It is heuristically useful to consider the bookends. The extreme models are that psychiatric disorders are caused by (a) the cumulative impact of hundreds or thousands of common genetic variants each of small effect (common-disease/common-variant model) or (b) many different gene-disrupting variants of strong effect (multiple rare variant model). In the latter model, every person with a serious psychiatric disorder would have a strong effect variant and these would cluster in a set of genes important to brain development and function.
