Patients with schizophrenia or mood disorders exhibit an exceedingly wide range of symptoms from a variety of domains. The cardinal features in schizophrenia are abnormal ideas (such as delusions); abnormal perceptions (such as hallucinations); formal thought disorder (as evidenced by disorganized speech); motor, volitional, and behavioural disorders; and emotional disorders (such as affective flattening or inappropriateness). In addition to these diverse symptoms, it has become increasingly apparent that the disorder is, to variable degrees, accompanied by a broad spectrum of cognitive impairments. It has been known for 50 years that people with schizophrenia often perform very poorly on most cognitive tests. This poor performance has generally been shown not to be due to poor motivation, as some cognitive tests are performed at levels consistent with premorbid functioning. It has been argued that there are “overriding deficits”, which produce reduced scores on other measures.