This report summarizes a body of empirical evidence supporting the conclusion that the preponderance of variability in human behavior and performance is attributable to design factors in the performance environment. This may be considered the central doctrine of human factors science that establishes the scientific basis of human factors as an integral discipline. The report goes on to offer a conceptual interpretation, based on behavioral cybernetic theory, of how and why design factors influence human behavior and performance. [This is not an official Bureau publication. The views expressed are not necessarily those of the U.S. Bureau of Mines.]