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Orne and Holland (1968) stated that it was lacking internal validity. \n they claim that the participants simply did not believe the shocks were real as despite the fact that the learner cried out in pain, the researcher remained cool and distant. \n they suggest that participants guessed the purpose if the experiment, therefore demonstrating demand characteristics, this is why so many of the participants were prepared to administer all of the shocks.
it could therefore be argued that Milgram's study lacks internal validity as it may not have measured the participant's normal behaviour but rather how they thought the experiment wanted them to behave.