A recent study has found that it takes mental effort to lie, and so people whose attention is already being occupied by another task are less likely to skirt the truth, according to Forbes. The study was simple: researchers gathered people together and told them that they would roll a single die, and be paid a number of Euros equal to their result, giving them an incentive to lie. Then, they made it so the number would be self-reported on a computer, and the rolling itself would be done with a cup with a hole at the top, so only the subject would be able to know what the real number is, giving them reason to believe they wouldn't get caught if they lied. 

Some, however, were also asked to do a basic memory test at the same time they rolled and reported their die, while others were not. What they found was that people who did not have to do the memory test reported higher numbers than those who did, to the point where either a lot of people got very lucky or, more likely, they lied. 

And so, the researchers conclude, since lying takes more effort than telling the truth, perhaps the best time to get the truth out of someone is to ask while their minds are busy with something else.