Perhaps the most important principle built into ethics codes is the right of the participant to refuse to be in the study or discontinue participation at any time. Ethical researchers, therefore, cannot coerce participants to agree to be in the study or prevent participants from discontinuing the study. When participants are children who cannot legally give consent, are incapacitated by a mental or emotional disorder, or have difficulty understanding information or giving consent, there is an even greater obligation on the part of the researcher to make sure that participants' rights are protected.

We still need to ask those who may not legally be able to give consent if they wish to participate in the study; we call this obtaining assent. If they say no, their wishes should be respected even if others have given permission for them to participate in the study. Incentives to participate in a study cannot be so great that a reasonable person would feel that he or she could not refuse to participate. This is particularly important when substantial sums of money are offered to participants or in an institutional setting in which even small rewards have great value.