Mental Retardation (Intellectual Disabilities)

Robert M Hodapp. Learning and Memory. Editor: John H Byrne. 2nd edition. New York: Macmillan Reference USA, 2004.

In any society, certain individuals fail to make normal progress in intellectual, social, and linguistic growth and development, exhibiting marked difficulties in learning. The need to distinguish these individuals led the scientific community to develop intelligence tests, as well as to create an operational definition of mental retardation. Mental retardation (also called intellectual disabilities) is defined as significantly subaverage general intellectual functioning existing concurrently with deficits in adaptive behavior, and manifested during the developmental period. Clinicians typically interpret the condition to include those individuals who obtain an IQ (intelligence quotient) score two or more standard deviations below the mean (i.e., IQ less than 70) on an intelligence measure such as the Stanford-Binet or the Wechsler Adult Intelligence Scale. Clinicians usually include two other criteria—one including deficits in everyday functioning (so-called “adaptive behavior”) and onset during the childhood years (i.e., before age eighteen)—in the three-criteria definitions of mental retardation.

Within the population with mental retardation itself, researchers and clinicians have further classified individuals due to the individual’s degree of intellectual impairment. Thus, persons with mental retardation have often been described as having mild (IQ 55 to 69), moderate (40 to 54), severe (25 to 39), or profound (below 25) degrees of impairment. Especially for persons with severe or profound mental retardation, most individuals also show correspondingly lower levels of functioning on a wide variety of cognitive-linguistic and everyday tasks. For individuals with mild and moderate mental retardation, however, life success (e.g., performing well at work) seems much more determined by one’s personality, perseverance, socialization skills, and lack of maladaptive behaviors.

IQ-related classifications have limited utility for understanding how different intellectual abilities may or may not be connected. In particular, an overreliance on an omnibus IQ score fails to acknowledge either the composite nature of this score or perspectives on the multidimensional nature of intelligence, such as those espoused by Howard Gardner (1983) and Jerry Fodor (1983). In fact, a single IQ score (e.g., 50) may be associated with markedly different patterns of development, with one child failing to make progress on language and performing near age level on visuospatial tasks and another showing the exact opposite pattern.

Another way to classify persons with mental retardation, then, relies on the cause or etiology of one’s mental retardation. From the late 1960s until the early twenty-first century, several researchers have advocated what has been called the “two-group approach” to mental retardation (Hodapp, 1997). Adherents of this approach argue that the retarded population can be divided into those whose mental retardation is due to the same polygenetic and/or environmental factors causing IQ differences within typically developing individuals (cultural-familial mental retardation) versus those whose mental retardation is caused by a specific organic insult (organic mental retardation). This entry explores the cultural-familial and organic groups in turn.

Cultural-Familial Mental Retardation

The first group is comprised of those individuals whose mental retardation has no obvious organic etiology. Scientists do not know the cause of mental disability in approximately 50 percent of persons with mental retardation. One hypothesis is that this group simply forms the lower portion of the normal, Gaussian distribution of intelligence. Why such individuals are retarded remains unclear, but some (unspecified) interaction of biological, familial, psychological, social, and environmental risks seems likely. And, if (as in nonretarded persons) approximately half of the variation in IQ scores is due to the workings of many genes, such individuals may show their lower IQ scores from having received a poor “polygenetic draw.” With the success of behavior geneticists, at least one of these polygenes for intelligence has been identified (Chorney et al., 1998). Future work will tease apart the effects of genetic and various environmental factors—working separately or in tandem over time—on one’s overall IQ.

Organic Mental Retardation

Organic mental retardation includes all persons whose intellectual deficits can be attributed to one or more specific organic insults. Such insults can occur before birth (as in any of the 750-plus genetic disorders associated with mental retardation); at or around the time of birth (due to anoxia, prematurity); or in the years after birth (due to meningitis, head trauma).

Since the 1990s, scientists have made much progress in mental retardation by focusing on well-defined organic syndromes, particularly those related to such genetic disorders as Down syndrome, Prader-Willi syndrome, and Williams syndrome (Dykens and Hodapp, 2001). Such research ties these forms of mental retardation to particular patterns of cognitive, linguistic, adaptive, and maladaptive deficits. In examining functioning in persons with specific genetic etiologies of mental retardation, researchers also gain insights into the structure of human intelligence. Several of the major genetic forms of mental retardation are discussed in this entry, together with presenting symptoms, characteristic cognitive deficits, explorations of their neurobiological underpinnings, and attempts at treatment.

Down Syndrome

Down syndrome (DS), trisomy 21, is the best-known form of mental retardation and the most prevalent form known to be associated with a chromosomal abnormality, occurring once in every 800 to 1,000 live births. The syndrome was identified and described in 1866 by Langdon Down, and the extra twenty-first chromosome was discovered in 1959. Since that time, Down syndrome has been the focus of intensive genetic and behavioral research. Children with DS usually have characteristic physical features including the epicanthic fold (leading to its original label, “mongolism”), a protruding tongue, short stature, and hypotonia (weak muscle tone). DS often occurs together with such medical conditions as heart defects, leukemia, and gut atresia. Although in the past individuals with DS had a short life span and were often institutionalized, medical treatments have improved their life span and they are now typically raised at home. Persons with DS have received considerable attention for educational successes that have far outstripped earlier predictions. DS may be detected during pregnancy through chorionic villus sampling, or amniocentesis; this procedure is usually recommended in women above age thirty-five, who bear a substantially higher risk of carrying a Down syndrome baby (Pueschel, 2001).

Three behavioral characteristics appear in many (possibly most) individuals with Down syndrome (Rondal, Perera, and Nadel, 1999). The first involves a specific set of cognitive-linguistic strengths and weaknesses. In various studies, persons with Down syndrome appear particularly impaired in language. Such impairments, which are more pronounced than overall levels of mental age (MA) per se, occur in linguistic grammar, in expressive (as opposed to receptive) language, and in articulation. Conversely, persons with Down syndrome often show relatively higher performance on tasks of visual short-term memory.

A second behavioral issue involves the rate of development, with children with Down syndrome developing at slower rates as they get older. Such slowings of development may relate to age-related changes or to difficulties these children have in achieving certain cognitive tasks (e.g., language). A third, possibly related change concerns Alzheimer’s disease. It is known that neuropathological signs of Alzheimer’s disease appear to be universal in individuals with DS by age thirty-five or forty. Geneticists continue to explore the connection of DS with Alzheimer’s, and continue to learn more about pathological segments of chromosome 21 involved in overlapping conditions.

Prader-Willi Syndrome

Prader-Willi syndrome is caused by missing genetic material from the chromosome 15 derived from the father—either a deletion on the paternally derived 15 or two chromosome 15s from the mother (so-called maternal disomy). Most individuals with Prader-Willi syndrome are short in stature (about 5 feet tall in adulthood) and show extreme hyperphagia (overeating). Indeed, hyperphagia and resultant obesity have long been considered the hallmarks of Prader-Willi syndrome, and most cases of early death in the syndrome relate to obesity and its related heart and circulatory problems (Dykens and Cassidy, 1999). In addition to hyperphagia, many individuals show obsessions and compulsions that are similar in level to those with clinically diagnosed obsessive-compulsive disorder.

Intellectually, most children with Prader-Willi syndrome show relative weaknesses on tasks involving consecutive, step-by-step order in problem solving, or sequential processing. In contrast, these children perform well on tasks requiring integration and synthesis of stimuli as a unified whole, or simultaneous processing. In addition to these more general cognitive profiles, research demonstrates that many individuals with Prader-Willi syndrome exhibit particularly high-level abilities in jigsaw puzzles. Such skills, which on average are way above typical children of comparable chronological ages, are especially shown in those having the deletion—as opposed to the maternal disomy—form of this disorder.

Williams Syndrome

Williams syndrome is caused by a micro-deletion on chromosome 7. Children with this syndrome generally show a characteristic, “elfin-like” facial appearance, along with heart and other health problems (Dykens, Hodapp, and Finucane, 2000). As many as 95 percent of these children suffer from hyperacusis, or a hypersensitivity to sound. Along with an almost overly social, outgoing style of personality, children with Williams syndrome also show a wide variety of anxieties and fears. Although not every child with Williams syndrome shows any or all of these fears, the vast majority do appear to be overly fearful compared to most children with mental retardation.

Apart from these medical and psychiatric issues, scientific attention has strongly focused on the inter-esting—possibly unique—cognitive-linguistic profile shown by most of these children. Children with Williams syndrome show relative strengths in language; for many years researchers thought that these children might even perform at chronological age levels on a variety of linguistic tasks. Although late-twentieth-century research has found that age-appropriate performance in language occurs in only small percentages of children with Williams syndrome (Mervis, Morris, Bertrand, and Robinson, 1999), these children nevertheless show relative strengths in language, as well as in auditory processing and in some areas of music. Conversely, many children with Williams syndrome perform especially poorly on a variety of visuo-spatial tasks.

Taken together, these patterns of strengths, weaknesses, and maladaptive behaviors have led to calls for interventions that might be tailored to different etiological groups. If, for example, persons with Williams syndrome are social and show relative strengths in language, they might benefit greatly from various group and “talk” therapies (Dykens and Hodapp, 1997). In Prader-Willi syndrome, strict supervision around food and eating has historically proven effective, but various drug regimens (mainly involving growth hormone) and clinical interventions (for obsessions and compulsions) are increasingly being tried. Educationally, too, calls have recently been made for etiology-based interventions (Hodapp and Fidler, 1999). In England, for example, S. Buckley (1999) has long advocated reading instruction for children with Down syndrome. The idea is that if for most of these children visual skills are relatively strong and language skills relatively weak, then reading may help them as an entryway into language. Although few such suggestions have so far been tested, future work promises to specify etiology-based intervention proposals and to evaluate if indeed such strategies are more effective than contemporary, more generic intervention approaches.

Developmental Accounts of Mental Retardation

Despite the extreme variability of cause in retardation, various researchers have for many years examined development in children with mental retardation (see articles in Burack, Hodapp, and Zigler, 1998). Such studies have generally focused on two related topics. In the first, studies examine the sequences or orderings in development. With only a few exceptions, most children develop in the universal orderings of development in most cognitive and linguistic domains.

A second developmental concern relates to the structure of development, or the ways in which developments in various domains go together. Although individuals with cultural-familial mental retardation likely show equal or near-equal levels of abilities across a variety of cognitive-linguistic domains, such does not appear to be the case in several genetic mental retardation disorders. Thus, children with Down syndrome show worse-than-MA-level performance in linguistic grammar, those with Prader-Willi syndrome show relative strengths in simultaneous processing (and weaknesses in sequential, step-by-step processing), and those with Williams syndrome display relatively strong language and weak visuo-spatial skills. Furthermore, in many cases, this pattern of strengths and weaknesses becomes more pronounced with age, such that children showing slight patterns of strength-over-weakness at early ages show more extreme patterns as they get older.

Until the mid-1990s, some researchers felt that such patterns provided evidence for “modular” skills in several genetic mental retardation disorders. In Williams syndrome, for example, some researchers held that language occurred at normal, age-equivalent levels and was separate from other areas of cognition. Later studies have generally proven such notions to be incorrect. Specifically, only about 5 percent of children with Williams syndrome show age-equivalent levels of language, and language, while a relative strength, appears connected to at least certain other cognitive abilities. Thus, in Williams syndrome (as with typically developing children), abilities in auditory short-term memory are strongly related with abilities in linguistic grammar. Such studies, which continue in many of these syndromes, promise to tell researchers both about strengths and weaknesses and inter-domain connections within each specific syndrome, as well how various aspects of cognition and language might be organized in nonretarded persons.