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Lying is a pretty sophisticated behavior. When you lie to someone else, you need to understand that when you tell them something that you know is not true, that they come to believe the false information. That means that you need to understand that their belief and your belief will differ.

Psychologists have studied this ability to separate your knowledge from your own beliefs from your knowledge of other people’s beliefs under the name theory of mind. The idea is that it requires understanding quite a bit about the way minds believe things in order to recognize that another person has a false belief.

For example, classic research about how children learn about false beliefs looked at the appearance-reality distinction. If you walk into a novelty shop, you might see a sponge that looks like a rock. When you first see the object, it looks like a rock. But, when you pick it up, it clearly feels like a sponge. You can then have fun with your friends by throwing the object at them. They flinch thinking that they are about to be hit by a rock, but the sponge bounces harmlessly off them. The joke only works because you realize that your friends will think that the object is a rock when it is actually a sponge.

Preschool children have difficulty with this distinction. If you show them a rock/sponge, they will tell you they think it is a rock. If you then let them play with it, they see that it is soft like a sponge. When you later ask them what a new person will think the object is, they will say that they think it is a sponge. That is, they have trouble keeping track of what a new person who has never seen the object before will think it is.

Not only do young children have difficulty with the appearance-reality distinction, they also don’t lie. This observation has been taken as evidence that theory of mind is important for the development of lying. A paper in the November, 2015 issue of Psychological Science by Xiao Pan Ding, Henry Wellman, Yu Wang, Genyue Fu, and Kang Lee tested this proposal experimentally.

They tested 60 preschoolers in China using a task that would encourage children to lie. In this Hide-and-Seek task, children first hid their eyes while an experimenter hid a candy under one of two cups. The child selected a cup, and if they were right they got to keep the candy. Otherwise, the experimenter got to keep it (and acted excited that she got to keep it).

Next, the child was given the opportunity to win a sticker that they really wanted. (Children of that age love stickers.) They were told that they would get to hide the candy 10 times and the experimenter would pick the cup. If the child won all 10 times, they would get the sticker. In fact, the experimenter always picked the cup that had the candy under it. The child could tell the experimenter the truth (and give the candy to the experimenter) or lie (and keep the candy). So, there was a big incentive for the children to lie.

At the start of the experiment, children were given a pre-test and were only selected for the rest of the experiment if they never lied on the task.

After that, half of the children were given training in theory of mind. For six sessions over 12 days the children were given versions of the appearance-reality task and other tests involving false beliefs. The training explained to the children how one person could believe one thing while a second person could believe another thing. A control group of children was given training on other cognitive skills not related to false beliefs. After the training, the group that learned about theory of mind was much better on tests relating to false beliefs and the appearance-reality distinction than children who got the control training.

Then, both groups did the hide-and-seek task again.

The children who got theory of mind training lied to the experimenter on about 60% of the trials, while those who got the control training lied only about 10% of the time. The experimenters gave follow-up tests to the children one day later and again 26 days later. At both tests, the children who learned about theory of mind were much more likely to lie than those who did not get the training.

This study demonstrates powerfully that there is a relationship between understanding that people can believe different things and lying. Once children clearly understood that other people could believe things that were different from what they believe, they used that information strategically to others.

Of course, theory of mind has lots of advantages beyond just lying. It is impossible to be a good teacher, for example, until you can separate what you know and understand from what other people know and understand. Lying is just a side-effect of an important mental ability.

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