Why we believe false things more after hearing them twice

HostWe have all had that moment where we hear a strange rumor or a weird bit of news, and at first, we think there's no way that's true. But then, a week later, we hear it again from someone else, and suddenly it starts to sound like common sense. It feels like it belongs in the true pile in our heads. Why does our brain let that happen? Why do we start to trust things just because they feel familiar?

GuestIt's one of those quirks of how we're built. Our brains are always looking for ways to save energy. Thinking is hard work, so the mind looks for shortcuts to figure out what's true and what's not. One of the biggest shortcuts is how easy something is to think about. If you have heard something once, your brain has already carved a little path for it. When you hear it again, your mind zips along that same path much faster. That speed, that ease of thinking, gives us a little glow of the feeling that this must be right. We mix up the feeling of ease with the feeling of truth.

HostSo just because a thought is easy for us to handle, we assume it's a fact? That seems like a pretty big flaw in how we work.

GuestYeah, it really is. People who study the mind call it the illusion of truth. And it works even when the thing we're hearing is totally made up. You could hear someone say that a date is just a dried plum. That's wrong, but if I say it to you three times, you'll actually start to rate it as more likely to be true than if you only heard it once. The brain basically says, I have seen this before, I know how to handle this, so it must be solid ground.

HostBut if I know for a fact that a plum turns into a prune, not a date, how can my brain still fall for it? I have my own knowledge to lean on.

GuestYou would think your knowledge would protect you, but the feeling of ease is very strong. Even people who know the right answer can get tripped up if the lie is repeated enough. The feeling of "I know this" comes faster than the actual memory of "wait, that's wrong." It's like your brain takes a vote. One vote is from what you know, and the other vote is from how familiar the words sound. When you hear something a second or third time, the familiarity vote gets louder and louder. Sometimes it just outvotes the facts you have stored away.

HostThat's a bit scary. It makes it sound like we're just at the mercy of whatever we hear most often. Is it just about the ease of it, or is there something else going on with how we remember things?

GuestThere's another layer to it. When we hear a claim, we often store the claim itself in one spot and the source of that claim in another. Over time, the link between the two gets weak. You might remember hearing that eating carrots gives you night vision, but you forget that you heard it in a cartoon when you were five. When you hear it again as an adult, the claim feels familiar, but the warning label that said it was just a joke is gone. We're left with the ghost of the idea, and without the source to show it's wrong, we just accept it as a bit of wisdom.

HostSo we keep the junk but throw away the receipt that shows where it came from. But surely we can just try harder? If we know this is a trick, can we just tell our brains to stop doing it?

GuestIt's much harder than it sounds. Even when people are told that they're about to hear a bunch of lies, they still find those lies more believable after hearing them twice. Being smart or being on guard doesn't totally turn it off. The only real way to fight it's to slow down and actually check the facts. But our brains hate doing that. We want to glide through the day. If we had to fact-check every single thing we heard, we would never get anything done. So we lean on that feeling of "I have heard this before" as a stand-in for "this is true."

HostThis explains a lot about why bad ideas stick around for so long. If someone says a lie enough times, it just becomes part of the furniture of our minds.

GuestIt's exactly why ads work the way they do. A brand doesn't just tell you their soap is the best one time. They tell you a thousand times. They want that name to be the easiest thing for your brain to grab when you're standing in the store. The same thing happens with rumors. Once an idea is in there, and it has been smoothed out by being repeated, it's very hard to kick it out. We even see this in how we judge ourselves. If you tell yourself you're bad at something over and over, you start to believe it just because the thought is so easy to have.

HostIs there any upside to this? It seems like a recipe for a world where nobody knows what's real.

GuestWell, it's useful for things that actually are true. It helps us learn fast. If you hear that fire is hot and you should stay away, you want that to become a fast, easy thought that you don't have to double-check every time. The problem is just that the brain doesn't have a built-in filter to separate the good shortcuts from the bad ones. It treats all repeated stuff the same way. It's a system built for speed, not for getting things perfectly right every time.

HostSo we have to act like our own editors. We have to look at the easy thoughts and ask where they really came from.

GuestThe most recent studies show that even when the truth is right in front of us, the pull of a repeated lie can still change how we act weeks later.

HostThose little paths in the mind might make life feel fast and easy, but they can also turn a simple prune into a date if we aren't careful about what we let back into the house.