We now know that these genetic effects are relatively small and non-deterministic: most people with a strong family history are not themselves affected (as is also observed for most complex biomedical diseases). Moreover, most psychiatric disorders do not “breed true”. For example, the immediate relatives of people with schizophrenia have increased risks for schizophrenia but also for multiple other conditions (e.g., bipolar disorder, major depressive disorder (MDD), and autism). The diverse clinical manifestations and variable course observed for many common psychiatric disorders are consistent with complex and relatively small genetic effects. For adult-onset common psychiatric disorders in particular, development is often within normal limits, although there is often some impairment of higher components of cognition.
