Imagine you're given a test with 20 simple math problems. You have 5 minutes to solve as many as possible - and the better you do, the more you'll get paid. When time's up, you're instructed to drop the paper into a shredder. But it's not really going to be shredded.

Instead, the Duke University researchers who devised this experiment hold onto the test. So later, when you report how well you did, they can check to see whether you're lying.

And if you're like most people, you've probably fibbed a little bit.

That's because "scheming and dishonesty are part of what makes us human," according to the cover story in the June issue of National Geographic.

Contributing writer Yudhijit Bhattacharjee explores psychology, neuroscience and con artistry to explain 'Why We Lie: The Science Behind Our Deceptive Ways'.

Researchers think that pretty much as soon as humans could speak, we were bending the truth.

"The ability to manipulate others without using physical force likely conferred an advantage in the competition for resources and mates, akin to the evolution of deceptive strategies in the animal kingdom, such as camouflage," Bhattacharjee writes.

These days, there seem to be four main reasons we lie.

We do it to promote ourselves or protect ourselves. We do it to affect others, either to be kind or cruel. And then there are the situations that are inexplicable, even to us.

The littlest kids are the least likely to lie, probably because they're still learning how to do it. In an experiment at the University of Toronto, children are asked to guess the identity of a hidden toy.

The experimenter always leaves the room to take a phone call - which is, of course, a lie - and tells the kid not to peek.

"Most children can't resist peeking," Bhattacharjee writes, but how they react after that depends on age.

Toddlers usually admit to taking a look, while about 80 percent of eight-year-olds claim they didn't.

They also become gradually savvier about covering up their naughty behaviour. Younger kids who have lied about peeking typically give the correct answer about the toy, while older ones deliberately offer the wrong answer.

Studies of adults have shown that brains continue to get more adept at lying over time.

Given that we all basically grow up to be liars, what's really unbelievable is that we're also so trusting.

But there's an advantage to that, too, Bhattacharjee adds: "Without the implicit trust that we place in human communication, we would be paralysed as individuals and cease to have social relationships."

2017 © The Washington Post

This article was originally published by The Washington Post.