The development of social maturity, language, and arithmetic abilities was studied over a 3-year period in 103 mentally retarded children from high IQ (51-75) and low IQ (26-50) groups in special schools and special classes. Among findings were that the high IQ groups of both special classes and special schools showed much improvement over the 3-year period; and that while the low IQ group in special schools greatly advanced in social maturity, the low IQ children in special classes developed better in language and arithmetic. (Author/SBH)