why did southam choose prisoners as test subjects?

Brief Explanations:

Prisoners were often seen as a captive and accessible population in unethical medical - research history. They may have been considered more likely to comply due to their confined status and potential lack of alternatives. Also, researchers may have wrongly assumed they had less social power to object or seek redress.

Answer:

Prisoners were a captive and accessible population, potentially more compliant due to their situation and perceived lack of power to object.