 The first challenge for this view is to explain why the mere enjoyment of benefits, without some prospective agreement to respond in kind, obligates individuals to help others. Presumably, your doing a nice thing for me yesterday, without my knowledge or invitation, does not obligate me to do you a good turn today. This concern seems even greater with respect to pediatric research. Children certainly benefit from previous research studies, but typically do so unknowingly and often with vigorous opposition. The example of pediatric research makes the further point that justification of clinical research on straightforward contractualist grounds will be difficult at best. Contract theories have difficulties with those groups, such as children, who do not accept in any meaningful way the benefits of the social system under which they live (Gauthier 1990).