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...