Why do Trump's biggest fans still believe him when he lies? The answer is in the human brain

As the president declares a national emergency, let me introduce you to a phenomenon known as the 'illusory truth effect'

President Donald Trump has just declared a national emergency after signing a bipartisan spending bill to fund the government and avoid another shutdown.

The national emergency declaration, which will allow Trump to bypass Congress and free up more money for border wall construction, comes more than a month after Trump first proposed the idea of using an emergency order to fund the wall that he said Mexico would pay for.

Just two days ago, Trump told reporters that construction of the border wall was already well underway. It is not. But now, he wants you to believe that he needs to declare a national emergency to build the wall that he just said was already being built.

Is your head spinning yet? No one could blame you for saying yes.

Our brains have limited cognitive resources, and Trump’s constant drumbeat of misinformation quickly overburdens our ability to effectively parse truth from fiction.

When evaluating information, our brains rely on mental shortcuts to help us form judgments and make decisions without having to stop and think about each step along the way. While these heuristics allow us to sort through the constant flow of information we encounter, they also leave us vulnerable to cognitive biases and poor decision-making.

Trump may not be a great dealmaker, but he is quite effective at tapping into these vulnerabilities and exploiting them to his benefit. No issue demonstrates this more clearly than immigration, and perhaps no event demonstrates this more clearly than the National Emergency declaration, which was issued in response to a crisis that doesn’t actually exist.

When Trump tells a lie, he tells it repeatedly. When he wanted people to believe that the border wall was being built, he returned to the claim on a near-daily basis, just like he peddles the same falsehoods about immigration and crime at all of his rallies.

Even when we know the information we’re hearing is false, being exposed to the same statement repeatedly has been shown to increase its acceptance as true. This is due in part to a phenomenon known as the “illusory truth effect,” whereby people rate statements as more truthful and believable when they have encountered them previously than when they are new statements.

Unfortunately, the same effect makes us susceptible to repetition of any kind—including the restatement of false claims during the process of fact-checking. Yes, you read that correctly: The more times a falsehood is repeated, even for the purpose of refuting it, the more likely it is to be accepted as true.

Research also shows that people tend to perceive messages from multiple sources to be more credible than those from a single source. As a result, hearing the same thing from multiple people or groups increases the likelihood that it will be accepted as true, even if it’s not.

Trump has been able to tap into that vulnerability, too, thanks to the assistance of other members of his administration, as well as agencies like the Department of Homeland Security (DHS), which has repeatedly backed his false claims about new border wall construction.

Furthermore, because agencies like DHS are perceived as authoritative sources of information, hearing Trump’s lies repeated by such a source makes it more likely that the false claims will be accepted as true and passed along to others.

On top of that, messages that create emotional arousal like fear or disgust are far more likely to be passed on, whether or not they are true. It’s not a coincidence, in other words, that fear-mongering is a key feature at Trump’s rallies.

Finally, Trump’s supporters may be particularly susceptible to believing his lies because the falsehoods confirm pre-existing beliefs about immigrants, crime, and the need to do something about it.

Although his supporters may be aware that Trump is not factually correct when he says that the wall is being built or that gang members are pouring over the border, they believe in the message he’s telling—and they’re willing to lie to themselves to hear it.