IQ decline in cross-sectional studies of schizophrenia: Methodology and interpretation

Abstract

Some researchers have compared neuropsychological performance in schizophrenia groups with and without presumed IQ decline. Inherent in this approach is an assumption that group differences are due to different IQ trajectories (stable vs. declining), but neuropsychological differences could be a function of current IQ regardless of the presence or absence of previous IQ decline. We examined this issue in 93 normal controls and in 80 patients classified as having preserved (27.5%), deteriorated (50%), or compromised (22.5%) intellect based on IQ and reading recognition-IQ difference scores. We also examined group differences in verbal and performance IQ. Deteriorated patients had the largest verbal performance-IQ differences. They were more neuropsychologically impaired than the preserved group (average effect size=0.43), but deteriorated patients also had significantly lower current IQs. When subgroups of preserved and deteriorated patients with equivalent current IQs were compared, neuropsychological differences were essentially eliminated (average effect size=0.10); however, both groups were significantly more impaired than controls with similar IQs. Neuropsychological impairment, even in patients with apparently preserved IQ, is consistent with a prefrontal-dysexecutive syndrome. Overall, these results strongly suggest that differences in current neuropsychological function in schizophrenia are attributable primarily to current IQ instead of to IQ trajectory over time.

Cannabis users have higher premorbid IQ than other patients with first onset psychosis

“…Our cannabis-using patients also had a smaller difference between current IQ and premorbid IQ than non-using patients. Kremen et al (2008) point out that premorbid estimates should be understood as a measure of "potential" had a given subject not been destined to develop schizophrenia. Thus, individuals with a high premorbid IQ could be seen as less predisposed.…”

IQ Subtypes in Schizophrenia

“…In fact, recent population-based estimates of intellectual deterioration indicate rates as high as 98% for patients with schizophrenia, who show measurable decline in intellect from premorbid levels (Keefe et al, 2005). Typically, studies use a psychometric index of intellectual deterioration computed on the basis of IQ-oral reading difference scores (e.g., Kremen et al, 2008;Weickert et al, 2000). Oral reading is considered a reliable and valid proxy estimate of premorbid intelligence, because it is both highly correlated with IQ and preserved in a variety of adult onset cognitive disorders.…”

“…Studies of schizophrenia using this index have reliably classified patients as having preserved, compromised, or deteriorated IQ (Badcock et al, 2005;Kremen et al, 2008;Weickert et al, 2000). For example, Kremen et al (2008) recently classified 80 patients as either intellectually deteriorated (50%) with subaverage IQ but normative oral reading, intellectually compromised (22.5%) with similarly subaverage IQ and oral reading, or intellectually preserved (27.5%) with average or above IQ and oral reading.…”

Applied Short-Form WAIS-III to Explore Global Cognitive Profile of the Patients with Schizophrenia

“…The Wechsler Adult Intelligence Scale (WAIS) has been used exten sively to assess cognitive functions in patients with schizophrenia for both research and clinical practices [3,[13][14][15][16][17][18]. The Wechsler Adult Intelligence Scale-III (WAIS-III) has been updated from the Wechsler Adult Intelligence Scale-Revised (WAIS-R).…”

“…Previous studies found that patients with schizophrenia tend to have higher VIQ than PIQ [18,23]. In a study with 42 participants diagnosed with schizophrenia, the average IQ scores were in the low average range, indicating a decline in performance compared to the general population [20].…”