The effects of family background on adolescents' IQ, aptitude, and school achievement test scores challenge some of the usual beliefs about the fairness of achievement rather than IQ tests, and the role of genetic differences among individuals and social class groups in academic achievements. Subjects included 115 adoptive families with adolescent children, adopted in the first few months of life, and a comparison sample of 120 biological families with their own adolescent offspring. Families ranged from solid working to upper middle class. Regressions of adolescents' IQ, aptitude, and achievement test scores on measures of family background revealed that differences among social class environments have little effect on IQ scores but larger effects on school achievement scores. Working class adolescents are at a greater disadvantage relative to upper-middle class adolescents when the tests sample recently taught material. Genetic differences among individuals and among social class groups were found to be approximately the same for all types of tests. Correlations of related and unrelated siblings reared together suggest that the heritabilities of IQ and school test scores are in the range of .2 to .6 in a white population of working to upper-middle class families. (Author/RL)