YOU ONLY USE 10 percent of your brain. Eating carrots improves your eyesight. Vitamin C cures the common cold. Crime in the United States is at an all-time high. None of those things are true. But the facts don’t actually matter: People repeat them so often that you believe them. Welcome to the “illusory truth effect,” a glitch in the human psyche that equates repetition with truth. Marketers and politicians are masters of manipulating this particular cognitive bias—which perhaps you have become more familiar with lately.