders or traits fall.
