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What is IQ? 

Intelligence quotient (IQ) is derived from standardised tests used to measure general cognitive functioning. IQ is most commonly measured using the Wechsler Adult Intelligence Scale (WAIS). The WAIS is designed to measure all aspects of cognitive functioning, and is divided into subtests measuring verbal IQ (verbal comprehension and working memory) and non-verbal IQ (perceptual organisation and processing speed). Other tests used to assess IQ include the Mini-Mental State Examination (MMSE), which assesses cognitive impairment; the National Adult Reading Test (NART), which assesses premorbid intelligence; the Wide Range Achievement Test (WRAT), which assesses both verbal and mathematic ability; and the Raven’s Progressive Matrices, which assesses general intelligence.

What is the evidence for IQ?

Compared to people without schizophrenia, moderate to high quality evidence suggests a large effect of lower IQ in people with schizophrenia, including people with first-episode, youth-onset, or late-onset schizophrenia, with late-onset samples showing the greatest impairment. There is also a small to medium-sized effect of lower current IQ, but not IQ measured prior to the onset of the disorder, when compared to people with affective psychoses (e.g. bipolar disorder). High quality evidence suggests a small effect of lower IQ in people with schizophrenia who have antisocial traits compared to people with schizophrenia without antisocial traits, and to people without schizophrenia who have antisocial traits.

High quality evidence shows small to medium-sized associations between more severe negative and disorganised symptoms and lower IQ. Moderate quality evidence also suggests a small association with more severe reality distortion symptoms. High quality evidence suggests a medium-sized association between higher IQ and better insight into the disorder.

High quality evidence shows greater improvements in cognition in people taking second generation antipsychotics compared to people taking first generation antipsychotics. Moderate to high quality evidence suggests improvements in cognition with quetiapine, olanzapine, clozapine or risperidone, and low dose, but not high dose, haloperidol. High quality evidence suggests a small effect of better cognition in people with a psychotic disorder and a substance use disorder, than in people with a psychotic disorder with no substance use disorder. High quality evidence suggests people at familial high risk of psychosis are more impaired on premorbid and current IQ than people at clinical high risk of psychosis.


April 2016

Page last updated: 4:23  6 September 2017

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