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Biological and Social Factors Contnbut~ng to Mild Mental Retardation JACK P. SHONKOFF The concept of mental retardation has eluded clear definition for centu- r~es. From the simplistic moralisms of preindustrial times to the complex "scientific" determinations of contemporary societies, the mentally re- tarded population has been to a great extent a cultural creation. As social and economic demands have changed, so have the names and the charac- teristics of the categories of intellectual deficit. The debate over the relative etiological contributions of biological attri- butes in the individual, both inborn and acquired, and sociocultural factors in the environment has raged fiercely. It assumes particular significance in American society today with regard to the phenomenon of mild mental retardation. This paper provides an overview of recent research in areas directly relevant to these issues, formulates the current state of the art, and provides a framework for conceptualizing the available data in their imperfect form. In so doing, it attempts to specifically examine the contri- bution of biological and social factors to the disproportionate representa- tion of minority students and males in education programs for the mildly mentally retarded. HISTORICAL OVERVIEW Shifting criteria for mental defectiveness have clearly mirrored changes in society. In early Western civilizations, handicapped children were fre I am grateful to Ian Canino, C. Keith Conners, Allen Crocker, Leon Eisenberg, Robert Hag- gerty, Jane Mercer, Julius Richmond, and Arnold Sameroff for constructive reactions to an earlier draft of this paper. 133

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134 SHONKOFF quently put to death, and those left to survive were often ostracized and cared for by the clergy (Menolascino, 1977~. Before the development of the industrial revolution and universal public education, almost all of those now categorized as mildly retarded were undoubtedly indistinguish- able from the general population. In medieval England, for example, a person merely had to be able "to count twenty pence, to tell one's age, and to name one's parents" in order to avoid designation as an idiot and thereby retain the right to the profits of his own property (Kirman and Bicknell, 1975:51. In the aftermath of the political consciousness of individual rights stirred up by the American and Prench revolutions in the 18th century, attention began to be directed toward the human needs of the mentally handicapped. During much of the 19th century, medicine greatly influenced the societal response to the problem of mental deficiency. While detailed classifica- tions of brain pathology were being compiled by such eminent neurolo- gists as Jean Martin Charcot, the possibilities of education for the "feeble- minded" were being championed by such physicians as Edward Seguin (Blanton, 19751. In an era when universal public education was viewed in the United States as a solution to the growing social problems associated with the industrialization, urbanization, and ethnic diversity resulting from increased immigration from Europe, institutions for the feeble-minded were established in a spirit of educational optimism, not simply as custodial enterprises. As the belief in the reversibility of significant mental retarda- tion weakened, however, the climate of hope and idealism diminished. With the growth of the intelligence testing movement at the turn of the 20th century came fierce battles over the need to protect society from the threat of its defective members who could now be more readily identified. Inspired by the tenets of social Darwinism, some of the most influential American psychologists of the early 20th century, including such luminar- ies as Lewis Terman, Henry Goddard, and Robert Yerkes, joined well-or- ganized efforts to advance the eugenic philosophy, popularized by Sir Francis Galton, by advocating compulsory sterilization and severe restric- tions on immigration. Terman singled out the mildly mentally impaired as a serious threat to the health of the society. In the first edition of the man- ual for the Stanford-Binet scales, he wrote (Terman, 1916:6-71: Intelligence tests will bring tens of thousands of these high grade detectives under the surveillance and protection of society. It is hardly necessary to emphasize that the high grade cases of the type now so frequently overlooked, are precisely the ones whose guardianship it is most important for the state to assume. Mildly retarded people were feared for their assumed tendencies toward immorality, delinquency, criminality, and the propagation of "defective"

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Biological and Social Factors 135 children who would further dilute the competence and vitality of American society. The residential institutions that originated in a spirit of salvation evolved into bastions of isolation and educational vacuum. In the years following World War II, encouraged by the work of such re- searchers as Heinz Werner and Alfred Strauss, interest in special educa- tion had a rebirth. In the decades that followed, with the increasing mili- tancy of many parents of handicapped children, the dramatic focus in the 1960s on civil rights for victims of institutionalized discrimination, and the critical support given by President Kennedy to the needs of mentally re- tarded persons, a revolution began in the status of the developmentally dis- abled population in American society. The widely held belief in the bene- fits of segregated special education gave way to arguments for normalized "mainstreaming" in the public school system (Dunn, 1968), which culmi- nated in the passage of the Education for All Handicapped Children Act of 1975 (P.L. 94-142). Historically, the problem of the classification of children for educational purposes has been problematic. In England the passage of the Defective and Epileptic Children (Education) Act in 1899 authorized special classes for children who were deemed incapable of performing adequately in ordi- nary classes but who were not seriously enough impaired to be assigned to an institutional setting. The Education Act of 1921 specifically addressed the needs of the mildly retarded by creating a category of mental defect re- stricted to children ages 7-16 and based on educational but not social de- ficiencies (Blanton, 1975~. At the turn of the century, when the French minister of education com- missioned Alfred Binet to develop a test to facilitate the early identification of children who could not meet the demands of regular schooling, the die was cast and the classification of school children was irrevocably altered. Although Binet himself believed in the value of compensatory instruction, his instrument has sometimes been used as a tool for limiting the educa- tional options for intellectually impaired youngsters. The Binet-Simon scales were adapted for use in school systems throughout Europe and the United States. Data obtained in Belgium and Italy revealed significant dif- ferences in scores related to social class, and eminent cultural anthropolo- gists argued that this "scientific" concept of measured intelligence was very much culturally determined (Blanton, 1975~. In the United States, revisions of the Binet scales were developed by Goddard, Kuhlman, and Terman, and the history of the use of these and other intelligence tests for the educational classification of children has been rich and controversial. At the heart of much ongoing debate has been the conflict between the "scientific," quantitative data obtained from stan- dardized tests and the practical matter of educational classification and

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136 SHONKOFF class placement, which is always affected by social values, attitudes, and beliefs. The changing nature of these values has been reflected in the changing definitions of mental retardation. In a presentation to the Na- tional Education Association in 1910, Goddard defined a "subnormal child [as] one who is unable to do school work at the usual rate, or any child who is behind his grade" (Goddard, 1910:242~. He suggested the fol- lowing classification (p. 242~: The temporarily subnormal ... whose backwardness is due to sickness, physical impairment, or unfavorable environment, [and the] permanently subnormal or "feeble-minded" which consists of three subgroups "idiots" twho] are totally ar- rested before the age of three, [the "imbeciles" who] become permanently arrested between the ages of three and seven, [and the "morons" who] become arrested be- tween the ages of seven through twelve. Little attention was paid to individual differences in the mentally retarded population. Generally speaking, a simple qllantitat*e concept of back- wardness was accepted in educational circles, and similar curriculum ma- terials were applied for a variety of children with diverse learning handi- caps. It was not until Werner and Strauss (1939) began to talk about the importance of functional analyses of individual strengths and weaknesses rather than standardized test scores that the concept of mental retarda- tion as a homogeneous condition was seriously challenged. Their popular- ization of the notions of endogenous (familial) and exogenous (secondary to prenatal, perinatal, or postnatal brain insult) mental retardation ushered in a new era of special education and laid the foundation for many of the modern concepts of specific learning disabilities. In 1953 a committee of the World Health Organization defined mental deficiency as incomplete or insufficient general development of the mental capacities secondary to biological factors and defined mental retardation as the same condition secondary to social factors. The upper boundary of deficit was conventionally defined as two standard deviations below the mean on a standardized intelligence test. In 1959 the American Association on Mental Deficiency (AAMD) pro- posed a system of classification that included a requirement for assessing adaptive behavior and created the category "borderline retardation" for those individuals with "subaverage intellectual functioning" as defined by a test score of between one and two standard deviations below the mean. Among the novel features of this model were its emphasis on current level of functioning and its focus on individuals whose deficits are manifested during the developmental period (Heber, l9S9~. In 1973 the AAMD announced that "since 1959 numerous changes have

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Biological and Social Factors 137 taken place in the field and in the society which necessitate a new manual to reflect the knowledge and philosophy of the seventies" (Grossman, 1973:4-5~. This new definition of mental retardation, which is still cur- rent, required "significantly subaverage general intellectual functioning," which was defined as two standard deviations below the mean, thereby eliminating the category of "borderline retardation." In their acknowl- edgment of "changing concepts regarding the social capability of persons with low intelligence" (p. 5), the AAMD arbitrarily transferred a segment of the mentally retarded population back into the "normal" fold with a simple stroke of the pen. As observed by MacMillan et al. (1980:112), "many of the children in a mildly retarded sample study conducted in 1965 would be 'nonretarded control' subjects today if they achieved an IQ of 75 to 85." Diagnostic systems for retardation have changed in their conceptual as well as their quantitative dimensions. They have alternately stressed the functional interests of psychometricians and educators and the etiological curiosities of the medical profession. Perhaps the best analysis of the dif- ferential impact of diverse models of diagnosis is that of Mercer (1971~. She defines the clinical perspective as one that considers retardation to be an intrinsic handicapping condition. The current AAMD definition re- flects this perspective. It is a statistical and pathological model designed to serve the needs of the helping professions, e.g., medicine, psychology, and education. The clinical perspective implies that a person who fits the criteria is in fact mentally retarded, even if no one is aware of that fact and a definitive diagnosis has not been made. The social system perspective, by contrast, implies that the status is assigned to an individual within a specific social milieu. The implication of this sociological model is that a person is in fact mentally retarded only when he or she is designated as such by a social system and therefore is perceived that way by its other members. Generally speaking, the school has traditionally been the sys- tem that most frequently assigns the social status of mental retardation. It is therefore critical that we gain greater insight into the factors that con- tribute to those administrative decisions that can so dramatically affect children's lives. The need to recognize that we are dealing with values and not objective truths is an important beginning. In summary, the concept of mental retardation is fluid and defies perma- nent definition. In its mild manifestations, it is less a vehicle for understand- ing those people whom it labels than a mirror of the society that determines its boundaries (Sarason and Doris, 1979~. In this context of uncertainty this paper explores the data regarding the biological and social roots of mild retardation.

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138 EPIDEMIOLOGY OF MILD MENTAL RETARDATION SHONKOFF In view of the continually changing definition of the mildly retarded popu- lation, it is not at all surprising to discover that this group is very difficult to count. Indeed, the search for valid epidemiological data has been fraught with frustration and inevitable limitations. Some of the confounding factors are related to methodological difficulties, while others are inherent in the chameleonlike nature of the condition itself. TYPE S OF DATA Two types of data have been the focus of study: incidence and prevalence rates. Incidence refers to the number of new cases of a condition that occur in a given time interval. These kinds of data have been particularly prob- lematic for the study of mental retardation because of the difficulty in de- termining the point at which the condition begins to exist. For children whose diagnosis is specific and unequivocal (e.g., Down's syndrome), this question has been relatively easy to answer. For the mildly retarded popu- lation, however, the point at which the diagnosis may appropriately be made is often difficult to ascertain. The empirical observation that an indi- vidual may move in and out of the mildly retarded category further clouds the usefulness of incidence data. Prevalence refers to the number of individuals who have a given condi- tion at a specific point in time. Although they are related to incidence data, prevalence rates are affected by the duration of a condition and are there- fore lowered by the removal of persons from the target population through death, "cure," or diagnostic revision. This paper focuses primarily on prevalence data, as these numbers are the most relevant for defining and planning intervention services. LIMITATIONS OF THE DATA The most fundamental dilemma is clearly related to the absence of a con- sistent definition of mild retardation. Whereas moderate and severe men- tal retardation have been relatively easy to identify, regardless of changing nosologies, the boundary between "mildly defective" and "low normal" re- mains ambiguous and tentative. As discussed above, diagnostic criteria have been altered as the values of the society have changed, and it is likely that further modifications will be developed in the future. Moreover, the present emphasis on concurrent adaptive behavior requires consideration of abilities that have traditionally eluded reliable and valid quantification. In the absence of a permanent, universally acceptable definition, it is

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Biological and Social Factors 139 not at all surprising that much of the available epidemiological data on all levels of mental retardation have been significantly influenced by the era during which they were collected, the target groups studied, and the disci- plinary orientations of the investigators. Clinical and school populations, for example, are not at all comparable. On one hand, medically based studies are generally skewed by populations with a disproportionate num- ber of "patients" with medically diagnosable conditions characterized by abnormal neurological signs and well-described clusters of findings (syn- dromes). Educationally based studies, on the other hand, understandably rely heavily on classifications related to school placement and pedagogical strategies. Thus, in some instances, a reported low prevalence of mental retardation may simply reflect limited resources for special education or a strong commitment to "mainstreaming" and individualized instruction; alternatively, a high prevalence rate may reflect artificially inflated figures designed to secure increased funding for service programs. MacMillan et al. (1980) examined the implications of these variations related to the sources of data for the planning and interpretation of relevant research. They differentiated between the mission of the school (which is to deliver education services) and the mandate of the psychological researcher (which is to build a model of retardation based on scientific rigor). The former is heavily influenced by variations in teacher behavior regarding re- ferrals, differences in the way those referrals are screened, and the range of alternative placements and education options available within each school system. The latter should be characterized by strict adherence to objective and highly reproducible data. Consequently, meaningful comparisons among studies clearly require explicit information on the criteria for selec- tion of each target group. Sociological and anthropological investigations have employed yet an- other framework whereby retardation is defined in terms of a broad ecolo- gical analysis of social status within a specific cultural milieu. Robinson (1978) noted that the reported prevalence of mild mental retardation in the People's Republic of China is essentially zero; their technologically unso- phisticated society places minimal value on individual achievement and maximal emphasis on social cohesion and mutual support. In Sweden, where industrial modernization and emphasis on achievement are more ev- ident, the reported prevalence of mild retardation is also relatively low, in part because of social acceptance of educational mainstreaming of intellec- tually limited children (Grunewald, 1979~. In both countries the preva- lence of mental retardation at all levels is significantly lower than reported in most studies because they primarily consider the severely impaired. From the clinical perspective, the mildly retarded have been overlooked; from a sociological perspective, they do not exist as a discrete group.

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140 SHONKOFF In addition to the problems of disciplinary variation and changes over time in the definitions employed, methodological rigor within disciplines and contemporary studies has been wanting. The bulk of the epidemiologi- cal literature does not conform to the AAMD requirement that a diagnosis of mental retardation be based on well-standardized measurement of both adaptive and intellectual deficits. Smith and Polloway (1979), for example, found the inclusion of adaptive behavior measures in less than 10 percent of the recent research efforts that they reviewed. Cleland (1979) reported that many studies mismatched individuals' test scores with the appropriate level of retardation. In an analysis of 566 articles in the American Journal of Mental Deficiency and Mental Retardation from 1973 through 1979, Taylor (1980) found that only 28 percent included terminology consistent with the AAMD classifications, confirming Cleland's assertions by demon- strating that almost 20 percent of the studies he reviewed included subjects who had been inappropriately classified based on data presented in the article itself. Interpretation of such information clearly presents major problems. The variety of data-collection methods employed has contributed addi- tional confusion to the literature. Lemkau et al. (1942) studied the preva- lence of mental disorders in Baltimore, Maryland, through an examination of the records of community and state agencies. Bremer (1951) surveyed the entire population ( 1,300 people) of a small Norwegian fishing village through interviews and personal observations. Wishik (1964) studied two Georgia counties through a combination of a communitywide campaign to solicit voluntary referrals and a canvass of 10 percent of the households in the area. Lapouse and Weitzner (1970) reviewed these and nine other epi- demiological studies, whose case-finding mechanisms ranged from reviews of school and other agency records to sample surveys, interviews with key community informants, and individual testing by the investigators them- selves. The prevalence rates for all levels of mental retardation generated by this wide variety of methods ranged from a low of 3.4/1,000 to a high of 77.0/1,000. When broken down by severity, the percentages of mild retar- dation within each group ranged from 63 to 92 percent, with a median of 80 percent. Clearly, the limitations of the available epidemiological data are formidable. With these caveats in mind, we now examine the numbers. PREVALENCE OF MILD MENTAL RETARDATION If intelligence were, in reality, normally distributed on a Gaussian curve, the prevalence of all degrees of mental retardation would be 2.28 percent. In fact, however, this is not the case. Several explanations have been of

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Biological and Social Factors 141 fered to identify the reasons for the empirically observed variations from the statistically predicted rates. Tarjan et al. (1973) have asserted that the true prevalence of mental re- tardation is closer to 1 percent. They explain the lower figure largely on the basis of the fact that not all people with IQ scores below two standard devi- ations from the mean have deficits in adaptive behavior (and therefore would not be appropriately classified as retarded). This position is sup- ported by Mercer (1973), who found a prevalence estimate for IQ scores below 70 of 21.4/1,000 in Riverside, California, but a rate of mental retar- dation of 9.7/1,000 when an evaluation of adaptive behavior was added to the diagnostic criteria. Further arguments advanced by Tarjan et al. (1973) to support the lower prevalence figure include the assumption that severely retarded individuals have a shortened life span and the observa- tion that "about two-thirds of the individuals diagnosed as "mildly] re- tarded lose this label during late adolescence or early adulthood" (p. 3721. Rutter et al. (1970) have added another consideration. They report an overall prevalence rate of 2.53 percent (based on IQ scores alone) among the 2,334 children ages 9-11 on the Isle of Wight and note that this con- firmed a slightly higher prevalence than theoretically expected (2.28~o) be- cause of the increased number with severe mental retardation. Given the small absolute number of retarded children in their population (59), the authors did not subdivide their group by levels of severity. The classic studies of Birch et al. (1970) in Aberdeen, Scotland, provide additional data, collected in a somewhat different fashion. Initial preval- ence rates were obtained by ascertaining the number of children (ages 8-10) who were identified as subnormal by the local school authorities and placed in special programs based on evaluation of their social competence, school performance, medical status, and psychometric test scores. These children, whose diagnoses were confirmed after reexamination by the in- vestigators, represented 1.26 percent of the population. Subsequent review of the scores of a psychometric test universally administered at school entry revealed an additional group of children who scored below the cutoff point at age 7 but who were not administratively designated as subnormal in the schools. This group represented 1.49 percent of the population of 8,274 children ages 8-10, giving a best estimate of overall prevalence of mental retardation of 2.75 percent. In the study, 50 percent of the children admin- istratively diagnosed as subnormal had IQ scores of 60 or more, compared with 77 percent of the total group. The authors noted that their prevalence data for Aberdeen reflect the "demands of a modern industrial society with free, universal, and compulsory education and the psychometric screening of virtually all children at 7 years of age" (Birch et al., 1970:9~.

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142 SHONKOFF In summary, valid prevalence rates for mild retardation are hard to come by. The overall prevalence of all levels of mental retardation is likely to be between 1 and 3 percent, with at least three quarters of that group probably falling within the range of mild impairment. Of all the method- ological weaknesses throughout this literature, however, the major factor that sabotages efforts to get better numbers is the problem of definition. If it is true that mild retardation will always be a reflection of contemporary cultural values, and if it is true that the boundary between normality and subnormality is inevitably blurred, the hope for more precise prevalence data is fantasy. VARIATIONS RELATED TO POPULATION SUBGROUPS Despite the problems and disagreements described above, a number of strong relationships have consistently been reported regarding the relative prevalence rates of mild retardation among specific demographic subgroups. SOCIOECONOMIC DIFFERENCES In 1962, The Report to the President of the President's Panel on Mental Retardation noted (p. 9~: Epidemiological data from many reliable studies show a remarkably heavy correla- tion between the incidence of mental retardation, particularly in its milder manifes- tations, and the adverse social, economic and cultural status of families in these groups in our population. These are for the most part the low income groups- who often live in slums and are frequently minority groups where the mother and the children receive inadequate medical care, where family breakdown is common, where individuals are without motivation and opportunity and without adequate ed- ucation. In short, the conditions which spawn many other health and social problems are to a large extent the same ones which generate the problem of mental retardation. The documentation of this phenomenon has been extensive and almost uniformly reproducible, although most reports have not included mea- sures of adaptive behavior. In a 1937 study of educational backwardness in children in the regular public schools of London, Burt reported a fre- quency of greater than 20 percent in the poor districts as compared with 1 percent in the well-to-do areas (cited in Rutter et al., 1970~. The New York State Department of Mental Hygiene (1955) in the early 1950s found a fourfold increase in the prevalence of mental retardation (loosely defined to include a variety of problems) from the highest to the lowest socioeco- nomic areas in Syracuse for children and youth under age 18. Stein and Susser (1969) collected data in the industrial city of Salford in northwest

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Biological and Social Factors 143 England and found very few children with IQ scores between 50 and 79 in school districts with "high social standing," in contrast to large numbers in districts of "low social standing." The Isle of Wight investigations con- firmed the reproducibility of these findings for small-town as well as inner- city populations (Rutter et al., 1970~. In their elegant studies in Aberdeen, Birch et al. (1970) reported a prevalence of mild retardation approaching zero in the upper socioeconomic classes, with an increase in prevalence rates by a factor of two for each step down the class ladder, resulting in a summary conclusion that the prevalence of mild retardation (based on IQ greater than or equal to 60) was nine times higher in the lowest class than in the highest class. When within-class differences were examined, it was found that approximately 91 percent of the lower-class population of re- tarded children were mildly impaired (IQ greater than or equal to 50), while 89 percent of the retarded children in the highest class were moder- ately to severely subnormal (IQ less than 50~. Detailed analysis of the data confirmed the fact that these marked discrepancies were accurate reflec- tions of the prevalence rates based on the diagnostic criteria accepted for the study and were not an artifact related to class differences in adminis- trative identification by the school system. Lapouse and Weitzner (1970) reviewed 12 epidemiological studies that further confirmed this inverse re- lationship between socioeconomic status and prevalence rates for mild retardation. A recent analysis of data on more than 35,000 children from the Collab- orative Perinatal Project of the National Institute of Neurological and Communicative Disorders and Stroke specifically looked at the relation- ship of race and socioeconomic status to the prevalence of mild retardation based on test scores of 50 to 69 on the (WISC-R at age 71. Rates for the white population were 3.34 percent for the lower socioeconomic group (bottom 25 percent), 1.31 percent for the middle group (middle 50 percent), and 0.30 percent for the upper group (top 25 percent), with an overall prevalence of mild retardation for the white children of 1.17 per- cent. Data for the black youngsters revealed a rate of 7.75 percent for the lower socioeconomic group, 3.59 percent for the middle group, and 1.19 percent for the upper group, with an overall rate of 4.83 percent (Broman, unpublished data, 1981~. Many investigators have tended to subsume the demographic character- istics of the lower socioeconomic classes under conceptualization desig- nated as the culture of poverty, which implies a pervasive psychological sense of hopelessness and the inevitability of competitive disadvantage. Others have observed that such a view merely serves as an excuse for policy makers and educators to expect minimal benefits from intervention efforts (Ryan, 1971, cited in Eisenberg and Earls, 1975~. Attempts to analyze var

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Biological and Social Factors 171 of the distribution of mild retardation within the entire population is largely a political task. A word must be said, however, about the distinction between intellec- tual impairment and cognitive differences that contribute to competitive disadvantage within- a specific sociocultural system. IQ tests, with all of their problems, have been shown to do reasonably well in their originally intended function of identifying children who have an increased probabil- ity of failing in school. Thus, although changes in the criteria for making a diagnosis of mild retardation will liberate many children from the stigma of such classification, their performance in a traditional school curriculum is still likely to correspond to their scores on a "standardized" psychomet- ric test. For many children whose life experiences differ from a typical mid- dle-class upbringing, however, discrepancies in test scores and school per- formance may very well reflect a different kind of cognitive ability that does not necessarily imply intelligence. The tyranny of the dominant cul- ture and its power over the standards of educational success will probably continue to undervalue such differences. There is, however, another aspect of this problem, which has its roots in the cultural sphere but extends far into the area of biology-the issue of central nervous system function and brain integrity. Poor and minority children are not the victims of social discrimination alone. A considerable body of data suggests that they also carry a disproportionate burden of bio- logical vulnerability that is largely related to the increased health risks of poverty. Much of the discussion of biological disadvantage among ethnic minorities and lower socioeconomic groups has traditionally focused on the issue of genetic differences in intellectual endowment. Biological dif- ferences in individuals, however, are determined by a great deal more than inherited traits. The developing brain, regardless of its genetic potential, is subjected to a variety of potentially damaging influences throughout its prenatal, perinatal, and postnatal life, which can have adverse effects on its ultimate functioning. Intrauterine factors such as cytomegalovirus and alcohol, complications during the newborn period related to prematurity and/or low birth weight, and early childhood insults such as malnutrition and lead intoxication can all inflict damage on an immature brain result- ing in significant impairment in later intellectual functioning. These threat- ening influences and many others exist with greater frequency among poor and minority populations. The unequal distribution of these risk factors is certainly influenced by social and economic forces, but their existence creates very real, intrinsic biological vulnerabilities in the children who are so afflicted. The ultimate roles of biological and social factors in the etiology of mild

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172 SHONKOFF retardation can best be understood in the context of a transactional model of development applied to a basically resilient human organism. The over- whelming majority of poor and minority children are not retarded. Most low birth weight babies do well developmentally. Of those children who were exposed to noxious agents during their prenatal or postnatal life, some will have impairment of their intelligence and others will appear to escape unharmed. Many of those whose brains have been injured will not demonstrate abnormalities on traditional neurological examinations. Ulti- mate developmental outcome for all children appears to be a function of a highly complex series of transactions among a great number of biological and environmental facilitators and constraints. Intelligence is determined by multiple factors, and its impairment rarely has a simple etiology. Some children are extraordinarily resilient and may have well-developed intellectual abilities despite minimal environmental supports. Others are constitutionally limited and will have significant defi- cits in the face of optimally facilitating experiences. Each child's abilities are dependent on the interplay between his or her biological equipment and life circumstances. Few individuals are without vulnerabilities most manage to adapt reasonably well. The distribution of vulnerabilities within the general population, however, is grossly unequal. Poor and minority children bear a proportionately greater burden of them in both a biological and a social sense. In conclusion, it is clear that mild mental retardation is largely a cul- tural invention and not an objective biological property. It reflects a so- ciety's expectations regarding intellectual performance and is subject to modification as values change. Children whose rearing and environmental resources differ from those of the dominant cultural group are at greater risk for having profiles of abilities that may very well be dysfunctional for the demands of the public school system. One must not underestimate, however, the fact that these same "disadvantaged" groups are victimized by a greater frequency of harmful biological factors that can adversely af- fect brain development in early life and later lead to very real intellectual deficits. Poor and minority children have the highest probability of sus- taining injuries through both nature and nurture. Attempts to assign quantitative weighting to the relative contributions of each are thwarted by the limitations of available data. The synergistic effects of cumulative vul- nerabilities in both spheres undoubtedly contribute to the greater preva- lence of mild retardation in these groups. Thus, the ultimate resolution of these inequities will have to go beyond the very important social battles over evaluation and classification procedures, extending into the realm of maternal and child health.

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