Accurate measurement of intellectual abilities of adults with developmental disabilities impacts key legal issues, including adjudicative competence, civil commitment, and death penalty litigation. This research compared standardized measures of intelligence in a multicultural sample of adults with developmental disabilities. Within subjects ANOVA revealed significantly higher Wechsler Adult Intelligence Scale-Third Edition IQs compared to Wide Range Intelligence Test (WRIT) IQs, with a median difference of 13.0 points. Underestimates provided by the WRIT could lead to adverse legal decisions, including exacerbation of malingered cognitive dysfunction cases and permitting individuals guilty of criminal acts to escape sentences. Policy implications exist for the methodology of intellectual assessment given that instruments yield discrepancies. The authors suggest utilizing standardized measures with strong psychometric integrity in Atkins hearings and incorporating relevant collateral information when generating clinical case formulations. This will give clinicians additional relevant data and afford greater precision in forming clinical judgments regarding diagnosis and cognitive level in forensic cases. Abstract published by arrangement with Taylor and Francis.