Several principles guide the selection of subjects for research:

• The burdens of research should be distributed equally among the persons who will benefit from it; therefore, overinclusion or underinclusion of any class of subjects must be justified (see Application for a Public Health Service Grant, PHS 398, for a discussion of research with women and minorities);
• Convenience alone does not justify using a particular class of subjects; the nature of the research should require or justify the use of a particular class of subjects;
• As a matter of social justice, there should be an order in the selection of classes of subjects: adults before children, competent individuals before incompetent individuals, and noninstitutionalized persons before institutionalized persons; in brief the least vulnerable classes of subjects should always be selected;
• Participation in research is voluntary; therefore, researchers are discouraged from using subjects who may have restricted freedom to refuse to participate (e.g., researchers' students, employees, or patients); and
• In order to ensure voluntary participation, subjects should be recruited through general announcements or advertisements rather than through personal solicitation.

(Selection of subjects is also discussed in Chapter 4 of this manual.)