In 1969, the psychologist Robert Zajonc published an article about a curious study. He'd posted a silly-sounding word—either kardirga, saricik, biwonjni, nansoma, or iktitaf—on the front page of some student newspapers in Michigan every day for several weeks. Then he sent questionnaires to the papers' readers, asking them to guess whether each word referred to “something 'good' ” or “something 'bad.’ ” Their answers were consistent, if a little strange: Nonsense words that showed up in print many times were judged to be more positive than those that appeared just once or twice. The fact of their repetition, said Zajonc, gave the words an aura of warmth and trustworthiness. He called this the mere exposure effect.
Maybe you've heard about this study before. Maybe you know a bit about Zajonc and his work. That's good. If you've already seen the phrase mere exposure effect in print, then you'll be more likely to believe that it's true. That's the whole point.
Psychologists have devised other ways to make a message more persuasive. “You should first maximize legibility,” says Daniel Kahneman, who describes the Zajonc experiment in Thinking, Fast and Slow, a compendium of his thought and work. Faced with two false statements, side-by-side, he explains, readers are more likely to believe the one that's typed out in boldface. More advice: “Do not use complex language where simpler language will do,” and “in addition to making your message simple, try to make it memorable.” These factors combine to produce a feeling of “cognitive ease” that lulls our vigilant, more rational selves into a stupor. It's an old story, and one that's been told many times before. It even has a name: Psychologists call it the illusion of truth.
See how it works? A simple or repeated phrase, printed in bold or italics, makes us feel good; it just seems right. For Kahneman, that's exactly what makes it so dangerous.