Although the opportunities for experimenting with human groups are somewhat restricted, social scientists and market researchers frequently assemble groups of people in order to ascertain their responses to program material, products, tasks of various kinds, and other situational stimuli. Since the results of such studies are commonly generalized to the broader populations from which the people were recruited, there is a basic question about the effect of the recruitment process on the representativeness of the results of the studies. This article reports the results of an examination of volunteer bias and thereby contributes specific information to the accumulating evidence needed to answer these questions.