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Sexual Assault of Adults and the Importance of Competencies versus IQ

Sexually assaulted mentally-defective adults in New Jersey have protections under the law similar to minors.

The definition of mental defectiveness was unpacked in New Jersey v. Olivio (1991). However, what is commonly thought of as a mentally-defective adult seems to differ from the following description provided by Judge Handler in New Jersey v. Olivio (1991): Mental defectiveness as related to “sexual assault on a mentally-defective person” is demonstrated when, “at [the] time of sexual activity the complainant was unable to comprehend [the] distinctively sexual nature of conduct or was incapable of understanding or exercising the right to refuse to engage in that conduct with another.”

When you imagine an adult with these inabilities, you may picture someone for whom everything is sexual or nothing is sexual perhaps as the result of a lack of exposure, teaching, or practice. You may picture an adult in a relationship with a significant power imbalance or an adult with an expressive language disability. Whatever type of adult you imagined, I would guess they have deficits in knowledge, abilities, and certain competencies. I would not presume they were necessarily mentally retarded, and would argue that they could be of average intelligence despite having profound deficits in some areas assessed by intelligence tests.

If I guessed correctly, it may surprise you that forensic psychologists routinely support their conclusions of mental defectiveness by demonstrating a Full Scale Intelligence Quotient (IQ) score in the mentally retarded range (New Jersey v. Olivio, 1991; New Jersey v. Scherzer, 1997; New Jersey v. Valerio, 2008), even though a diagnosis of Mental Retardation (MR) is not required to substantiate a mental defect (New Jersey v. Valerio, 2008). Beyond the fact the MR diagnosis is not required, why else should experts try an alternative approach to the assessment of a mental defect? For one, the MR diagnosis is insufficient in some cases, and for two, this approach is not parsimonious. The latter exposes experts to significant problems upon cross-examination when they are trying to bridge the gap between what a timed block manipulation IQ subtest has to do with sexual knowledge and ability to consent. Here are some additional reasons.

The area of intelligence has several competing theories (Sattler, 2008). According to the most widely accepted theory that forms the basis of David Wechsler’s intelligence scales, IQ can be misleading in the event there are significant discrepancies between the major underlying areas of intellectual functioning (Kaufman & Lichtenberger, 1999). Consider an autistic victim of sexual assault with an IQ in the Low-Average range. It is possible that their verbal intellectual abilities are in the MR range and their nonverbal intellectual abilities are exceptionally high. Therefore, even though they are not MR, their verbal deficits may satisfy the elements required for a mental defect.

New Jersey v. Olivio (1991) did not clarify how intellectual abilities are related to a mental defect beyond the extent intelligence is associated with cognition and fund of knowledge. For example, in New Jersey v. Valerio, (2008),Valerio was convicted even though one expert said the victim was MR and the other expert disagreed. This decision was not a matter of one expert appearing more credible than the other. Rather, Valerio was convicted and was unsuccessful on appeal because the victim’s specific deficits in knowledge and cognition took precedent over her IQ score.

Also, consider what we have learned from Atkins v. Virginia (2002), which established MR makes you ineligible for execution under the Eighth Amendment. IQ scores became a life or death matter, which is extremely problematic because of measurement error (Atkinson, 1991). All IQ tests have some measurement error that renders the true IQ undeterminable. The best a psychologist can do is report the degree of confidence they have that the true IQ falls within a certain range. Since Atkins v. Virginia (2002), several states have defined what IQ cut-off they would use for MR. Appeal courts have heard arguments based on the accuracy of IQ tests, causes of MR, and the impact of culture on the reliability of IQ testing (Widroff & Watson, 2008). These are fair cross-examination questions for an expert who based their finding of a mental defect on a diagnosis of MR.

Finally, it is difficult for any psychologist to explain to a lay audience how IQ assessment tasks like pattern identification, block manipulation, and knowledge of word meanings (Wechsler, 1999) are related to knowledge of what is sexual and the ability to consent. This lack of face validity is a limitation in the courtroom. The same is true for recent research conducted by Kennedy (2003) suggesting executive functioning tests (e.g., word fluency and card sorting) are better at identifying individuals with the capacity to consent to sex than other popular approaches. The best an expert can do is say that IQ is correlated with sexual knowledge and the capacity to consent (Murphy & O’Callaghan, 2004), but for that matter there is also a correlation between ice cream sales and violence.

Forensic psychologists conducting mental-defect evaluations should instead focus on the assessment of relevant knowledge, abilities, and competencies that address the elements of the law. For example, McCabe’s (1994) Sexual Knowledge, Experience, Feelings, and Needs Scale (SEX-KEN-ID) measures learned skills and knowledge. There are tests that assess abilities to identify circumstances that two parties are in agreement to have sex (Stavis & Walker-Hirsch, 1999), and others that assess knowledge of the sexual nature of the acts under consideration, and understanding of the choice to reject sexual advances and propositions (Lyden, 2007).

Adaptive behavior (AB) tests, such as the Vineland Adaptive Behavior Scales-II (Sparrow, Cicchetti, & Balla, 2005), assess practical skills needed for daily functioning. AB tests are frequently included in mental defect assessments because impairment in adaptive functioning is required to make a diagnosis of MR. However, the abilities assessed by AB tests can substantiate a finding of a mental defect without IQ scores. O’Callaghan and Murphy (2002) developed a type of AB test that specifically targets skills relevant to the substantiation of a mental defect. In later research, the authors found that intellectually disabled individuals (n=60) performed significantly worse on this AB task than a normative group of 16-year-olds (Murphy & O’Callaghan, 2004).

For the protection of mentally-defective individuals, it is crucial that experts approach this referral question with assessment tools that specifically address the elements of the law. Comprehensive intellectual assessment instruments are correlated with the elements, but lack a face-valid relationship. Low intellectual functioning should be treated as a warning rather than a proxy for mental defectiveness.


Atkins v. Virginia, 536 U.S. 304 (2002). Retrieved August 29, 2009, from, 1991

Kaufman, A.S. & Lichtenberger, E.O., (1999). Essentials of WAIS-3 Assessment, John Wiley & Sons, New York.

Kennedy, C. (2003). Legal and psychological implications in the assessment of sexual consent in the cognitively impaired population. Assessment, 10(4), 352-358.

Lyden, M. (2007). Assessment of sexual consent capacity. Sexuality and Disability, 25(1), 3-20.

McCabe, M. P. (1994). Sexual knowledge, experience and needs scale for people with intellectual disability (Sex Ken-ID) (4th Ed.). Psychology Research Centre, Deakin University, Melbourne.

Murphy, G.H., & O'Callaghan, A. (2004). Capacity of adults with intellectual disabilities to consent to sexual relationships. Psychological Medicine, 34(7), 1347-57.

New Jersey v Olivio, 123 N.J. 550, 589 A.2d 597 (1991)

New Jersey v. Scherzer, 301 N.J.Super. 363, 694 A.2d 196

New Jersey v Valerio, 2008 WL 2699843 (N.J.Super.A.D.)

O'Callaghan, A. C., & Murphy, G. H. (2002). Capacity to consent to sexual relationships in adults with learning disabilities. Final report to the Nuffield Foundation, UK.

Sattler, J.M. (2008). Assessment of children: Cognitive foundations (5th ed.). California: Jerome M. Sattler Publisher, Inc.

Sparrow, S.S., Cicchetti, D.V., & Balla, D.A., (2005). Vineland Adaptive Behavior Scales: Second Edition (Vineland II), Survey Interview Form/Caregiver Rating Form, Livonia, MN: Pearson Assessments.

Stavis, P, Walker-Hirsch, L.W.: Consent to sexual activity. In: Dinerstein, R. et al. (eds.) A Guide to Consent, pp. 57–67. American Association on Mental Retardation, Washington, DC (1999).

Wechsler, W. (1999). Wechsler Abbreviated Scale of Intelligence. The Psychological Corporation: California

Widroff, J., & Watson, C. (2008). Mental retardation and the death penalty: Addressing various questions regarding an Atkins claim. Journal of the American Academy of Psychiatry and the Law, 36(3), 413-415.

Disclaimer: While every effort has been made to ensure the accuracy of this publication, it is not intended to provide legal advice as individual situations will differ and should be discussed with an expert and/or lawyer.For specific technical or legal advice on the information provided and related topics, please contact the author.

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