Common gene variants (those with a prevalence of 1% or greater in the general population) play a large role in psychiatric disorders. Common variants are likely to have quite small effects (relative risk of ~1.1, or an increase in risk of 10% for carriers), whereas rare variants may have larger effects. The effects of common variants may be combined, and it is likely that the major psychiatric disorders result from the combination of variants in hundreds or even thousands of genes.
