Cognitive impairment has been at the forefront of schizophrenia research and clinical interest for the last 2 decades. The prevailing clinical impression is that individuals who meet criteria for schizophrenia also suffer from easy observable and at times severe cognitive impairment. However, when large populations of schizophrenics undergo classic psychological testing, the normal distribution of their composite scores is "shifted to the left" only moderately. There exist a very large overlaps between patients and controls in terms of cognitive scores regardless of the tests employed. An hypothesis that would reconcile the clinical observations with the research data on large population would suggests that the quality and degree of cognitive impairment that cognitive impairment in schizophrenia is heterogeneous both in quality and severity and that some subgroups of individuals perform within or above normal range on all aspects of cognition. Adding to this heterogeneity is the fact that for some individuals the cognitive impairment is static while for others it is progressively declining.

Since different aspects of cognitive impairment might have different biological substrate, investigating and sub-typing cognitive impairment could be essential to finding a therapeutic remedy.