Serious Problems with the Mexican Norms for the WAIS-III when Assessing Mental Retardation in Capital Cases

Abstract

A Spanish-language translation of the Wechsler Adult Intelligence Scale-III (WAIS-III), normed in Mexico, is sometimes used when evaluating Spanish-speaking defendants in capital cases in order to diagnose possible mental retardation (MR). Although the manual for the Mexican test suggests use of the U.S. norms when diagnosing MR, the Mexican norms—which produce full-scale scores on average 12 points higher—are sometimes used for reasons that are similar to those used by proponents for “race-norming” in special education. Such an argument assumes, however, that the Mexican WAIS-III norms are valid. In this paper, we examined the validity of the Mexican WAIS-III norms and found six very serious problems with those norms: (1) extremely poor reliability, (2) lack of a meaningful reference population, (3) lack of score normalization, (4) exclusion of certain groups from the standardization sample, (5) use of incorrect statistics and calculations, and (6) incorrect application of the true score confidence interval method. An additional problem is the apparent absence of any social policy consensus within Mexico as to the definition and boundary parameters of MR. Taken together, these concerns lead one to the inescapable conclusion that the Mexican WAIS-III norms are not interpretable and should not be used for any high-stakes purpose, especially one as serious as whether a defendant should qualify for exemption against imposition of the death penalty.