Intelligence in Schizophrenia: Meta-analysis of the Research

Abstract

This article combines a review and meta-analysis of research on IQ in schizophrenia, with emphasis on areas of convergence in the findings, as well as questions that remain to be answered. Taken together, the findings suggest that early-onset and adult-onset schizophrenia are associated with intellectual deficits across the lifespan. Preschizophrenic children, adolescents, and young adults perform below matched controls on a variety of standardized measures of intelligence. Significant IQ deficits are also apparent after the onset of the disorder. Moreover, IQ is positively related to several indices of prognosis, and, among hospitalized patients, there is negative within-subject covariance between intellectual performance and symptom severity. Although there is fairly consistent evidence that Verbal IQ is higher than Performance IQ among schizophrenic patients, a more specific pattern of subtest performance is not apparent. A central question raised by the results is whether IQ is an independently determined factor that can serve to mitigate the vulnerability of individuals who are constitutionally predisposed to schizophrenia, or whether intellectual deficit is one manifestation of the constitutional predisposition to the disorder. The findings also raise the question of possible sex differences in the developmental determinants of schizophrenia: Meta-analyses revealed that premorbid IQ deficits are more prevalent among males than females.

Precursors of cognitive impairments in psychotic disorders: A population-based study

“…Thus, although there is a negative link between FSIQ and psychotic disorders primarily low PIQ was strongly related to psychoses whereas this was not the case for VIQ. This finding in which subjects with psychotic illnesses perform more poorly on performance than on verbal intelligence tasks is consistent with earlier research showing that primarily fluid functions are negatively affected in psychotic individuals (Aylward et al, 1984;Ott et al, 1998;Amminger et al, 2000). This gap between deficits, however, is assumed to vary according to the stage of illness.…”

Research on Research: A Meta-Scientific Study of Problems and Solutions in Psychological Science

“…Studies into group differences compare existing (non-manipulated) groups and typically use Cohen's d or raw mean IQ differences as the key effect size. Examples include studies comparing mean IQs between males and females (Irwing & Lynn, 2005) or mean IQs between healthy controls and people diagnosed with schizophrenia (Aylward, Walker, & Bettes, 1984). Experiments and interventions are studies that attempt to improve IQ of either healthy or unhealthy groups.…”

Neurocognition and conversion to psychosis in adolescents at high-risk

“…Despite strong evidence of poor premorbid intellectual functioning (for a meta-analytic review, see Aylward et al, 1984;Seidman et al, 2006;Reichenberg et al, 2006) and literature suggesting subtle functional impairments in years preceding clinical onset (e.g., , only a few studies have examined neurocognitive functioning among high-risk youth in relationship to vulnerability for conversion to Axis I psychosis.…”

“…Interestingly, despite well-established sex differences in cognitive in schizophrenia (Lewine, Walker, Shurett, Caudle & Haden 1996;Hoff & Kremen 2002; for review, see Leung & Chue 2000), few studies have examined sex differences in neurocognitive functioning, including IQ, prior to illness onset (for review, see Aylward, Walker and Bettes, 1984;Weiser et al, 2000;Offord 1974;Jones and Done, 1997;Bilder et al, 2000;Gittelman-Klein and Klen, 1969;Erlenmeyer-Kimling et al, 1984). The vast majority of studies indicate better premorbid functioning during childhood among females (compared to males) who later developed schizophrenia.…”