There has been an awareness of children with significant problems in learning and development since ancient times. Individuals with less severe impairments were more easily included in agrarian and generally nonliterate societies, but more impaired children were typically viewed as a burden (Trent, 1994). With a few exceptions, care was poor and mortality rates were high. Modern interest in what is now termed intellectual disability (ID) began around the Enlightenment period, when issues of nature/nurture and educational practices began to be debated, e.g., as reports of feral (“wild”) children began to appear (Candland, 1993). By the early 19th century, provisions had begun to be made for more humane care, and efforts to foster development began. The first state training schools were founded in the United States and provided sheltered institutional settings, often in rural areas, where cognitive demands were minimized. The delineation of specific medical syndromes associated with specific genetic or environment risk factors also led to considerable research interest in ID (Zigler & Hodapp, 2013).

Although the initial purpose of institutional settings was to provide rehabilitation and humane care, over time such facilities frequently became places for custodial care or worse—a program that has now led to emphasis on providing services within homes and communities. Conditions in these institutions deteriorated, and a series of legal cases and greater awareness of the importance of home and institutional care led to a movement, starting mainly in the 1960s, that focused on deinstitutionalization in favor of home and community care (Rothman, 2017). The passage of the Education for All Handicapped Children Act (Public Law 94-142) (see Dunn, 2013) in 1975 also mandated schools in the United States to provide free and appropriate education to all children, thus providing for inclusion of children with all disabilities in public school settings.

The development of better measures of intelligence and of adaptive skills provided better metrics for assessment and documentation of progress. In this regard, the early work of Alfred Binet was particularly important. Binet developed the idea of the “mental age” by looking at knowledge that was normatively expectable for children at certain ages. Subsequently, the concept of intelligence quotient or IQ (originally produced by dividing mental age by chronological age and multiplying by 100 and now usually derived from standardization tables) made it much easier to compare children of different ages and levels of ability (see Gould, 1996 for an excellent review). Unfortunately, in some instances the notion of IQ,
particularly when it was viewed as fixed or incorrectly and inappropriately assessed, was also misused, especially within the eugenic movement.

There was considerable faith in the IQ as a valid predictor of subsequent development, that is, as a fixed measure, and this led to a number of problems.

As children were followed longitudinally, it became clear that IQ scores did not become particularly stable, in large groups of children, until around the time the child entered school (this is not surprising because IQ tests were originally designed to predict success in school) (Hunt, 1963). Studies conducted in the 1930s and 1940s, often on children in orphanages or other institutions, began to show significant effects of experience (see Box 6.1). It also became apparent that IQ alone was not an adequate predictor of adult self-sufficiency. An awareness of the importance of appropriate self-care or “adaptive” skills in real-world settings (essentially “street smarts”) led the psychologist Edgar Doll to develop the Vineland Social Maturity Scale. The current version of this scale continues to serve as an important tool in the assessment of children with ID (Sparrow et al., 2016). Importantly, in contrast to IQ, adaptive skills can be readily taught, and, as will be discussed shortly, it is clear that adaptive skills have become a major predictor of adult outcome in ID.

Another line of work centered on the association of intellectual deficiency with specific patterns of dysmorphology and developmental features and led to the specification of specific syndromes of ID that we know today. These often emerged well before an awareness of the importance of human genetics or specific mechanisms, but such conditions continue to be valid until today (Jones et al., 2013). A good example is the work of Dr. Langdon Down, who reported a syndrome (now usually referred to as trisomy 21, or earlier as Down syndrome), the result of a trisomy of chromosome 21. At the time of his report, Dr. Down, of course, had no notion of chromosomes. Although his initial theoretical concept of the condition (i.e., reversion to an earlier and inferior evolutionary type) is very fundamentally flawed, his clinical observation has been remarkably robust.

Since the 1960s, a series of court cases and legal and social initiatives began to change the care of individuals with ID significantly. There has been an increasing tendency to avoid institutional placements and segregated education settings in favor of community-based school and home care. For more severely cognitively impaired individuals, this is, of course, more challenging, and special programs within school or in more segregated settings are sometimes needed. It is important to mention noteworthy variations in practice from state to state within the United States. As will be discussed shortly, important challenges for both medical and mental health care remain to be addressed—particularly in adults with ID (Whittle et al., 2018).

Intellectual disability has had many previous diagnostic labels, mental retardation being the most recent, and earlier on terms like feebleminded, idiot, or cretin were used. The current diagnostic approaches (DSM-5) all define the condition essentially on the basis of the combination of subnormal intellectual functioning (see Harris, 2013), typically identified as a full-scale IQ score of 70 or below and commensurate deficits in adaptive functioning with onset before 18 years. The concept of intelligence is generally recognized to include a series of mental abilities involving problem-solving, abstract thinking, comprehension and expression of ideas, academic learning, planning, and learning from experience and so forth (Gottfredson, 1997). A number of well-developed tests of both intelligence and adaptive functioning are now available, as are guidelines for assessment and treatment (see Figure 6.1). (Siegel et al., 2020)

The current approaches in DSM-5 (APA, 2013) attempt to de-emphasize reliance on IQ by mentioning it in text rather than criteria and to emphasize the importance of clinical assessment and the importance of various areas of functioning (conceptual, social, and practical). As in the past, various levels of ID have been specified: mild (IQ 50-70), moderate (IQ 35-49), severe (IQ 20-34), and profound (IQ < 20). Flexibility is allowed for clinical judgment. Most persons with ID in childhood are those with mild ID (about 85% of cases); the remainder of cases are comprised of those with moderate (about 10%), severe (about 4%), and profound (1%-2%) ID. In the past, the distinction was made between educable (IQ 50-70) and trainable (IQ < 50).

This distinction had an important function in that persons with mild ID often have psychiatric difficulties that are fundamentally similar (if generally more frequent) to those seen in the general population; this is not true for more severely impaired persons (Bouras & Holt, 2007). As noted in DSM-5, many individuals with mild ID can live independently as adults with minimal supports. Those with moderate ID may be independent but may also be semi-independent, needing higher levels of support. Persons with severe and profound ID need significant levels of support, the most disabled needing round-the-clock care. It is also the case that specific medical conditions associated with ID are more likely in the group with an IQ lower than 50, whereas poverty and lower socioeconomic status are more frequent in the group with mild ID (Volkmar et al., 2018). The proportion of persons with severe and profound ID is higher than would be expected given the normal curve, reflecting the impact of genetic disorders and severe medical problems on development (see Figure 6.1). Boxes 6.2 and 6.3 provide brief clinical examples of levels of ID.

There are important reasons for using both subnormal intellectual functioning and deficits in adaptive behavior in the definition of ID. If the IQ criterion alone is used, the expectation, based on the normal curve, would be that about 2.3% of the population should exhibit ID. In reality, this number is more reflective of children than adults. To some extent this reflects the fact that for some cognitive gains can occur and the IQ is not as fixed as was once thought and, importantly for many adults, gains in adaptive (real-world) skills mean that the person can be independent or semi-independent if the adaptive criterion is included. Figure 6.2 summarizes the differences in rates on the basis of whether or not both IQ and adaptive deficits are used to identify cases in the classic studies of Rutter and colleagues on the Isle of Wight (Rutter et al., 1976).

For severe ID, prevalence ranges from 3 to 4 cases per 1000, and for mild ID rates range from 5 to 10 children per 1000. More boys than girls are affected. Age of diagnosis varies depending on the severity of the cognitive deficits and parental awareness of normative developmental abilities. Thus, profound or severe ID is likely to be recognized early in life,
whereas mild ID frequently goes unrecognized until learning difficulties are noticed in school. It should be noted that various risk factors such as prematurity, exposure to environmental toxins, as well as genetic factors can pose a risk and that there can be complex interactions between environmental and genetic factors as well (Huang et al., 2016; Simonoff et al., 1996). There are complex interactions of poverty and ID, for example, greater exposure to lead-based paint products in lower income families. For many known genetic conditions, prenatal testing can be used. At the same time, more precise methods for genetic testing have range also revealed some individuals with specific genetic syndromes who function above the ID.

The degree to which experience (nurture) or biology (nature) is involved in the pathogenies of ID has been a matter of considerable debate. A considerable body of work has focused on the presence of genetic factors or early adverse neurologic experience, for example, prematurity and obstetric risk (Huang et al., 2016). Particularly for individuals with more severe ID, medical causes are much more likely to be found—probably in about 50% of cases. The range of risk factors associated with ID is extensive, for example, from alcohol or toxin exposure in utero, to problems of early hypoxic events and genetic factors. Often, physical examination looking for the presence of specific dysmorphic features gives important clues to etiology, and an excellent clinical guide for this is available (Smith et al., 2013). For the group of individuals with mild ID, the role of experience, that is, social-cultural factors, may predominate (Zigler & Hodapp, 1986). In practice, these issues are difficult to disentangle. Clearly, some cases of mild ID undoubtedly represent the tail end of the normal, Gaussian distribution of intelligence. In many respects, work on the etiology of ID seems poised to enter a new era, with advances in genetics and neuroscience offering the potential for new models and theories (Dykens et al., 2000). Over 1000 genetic etiologies have now been identified, and this number will undoubtedly increase. Table 6.1 summarizes some of the specific forms of ID and their etiology and relevant clinical features.

Advances have also occurred in the approach to behavioral characterization, and these features may, in turn, contribute to our understanding of gene or brain function. As noted in Table 6.1, some syndromes feature unique psychiatric vulnerabilities and thus hold promise for helping to identify pathways to these psychiatric end points. There has been increased interest in relating psychiatric and behavioral difficulties to specific genetic or other etiologies. For example, the hyperphagia and compulsivity in Prader-Willi syndrome, attentional and social problems in fragile X syndrome, inappropriate laughter in Angelman’s syndrome, the unusual cry in 5p-syndrome, and the self-hug in Smith-Magenis syndrome (see Dykens et al., 2000). Sometimes, aspects of syndrome expression can be related to the genetic vulnerability underlying the condition, that is, the severity of ID in fragile X syndrome and the type and severity of maladaptive behaviors in Prader-Willi syndrome. Furthermore, some of these connections between genetic disorder and behavioral outcome appear unique to a single syndrome, whereas others are “shared” between two or more syndromes. Thus, in some instances, features are relatively syndrome specific, such as the unusual hand-washing stereotypies of Rett’s syndrome or the extreme hyperphasia in Prader-Willi syndrome. More often, however, features are shared in two or more conditions. Thus, attentional problems are frequent in fragile X and Williams syndrome. Understanding the nature of “dual diagnosis” is of considerable interest given its potential for shedding light on fundamental mechanisms of psychopathology (Hodapp, 1997; Mazza et al., 2020). These issues are, however, complex because the causal direction of most risk factors is unclear. Poor peer or social relations, for example, may be a precursor of psychopathology or a consequence of disruptive behavior. Other complications that increase risk include the presence of seizure disorders and impairments in sensory or motor impairments among persons with ID. For some conditions, specific genetic/biochemical or neurologic anomalies are associated with specific patterns of unusual behaviors such as severe self-injury. Demonstrable brain abnormalities, for example, as with neuroimaging, become more frequent as the degree of cognitive impairment increases.

Some conditions such as Rett’s disorder are associated with gross brain atrophy, although more subtle difficulties may also be detected—particularly with the technical advances of recent years.