In 1968, social psychologist Robert Zajonc published a paper that challenged one of the most deeply held assumptions in marketing, politics, and persuasion: that people’s preferences are primarily determined by the objective qualities of what they prefer. Zajonc’s research demonstrated that mere repeated exposure to a stimulus is sufficient to increase positive affect toward it, independent of any evaluation of its actual qualities. He called this the mere exposure effect, and it has since been replicated hundreds of times across cultures, age groups, and stimulus types ranging from geometric shapes and Chinese characters to faces, words, music, and brands. The...