2.63 percent of all black students receive special education services due to the MR disability, a rate that is 2.35 times the rate for white students. White and American Indian/Alaskan Native students are in MR programs at very close to the same rates, while Hispanic students have slight underrepresentation and Asian/Pacific Islanders have substantial underrepresentation (see Chapter 2). Disproportionate MR representation has been the most controversial and intractable pattern over the past few decades.

Many changes in MMR have occurred since the 1982 NRC report. While MR was the disability category of interest in that report, during the intervening period many of the mild cases have ceased to be identified as mentally retarded in many states (MacMillan et al., 1996d). It is instructive to note the “vacillating prevalence” of MR among schoolchildren in the past half century (Mackie, 1969). Mackie reports that between 1948 and 1966 there was a 400 percent increase in the number of children served in public school programs for students with MR. During the latter phase of that time period, the American Association on Mental Deficiency adopted the Heber (1959, 1961) definition that set the upper IQ cutoff score at -1 SD (roughly IQ 85), leading Clausen (1967) to note that this was the most liberal, inclusive definition ever of the concept of mental deficiency. In the mid-1960s there was no LD category recognized in federal law, and public schools encountering a youngster with severe and chronic low achievement had few options for helping that child—either they classified him or her as MMR, or services were restricted to the interventions available in general education.

The existence of two groups of individuals with MR has long been recognized (Dingman and Tarjan, 1960; Zigler, 1967). One is a more patently disabled group of individuals whose MR more often has a biological basis (referred to as “organic” by some) and whose IQ is commonly very low (i.e., below 50). Zigler (1967) proposed that this group of individuals represents a separate IQ distribution with a mean of approximately 35 and ranges from an untestable level up to an IQ of about 70. Zigler said that the intellectual functioning of this group of mentally retarded children reflected “factors other than the normal polygenic expression”—that these people had an “identifiable physiological defect.” A second group of individuals, referred to as “familial cases of mental retardation” evidence no organic impairment and are believed by Zigler to represent the lowest portion of the normal curve of intelligence. Predictions derived from these hypotheses generated by Dingman et al. were tested by Mercer (1973b), examining the presence of physical disabilities (e.g., seizures, ambulation, vision, and hearing problems) in individuals clinically identified as MR with IQ scores in the range of about 55-75, i.e., the familial type. She concluded: “Clearly, persons whose IQs are more than 3 standard deviations below the mean of



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