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<div><p>In recent years, powerful new forms of influence have been discovered that the internet has made possible. In the present paper, we introduce another new form of influence which we call the “opinion matching effect” (OME). Many websites now promise to help people form opinions about products, political candidates, and political parties by first administering a short quiz and then informing people how closely their answers match product characteristics or the views of a candidate or party. But what if the matching algorithm is biased? We first present data from real opinion matching websites, showing that responding at random to their online quizzes can produce significantly higher proportions of recommendations for one political party or ideology than one would expect by chance. We then describe a randomized, controlled, counterbalanced, double-blind experiment that measured the possible impact of this type of matching on the voting preferences of real, undecided voters. With data obtained from a politically diverse sample of 773 eligible US voters, we observed substantial shifts in voting preferences toward our quiz’s favored candidate–between 51% and 95% of the number of people who had supported that candidate before we administered and scored the quiz. These shifts occurred without any participants showing any awareness of having been manipulated. In summary, in the present study we show not only that OME is a large effect; we also show that biased online questionnaires exist that might be shifting people’s opinions without their knowledge.</p></div>