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You've probably heard of all kinds of tricks you can use to tell if someone's lying. Like from me.

Six years ago. -Liars often use more formal language to deny something that they've actually done. Oh my god! [laughs] But separating lies from the truth isn't as simple as learning all those techniques, because most of them are based on the assumption that there's an inherent psychological difference between lying and telling the truth. And that is not always the case.

Which means a foolproof lie detector is unlikely, if not impossible. Let's say someone is letting their dog poop on your lawn, but none of your neighbors are copping to it. If you're the type of person who faithfully watches every episode of Lie to Me or Law and Order, you might think you know a thing or two about detecting deception.

But even if you tried those techniques, you might not be able to catch the liar — or worse, you might end up accusing the wrong person. Most of these things involve looking for signals of stronger emotions, especially the fear of being caught. Polygraph machines, for example, are designed to detect lies based on changes to things associated with anxiety, like an increased heart rate or sweating.

And they're notoriously unreliable. There's also the idea that minute facial movements, also known as microexpressions, can reveal someone's innermost thoughts. But microexpressions actually have very little empirical basis, especially if you're talking about reading people in real time.

While there is some potential for developing computerized methods of lie detection based on it, it's not really a skill you can pick up and then use when you go confront your neighbor about that new mystery lawn turd. In fact, microexpression training can make you worse at detecting certain kinds of lies, for the same reason polygraphs don't work: emotions just aren't a good way to tell if someone is lying. Like, it's not that weird to think that you might be super anxious if you're in a tiny windowless room being intensely interviewed while hooked up to a weird machine—whether or not you're telling the truth.

On the flip side, a super confident liar might not feel anxious at all. And even if you do feel stronger emotions when you lie, they can vary based on your motivation for lying and what's at stake if you're found out. So more recent research into lie detection has stepped away from looking at emotional signals and focused more on the idea that lying increases what psychologists call cognitive load.

If you think of your brain like a computer— which you mostly shouldn't—then cognitive load is, like, how much of your RAM is in use. The idea is that when you lie, you're inventing things, juggling the real and the fake, carefully watching for hints that you've been found out, and generally using more working memory than you would be if you just told the truth. And that's supported by brain scanning studies.

Since you only have so much RAM to work with, cognitive lie detection relies on challenging the suspected liar's brain and gauging how well they handle the additional work. They use tactics like having people tell their stories in reverse, draw diagrams of the scene, or perform some other physical task while they're talking. Then they look for inconsistencies between accounts or changes to verbal and nonverbal cues, like pauses or body language.

This method works somewhere between 2/3 and 3/4 of the time, which is better than flipping a coin, but still not great. And like stronger emotions, having a high cognitive load isn't unique to liars. Someone can have a lot going on in their head for completely unrelated reasons.

There's some newer research using brain scanners and other high-tech machines to look for specific brain pathways or activation patterns that are unique to lying. But so far, they have not found any magic bullets, and it's unclear if they ever will. So there's not as much distinguishing a lie and the truth as you might think.

Part of the reason might be that when people lie, they don't usually make everything up—they often tell embedded lies, where most of what they're saying is true. Like, if you asked that person about that dog's last walk, they might tell you about another day where they didn't stroll past your house, and be like, “I never walk past your house! That day I was in a completely different neighborhood.” So, they are lying, but they're also giving a totally true account of a walk they really took.

And lying is an essential part of our psychology—we lie for all kinds of reasons, and we don't just lie to others, we lie to ourselves, so much so that what we see as the truth is constantly shifting. While all of these studies are super important for things like our justice system and how we train law enforcement, they aren't really things that you can use to figure out if your neighbor is hiding the truth about their dog pooping on your lawn. Which they are.

They definitely are. Like, good luck getting them to sit down for some extensive, taped one-on-one interviews so you can try to analyze their speech patterns or check details while you're getting them to retell the story of their dog walk for the sixteenth time. Besides, trying to become a better lie detector often backfires.

Simply trying to detect a lie or thinking you have the skills to do so can make you more likely to think that a lie is being told—a phenomenon known as lie bias, which we're all familiar with if we play the game “Werewolf.” When put to the test, professionals who detect lies as a part of their jobs—like cops—don't generally do a better job of telling fact from fiction, even though they're more confident in their assessments. Studies have also found that people who undergo lie detection training can actually be worse at sniffing out liars than those who don't. Either way, I'd hazard a guess that your neighbors aren't so keen on being interrogated.

So setting up a camera to catch that dog in the act might be a safer bet. Thanks for watching this episode of SciShow Psych! If you want to learn more about how the truth can change over time, you might like our episode on how your memory can be tricked.

And if you want to keep learning more about your brain and how it works, be sure to click on that subscribe button! [OUTRO ♪].