Cognitive impairment was recognized in the ancient and medieval worlds, but little interest was exhibited in pursuing the problem beyond the bare minimums necessary to resolve questions of property rights. Radical conceptual changes in the philosophy of human nature and significant advances in science had to occur before the right questions could be posed. The requisite progress came together at the time of the French Revolution. Victor, a “feral child” found in the woods at Aveyron, was pronounced an idiot by Pinel, one of the founders of modern psychiatry. Itard received one of the first government research grants to attempt to educate this significantly delayed boy. Over a 5-year period, Itard virtually invented the discipline of special education and pioneered much of behaviorist psychology. His student, Seguin, continued working on habilitation techniques and became an influential leader of the new residential school movement when he emigrated to the United States. With few changes, the special education methodologies these two physicians devised to help persons with mental retardation became the foundation for the early childhood education system popularized in the first half of the twentieth century by a third physician, Maria Montessori.

In the early nineteenth century, Esquirol differentiated “imbeciles” from three classes of more severely limited “idiots” by their functional ability to use language. In 1877, Ireland published the first modern medical textbook on mental retardation, On Idiocy and Imbecility, in which he proposed an etiologic classification that would remain valid for most of the next century. Ireland’s book also publicized the 1866 description by Down of one of the first specific mental retardation syndromes. However, until 1959 when Lejeune identified the specific chromosomal trisomy, difficulty in clinically differentiating Down syndrome from cretinism and other disorders persisted. The pseudoscientific eugenics movement of the later nineteenth and early twentieth centuries precipitated sterilization laws and euthanasia practices and distorted the initial idealism of the educational movement pioneered by Itard and Seguin. The flowering of the age of syndrome identification and molecular genetics in the latter half of the twentieth century somewhat redeemed the contribution of scientific genetics to the study of intellectual disability.

Major progress in the medical and behavioral areas depended heavily on the development of well-designed psychometric instruments for accurate classification. In 1905, the psychologist Binet and the physician Simon published the first standardized intelligence test (later translated into English at Stanford University). This advance ushered in more than half a century of use and abuse of psychometric instrumentation that only partially filled the authors’ original intentions of introducing an objective and unbiased measurement device to replace the subjective and sometimes arbitrary opinion of the classroom teacher and untrained professionals. The pediatrician and psychologist Gesell was the first to extend the quantification of development to the period of infancy and early childhood, and his assessment approach avoided some of the major pitfalls of IQ mismeasurement: (a) test scores did not stand alone but acquired meaning only within the broader context of past history, biological risk and repeated assessments, and (b) the dissociation between the various streams of development was used to focus the medical evaluation.