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.

President Trump is a "great businessman," he says over and over again. Some evidence suggests that might not be true. Or look at just this week, when the president signed three executive orders designed to stop what he describes—over and over again—as high levels of violence against law enforcement in America. Sounds important, right? But such crimes are at their lowest rates in decades, as are most violent crimes in the US. Not exactly, as the president would have it, "American carnage."

The effect is more powerful when people are tired or distracted by other information. So ... 2017, basically.

"President Trump intends to build task forces to investigate and stop national trends that don’t exist," says Jeffery Robinson, deputy legal director of the American Civil Liberties Union. He's right that the trends aren't real, of course. But some number of people still believe it. Every time the president tweets or says something untrue, fact-checkers race to point out the falsehood—to little effect. A Pew Research poll last fall found 57 percent of presidential election voters believed crime across the US had gotten worse since 2008, despite FBI data showing it had fallen by about 20 percent.

So what's going on here? "Repetition makes things seem more plausible," says Lynn Hasher, a psychologist at the University of Toronto whose research team first noticed the effect in the 1970s. "And the effect is likely more powerful when people are tired or distracted by other information." So ... 2017, basically.

Brain Feels

Remember those "Head On! Apply Directly to the Forehead!" commercials? That's the illusory truth effect in action. The ads repeated the phrase so much so that people found themselves at the drugstore staring at a glue-stick-like contraption thinking, "Apply directly to MY forehead!" The question of whether it actually alleviates pain gets smothered by a combination of tagline bludgeoning and tension headache.

Repetition is what makes fake news work, too, as researchers at Central Washington University pointed out in a study way back in 2012 before the term was everywhere. It's also a staple of political propaganda. It's why flacks feed politicians and CEOs sound bites that they can say over and over again. Not to go all Godwin's Law on you, but even Adolf Hitler knew about the technique. "Slogans should be persistently repeated until the very last individual has come to grasp the idea," he wrote in Mein Kampf.

The effect works because when people attempt to assess truth they rely on two things: whether the information jibes with their understanding, and whether it feels familiar. The first condition is logical: People compare new information with what they already know to be true and consider the credibility of both sources. But researchers have found that familiarity can trump rationality—so much so that hearing over and over again that a certain fact is wrong can have a paradoxical effect. It's so familiar that it starts to feel right.

"When you see the fact for the second time it's much easer to process—you read it more quickly, you understand it more fluently," says Vanderbilt University psychologist Lisa Fazio. "Our brain interprets that fluency as a signal for something being true"—Whether it's true or not. In other words, rationality can be hard. It takes work. Your busy brain is often more comfortable running on feeling.

You are busy, too, so let me get back to Trump's latest executive orders, which are mostly symbolic. They certify that the government will do what it can to keep law enforcement officers safe. They contain vague language that civil rights advocates worry could lead to the criminalization of protest. But while perhaps unnecessary, the orders are hardly pointless—they reinforce the idea that America is unsafe, that law enforcement officers are at risk, that the country needs a strong "law and order" president. Data be damned.

As with any cognitive bias, the best way not to fall prey to it is to know it exists. If you read something that just feels right, but you don't know why, take notice. Look into it. Check the data. If that sounds like too much work, well, facts are fun.

Facts are fun.

Facts are fun.

Facts are fun.