If a question must be answered that has significant negative psychological impact on the subject, such as in the Stanford prison study, is it more ethical to use human or animal subjects?

Human research is common in the field of psychology, which raises many ethical questions.
1). If a question must be answered that has significant negative psychological impact on the subject, such as in the Stanford prison study, is it more ethical to use human or animal subjects?
2). Are there some research questions that should simply not be evaluated because of the potential for negative psychological impact the research would have on the human subjects?

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