- Prisoners are classified as a vulnerable population in clinical research because their ability to give voluntary and uncoerced consent is often compromised due to their incarceration status.
- They may experience undue influence or coercion from prison authorities or peers, which necessitates additional safeguards to protect their rights and welfare.
- Regulations require extra ethical oversight to ensure their participation is truly voluntary and ethically justified.

Reference: The Belmont Report, Volume 1, Chapter on "Ethical Principles and Guidelines for the Protection of Human Subjects of Research"