Classification Discrepancies in Two Intelligence Tests: Forensic Implications for Persons with Developmental Disabilities

Abstract

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. We 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.

Keywords:

• WAIS-III
• WRIT
• intelligence
• death penalty
• Atkins
• developmental disabilities

ACKNOWLEDGEMENTS

We extend special thanks to Elise Devecchio-Cavagnaro, PhD, for all of her assistance and inspiration in this project. Also we thank each of the psychology interns, including Cindy Hirbour, MA, who participated in this research.

Notes

¹ In the present study, Cohen's d was selected to gauge effect size for ANOVA comparisons involving two means. Eta-squared (η² ) was selected when comparisons involved three or more means.

² Interpretation guidelines of the standard mean difference (Cohen's d) are as follows: 0.20 = a small effect; 0.50 = a medium effect; and 0.80 = a large effect (Cohen, 1988).

³ According to Cohen (1988), a value for η² of 0.0099 represents a small effect, a value of 0.0588 a medium effect, and a value of 0.1379 a large effect.

⁴ Although this effect size value might be conceptualized as representing a medium effect, the effect must be put into context and interpreted with caution given the small sample sizes of some of the ethnicities (Ferguson, 2009).

⁵ As expected, age showed no significant relationship with WAIS-III Full Scale IQ (r = –0.20), WRIT General IQ (r = –0.19), or with difference scores between WAIS-III Full Scale IQ and WRIT General IQ (r = 0.10). There were significant relationships between age and WAIS-III Verbal IQ (r = –0.27, p < .05) as well as between age and WAIS-III Verbal Comprehension Index (r = –0.35, p < .01). No significant associations of age with any of the other IQ indices occurred.

⁶ Two individuals obtained IQ scores in the average range. These persons did not have diagnoses of mental retardation or borderline intellectual functioning but had diagnoses of “other disabling conditions” described earlier.

FIGURE 1 Relationship of WAIS-111 Full Scale IQ and WRIT General IQ.