The participants were asked whether they had ever stolen anything, lied or taken drugs. Some were told that their answers would be published in a research bulletin, others were asked for explicit permission to publish those answers, and still others were asked for permission to publish the answers as well as their age, sex and country of birth.

The results revealed the imperfection of human reasoning. Those who were offered the least control over who would see their answers seemed most reluctant to reveal themselves: among them, only 15 percent answered all 10 questions. Those who were asked for consent were nearly twice as likely to answer all questions. And among those who were asked for demographic information, every single person gave permission to disclose the data, even though those details could have allowed a complete stranger a greater chance of identifying the participant.

Mr. Acquisti took note of the paradox: fine-grained controls had led people to “share more sensitive information with larger, and possibly riskier, audiences.” He titled the paper, which he wrote with his colleagues Laura Brandimarte and George Loewenstein, “Misplaced Confidences: Privacy and the Control Paradox.”

“What worries me,” he said, “is that transparency and control are empty words that are used to push responsibility to the user for problems that are being created by others.”

That sense of control can be undermined in other ways, too, principally by distractions: they apparently play the most powerful tricks of all.

In a study called “Sleights of Privacy,” Mr. Acquisti’s subjects — students at Carnegie Mellon — were divided into two sets of two groups. Each group was asked to evaluate professors and were given additional questions about cheating. In the first set, half were told that only other students could see their answers; the others were told that faculty members, as well as students, could see the responses. As one might expect, the group with student-only viewers was more forthcoming than the group with student and faculty viewers. The participants seemed concerned about who could see their evaluations.

With the other set of students, Mr. Acquisti offered the same questionnaire — but played a little trick. After again explaining the response rules and procedures, he asked an unrelated question: Would they like to sign up to receive information from a college network? That little distraction had an impact: This time, the two subgroups were almost equally forthcoming in their answers.