Although there continues to be controversy about the magnitude of genetic and environmental effects on human behavior, it is generally agreed by various scientific fields that individual differences in brain function and behavior must follow the same laws of variability as other human characteristics. Whether or not racial and ethnic group differences in behavior result in part from genetic differences is a matter of less consensus and little direct evidence. The continuing controversy about the role of genetic differences in behavior is fueled by two common misunderstandings: (1) sources of differences among people are confused with the causes of development, (2) the relative standing of an individual in comparison to his peers is confused with his actual level of performance. Thus, the malleability of behavioral development under more and less favorable environments is often underestimated when genetic differences are found to affect the rank order of individuals, as in a correlation coefficient. This paper examines several recent studies of biological families, adoptive families, and twins of various ages. There are three general conclusions: (1) that IQ levels and cognitive skills are malleable, (2) that the relative standing of a person in his intellectual peer group depends more on genetic than environmental differences, and (3) that the major source of differences in intellectual performance between blacks and whites is not genetic differences. The results are discussed in light of contemporary population biology, developmental theory and behavior genetic studies. (Author/AM)