The U.S. federal regulations require investigators conducting nonbeneficial research to obtain the assent of children who are capable of providing it. Unfortunately, there has been no analysis of which children are capable of assent or even what abilities ground the capacity to give assent. Why should investigators be required to obtain the positive agreement of some children, but not others, before enrolling them in research that does not offer a compensating potential for direct benefit? We argue that the scope of children's research decision making should be based on the principles of respect for autonomy and nonmaleficence. These principles imply that the threshold for assent should be fixed at 14 years of age, and a dissent requirement should be adopted for all children in the context of nonbeneficial research.