The study of ability differences between populations and subpopulations, including cross-cultural studies, remains largely descriptive. It should not be forgotten that correlations between various cultural and environmental factors, on the one hand, and abilities, on the other, are merely descriptive, in much the same way that population means and variances are merely descriptive. Inferences concerning the causes of ability differences between populations depend upon experimental control and behaviorgenetic analysis. The particular hypotheses and the most suitable experimental and genetic methods for testing them may differ markedly depending upon the particular populations and the particular behavioral traits being compared. It is useful to distinguish between cross-cultural studies as contrasted with studies of ability differences among various subpopulations within the same culture. This may involve an arbitrary division of a continuous variable, but at least at the extremes the distinction is obvious. Studying differences between Eskimos and Australian Aborigines seems very cross-cultural. Studying ability differences between Negro and white children born in the same city, attending the same schools, watching the same television programs, speaking the same language, shopping in the same stores, and eating the same foods, does not seem very cross-cultural.