Much of our thinking about contemporary social problems reflects tacit presuppositions regarding the operation of heredity and environment in human behavior. These beliefs have important implications for practical decisions. Advances in genetics, psychology, anthropology, and other disciplines have contributed much to a clarification of the operation of hereditary and environmental factors in human development. Thus, it behooves us to bring the presuppositions into the open and reexamine them periodically in the light of pertinent research findings. Failure to understand what a heritability ratio can--and what it cannot--tell us may lead to highly fallacious conclusions, as illustrated by some of the popular misinterpretations of the Jensen article. Heritability ratios are applicable to populations, not individuals; they are descriptive of existing conditions in a specified population and cannot be generalized to other populations or to the same population under different conditions; and they do not indicate the degree of modificability of a trait. The operation of heredity concerning behavior can be described in terms of a continuum of indirectness. In most situations, the appropriate question is not whether heredity or environment operated, nor the proportional contribution of each--both are meaningless questions from the standpoint of the individual. Rather what we need to identify is the specific etiological mechanism or causal chain that led to a given condition. (Author/JM)