To be sure, saying no to a police officer is different from a saying no to an experimenter in a laboratory study. So we also tested whether people underestimate the pressure to comply with the police as well.

We recruited a separate group of survey respondents and offered them a monetary bonus if they could predict how often drivers grant consent when stopped by the police. According to traffic data, upward of 90 percent of drivers say yes when the police ask to search their car. But our survey respondents’ average guess was far lower: They thought that only about 65 percent of drivers say yes. Again, people vastly understated compliance.

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This tendency to underappreciate the power of social influence is one of the most enduring and important findings in all of social psychology. In Stanley Milgram’s famous studies on obedience, for instance, research participants were willing to heed an experimenter’s instructions to administer dangerous electric shocks to an innocent, protesting victim.

Mr. Milgram showed that normal people would commit violent acts — not because they were sadists, but because they were loath to disobey an authority figure’s directives. This was a result that no one, including expert psychologists, expected.

Critics of consent searches have also been approaching the issue in the wrong way. Groups like the American Civil Liberties Union have focused on advocating that the police be required to notify citizens of their right to refuse consent, much as the police are required to read custodial suspects their Miranda rights. But this is unlikely to address the psychological factors at play.

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