ABSTRACT
Significantly diminished intellectual functioning, as indicated by appropriately administered IQ tests with scores below 70, is a frequent mental handicap leading to severe social disadvantages and serves as a paradigm for molecular genetic research of complex disorders and traits due to its multitude of known and unknown, genetic as well as environmental causes. Since the number of confounding variables is expected to be considerably reduced in the normal population at the opposite ends of the IQ distribution, we employed a contrast of extremes approach by comparing adults of high (N = 71) and average IQ (N = 78) in association studies to search for genes involved in the multigenic forms of familial mental retardation. The dopamine D2 receptor gene (DRD2) was chosen as a candidate gene for general cognitive ability (g) since it has been found to be associated with visuospatial ability which in turn is highly correlated with g. Confirming two similar studies in children, however, no significant differences were obtained. Given three negative studies, the DRD2 gene is unlikely to pay a major role in g.
