Discussed are the theoretical explanations of the observation that low intelligence quotient (IQ), low socioeconomic status children appear to be brighter in certain ways than low IQ middle class youngsters. The two different theories on IQ as a function of socioeconomic status--environmental or cultural vs. genetically determined biological potential factors--are evaluated. Also presented is a discussion of the importance of cultural bias in tests and of the various correlations of IQ and learning tests. It is felt that "heritability" offers a more useful criterion to account for the extent of the cultural loading dimension. Postulated is the simplest possible model to explain IQ differences, a hypothesis which formulates two types of mental processes, Level I (associative learning) and Level II (abstract problem solving and conceptual learning). Empirical findings are then more understandable when three further hypotheses are considered--(1) there is a genotypic independence of Level 1; and (3) genotypes are differentially distributed in upper and lower social classes. The practical educational consequences of this theory are briefly mentioned. (NH)