In this response to Bellini's (2007) commentary on “Defining Mental Retardation: A Matter of Life and Death” (Lichten & Simon, 2007), we summarize the major issues in defining mental retardation and respond to several points raised by Bellini (2007). We are also aware of and support the change in nomenclature from mental retardation to intellectual disability, as advocated by the American Association on Intellectual and Developmental Disabilities (AAIDD). However, during this transition period, the existing diagnostic guides and state statutes have not yet changed to the more current term, intellectual disability. Therefore, we base our discussion on AAIDD's (formerly the American Association on Mental Retardation [AAMR]) Mental Retardation: Definition, Classification and Systems of Supports (Luckasson et al., 2002) and use that nomenclature throughout.
Since Heber (1959) introduced the requirement that both IQ and adaptive behavior be a part of the diagnosis of mental retardation, the AAIDD manual (Luckasson et al., 2002) has moved gradually in the direction of relying on standardized tests of adaptive behavior. However, the 2002 manual does not define how both measures should be combined. The manual states for IQ scores that, “The criterion for diagnosis is approximately two standard deviations (SD) below the mean” (p. 14). For measures of adaptive functioning, it states that:
For the diagnosis of MR, significant limitations in adaptive behavior should be established through the use of standardized measures … significant limitations in adaptive behavior are operationally defined as performance that is at least two SD below the mean of either (a) one of the following three types of adaptive behavior: conceptual, social, or practical, or (b) an overall score on a standardized measure of conceptual, social, and practical skills. (p. 14)
Lichten and Simon (2007) pointed out that imposing both conditions (IQ and adaptive behavior), as Luckasson et al. (2002) directed, would drastically reduce the prevalence of mental retardation in the general population. The exact degree of this reduction would depend on the correlation coefficient between the standardized IQ and adaptive measures used. Because correlations between IQ and adaptive measures are usually small, we provided an example in which the correlation was zero. We then found that the current prevalence of 2.3% (approximately 6.8 million people with mental retardation in the United States) would be reduced to the absurd 2.3% of 2.3%, or 0.053% (approximately 160,000 people with mental retardation in the United States). (With a correlation of .2, the number of persons with mental retardation would equate to approximately 400,000, still an absurd figure; Lichten & Simon, 2007.) Clearly, for everyday clinical diagnostic use, the Luckasson et al. (2002) criteria need revision.
In our article (2007), our solution to the above problem was to use the standard method in constructing scales, such as for IQ or adaptive behavior. Scale authors, such as Wechsler (1974) or Sparrow, Balla, and Cicchetti (2005), started with multiple measures of the traits they wished to study. They then combined these measures into a single scale. Conventionally, this combined scale has a mean of 100 and standard deviation of 15.
We followed the same method in combining IQ with adaptive functioning. The example we chose combined commonly used and well-standardized IQ (Wechsler, 1974, and/or Stanford-Binet [Roid, 2003]) and adaptive behavior measures (Vineland Adaptive Behavior Scales; Sparrow, Balla, & Cicchetti, 2005). We preferred these scales for our example largely because the data quality seemed best to us. Nevertheless, our method was general and required only the correlation coefficients among the scales to be combined. Thus, the method can handle any of the types of standardized measures specified by Lukasson et al. (2002, see previous quotation), if the requisite correlation coefficient is available.
“Their assertions may be accurate within the narrow bounds of their own assumptions, but these do not accurately reflect contemporary guidelines. Luckasson et al. (2002) did not endorse the use of a total adaptive behavior score for this purpose. Rather, ‘significant limitations are operationally defined as performance that is at least two SDs below the mean of either (a) one of the following three types of adaptive behavior: conceptual, social, and practical skills’ (p. 14).” (p. 348)
Part (b), omitted by Bellini (2007), allows use of total adaptive behavior scores. Bellini's comment is a selective presentation of the full definition. There is no disagreement between our assumptions and the AAIDD handbook (Luckasson et al., 2002).
Furthermore, the use of the Vineland Adaptive Behavior Composite score is not necessary for our argument; we used it as an example. One can use either recommendation of Luckasson et al. (2002; i.e., use of a single domain score or a total adaptive behavior score) in constructing the Total Quotient (TQ). The only required information is the correlation coefficient between IQ and the chosen measure. Therefore, there is no disagreement between us and Luckasson et al. (2002).
In his commentary, Bellini (2007) also criticized the TQ index as being inadequate. He overlooked repeated statements in our article that expressed the need for clinical judgment in addition to our quantitative measure. For example,
Of course, no mechanically applied measure can be trusted to substitute for informed clinical judgment. … We do not mean to suggest a blind application of the joint TQ criteria without taking into account other psychological/sociological factors. Total Quotient also does not obviate the use of sound clinical judgment. (Lichten & Simon, 2007, p. 340)
We find no real disagreement between our article (2007) and Bellini's (2007) commentary. We certainly agree with Bellini that “institutional decisions … in the lives of individuals should be based on a comprehensive assessment” (p. 348). Nevertheless, past history has shown that diagnoses rely heavily on quantitative data, which is why we constructed the TQ.
Bellini (2007) further criticized TQ as follows: “Use of TQ as a diagnostic benchmark would likely create greater ambiguity in classification because individuals with other disabilities … who currently do not meet all relevant criteria for mental retardation could qualify using the TQ benchmark” (p. 349). Our discussion (2007) of TQ also clearly pointed this out: “TQ also does not obviate the use of sound clinical judgment by trained professionals in assigning a diagnosis of MR that would otherwise result in misdiagnoses in such cases as depression, physical and sensory challenges, or the inappropriate use of standardized tests” (p. 341). Bellini (2007) also posited the importance of context, such as the level of support, in evaluating mental retardation. He then suggested that TQ is inadequate because it fails to take context into account. We do not disagree with the possible importance of such factors. However, present definitions of mental retardation (Luckasson et al., 2002) emphasize standardized measures: IQ and adaptive functioning. What we have done is to solve the problem of entering both factors into the definition of mental retardation without changing its prevalence—nothing more, nothing less (Note: Bellini presented the well-known distinction between precision and accuracy. We don't disagree with his distinction but find it irrelevant to our discussion of TQ. The word precision never occurred in our article.)
We have implemented the present definition of mental retardation (Luckasson et al., 2002) with a simple combination of both IQ and adaptive functioning scores (Total Quotient), which keep the prevalence of mental retardation constant. We find that Bellini's (2007) criticisms were based on a selective reading of our article (Lichten & Simon, 2007) and that of Luckasson et al. (2002). He criticized our article for not going beyond the current definition of mental retardation by Luckasson et al., by including additional factors that he felt should be included in the definition. Yet we believe that we made a positive contribution to the definition of mental retardation by enlarging the number of multiple indicators. TQ, as exemplified by us, extends that number from 10 in IQ to 21 in both IQ and adaptive functioning. We consider this an advance but not the last word, as the concept of intellectual disability continues to be specified.
William Lichten, PhD, Professor Emeritus, Koerner Center for Emeritus Professors, Yale University, 145 Elm St., New Haven, CT 06520-8368. Elliott W. Simon, PhD (firstname.lastname@example.org), Executive Director, Elwyn, 111 Elwyn Rd, Elwyn, PA 19063