Fitness to plead: the role of neuropsychological assessment and the experience of legal professionals

Abstract

Internationally, the Wechsler Adult Intelligence Scale (WAIS; Wechsler, 2008) is commonly used to aid in the determination of fitness to plead (FtP), yet the cognitive profiles of defendants deemed fit or unfit to plead remain poorly understood. This meta-analysis aimed to systematically evaluate the similarities and differences in performance on the WAIS between defendants deemed fit to plead (FtP) and unfit to plead (UtP), along with the use of performance validity tests in this population. Performance was compared between FtP (n= 979) and UtP (n= 690) defendants in eleven studies (n=1669). Full Scale IQ, Verbal IQ, Performance IQ and individual subtests were meta-analysed using a random effects model that indicated individuals deemed FtP scored significantly higher than UtP defendants across all indices and 9 subtests. The largest effect was observed in the comprehension subtest. Both groups scored within the ‘low average’ and ‘borderline’ classifications on most subtests, translating to a difference of approximately five standard scores or one scaled score between groups. This review extends previous literature that has reported significant differences only in Full-Scale IQ and, to our knowledge, this is the first meta-analysis examining WAIS subtest performance in this population. These findings are consistent with previous literature that suggest language and working memory are of central importance in the determination of FtP. Findings from this meta-analysis highlight both the utility and shortcomings of the WAIS in informing the determination of FtP.