The milgram obedience experiments were controversial because the

"teachers" were deceived and frequently subjected to stress.

Explanation:

The Milligram's experiment consisted of participants or "teachers" that were told to administer an electric shock every time the "learner" (that teachers were not conscious that were not actual participants) made a mistake, this electric shock was stronger each time and when the teacher stopped he was instructed to continue. This experiment was controversial because teachers were deceived and pushed to keep on going.

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