Coerced Sample Pools

Every college, university, and research hospital has what is called an institutional review board to oversee every proposed experiment involving human subjects, screening them for ethical problems or concerns. This all started with the uproar over Stanley Milgram's experiment in the early 60s showing that people will obey authorities, even when it means causing harm to others that one would ordinarily not consider oneself capable of causing. As a result, there is now a set of stringent policies in place regarding treatment of subjects, confidentiality of data, and how long one can keep one's samples and records before destroying them out of concern for the privacy of those contributing to the experiment.

Yet, despite the psychological experiment that started it all off, an experiment that is taught in every psych 101 class, psychology departments often have a policy for those very 101 students that they have to participate in some number of experiments run by members and other students in the psychology department. When I took psych 101 in college, it was five experiments throughout the semester. We were always read the post-Milgram statement that we were free to leave the experiment at any time if we felt uncomfortable. but how free were we really? After all, it was a Milgramesque authority who told me to go there in the first place. I need five of these to pass this class, am I really going to walk out? Would it get back to my prof? Would it cause harm to my grade?

Is there an ethical problem with this sort of sample pool or is the facts that the experiments are vetted by the IRB and students can leave sufficient to overcome the problem?