In the US, federally funded research institutions have to follow certain rules when doing human subjects research. When I was a researcher the training I took said prisoners generally could not be used as subjects just because they were convenient, and that the two situations where they could typically be studied were cases where prisons, crime, or something else that was inherently specific to prisoners was being studied and cases where some condition very highly over represented among prisoners was what was being studied (the example given was HIV).