The informed consent process is central to the ethical conduct of research involving human subjects and derives from a key ethical principle, "respect for persons". Respect for persons incorporates at least two ethical convictions: first, that individuals should be treated as autonomous agents, and second, that persons with diminished autonomy are entitled to protection.
"An autonomous person is an individual capable of deliberation about personal goals and of acting under the direction of such deliberation. To respect autonomy is to give weight to autonomous persons' considered opinions and choices while refraining from obstructing their actions unless they are clearly detrimental to others. To show lack of respect for an autonomous agent is to repudiate that person's considered judgments, to deny an individual the freedom to act on those considered judgments, or to withhold information necessary to make a considered judgment, when there are no compelling reasons to do so.
However, not every human being is capable of self-determination. The capacity for self-determination matures during an individual's life, and some individuals lose this capacity wholly or in part because of illness, mental disability or circumstances that severely restrict liberty. Respect for the immature and the incapacitated may require protecting them as they mature or while they are incapacitated." (see the Belmont Report
The person who obtains a prospective subject's consent to participate in a research project must not accept the subject's consent unless it is freely given by an informed subject who has sufficient capacity to consent. This duty remains even when a witness is present or when another person(s) has been authorized to assess the subject's capacity to consent.