There is a central issue and unifying theme of this book: the developmental-difference controversy in the area of mental retardation. Stated most simply, this controversy centers around the question of whether the behavior of those retarded persons with no evidence of central nervous system dysfunction is best understood by those principles in developmental psychology that have been found to be generally applicable in explaining the behavior and development of non-retarded persons, or whether it is necessary to invoke specific differences over and above a generally lower rate and asymptote of cognitive development. This controversy is of importance because at least 75% of all those identified as retarded have no evidence of organic brain dysfunction.