Deception is one of the distinguishing characteristics of human behaviour, although higher primates are also thought to go in for it in a more limited capacity. Lying requires an understanding of what psychologists call "false belief". You must perceive that what somebody else knows about a situation is incomplete or wrong; and then you knowingly allow that to continue - or even exploit it.
Parents are often shocked when they first find that their children can lie to them. Lying, however, is a sign of growing maturity. Donald Christie, senior lecturer in educational studies at the University of Strathclyde, says: "Young children, say, aged three, are capable of pretence. They will tease parents, pretending to do things. But there is complicity with the parent there."
For example, "peekaboo" games (in which the parent pretends to hide) amuse very young children. Both players know the other has not really gone. But neither is lying to the other.
Generally, children develop an ability to deceive intentionally at about five years old, though it will be earlier if they have more social interaction with other human beings.
Donald Christie says: "Deception means the child has an interest in maintaining a difference between its understanding of a situation and other people's. That's usually associated with an awareness of consequences - that telling the truth may get you into trouble, or gain you something."
That awareness of consequences often leads to the signs that adults can recognise in lying children - blushing, shiftiness and tension. The same effects, on a lesser scale, can be picked up in adults using lie-detectors that monitor blood pressure, heart rate and breathing. Though those are autonomic responses - not usually under conscious control - adults can learn to control them by minimising their perception of the consequences of lying. Thus lie detectors are not infallible.
That understanding of the mixture of pretence, consequence and false belief is known as "theory of mind". It could be paraphrased as the ability to see things from someone else's point of view.
Interestingly, people with certain mental disorders show important differences in their "theory of mind" and proclivity to lie.
Those with autism (which affects people's ability to interact with others) are often slow to develop the ability to lie, or even lack it altogether. That is a disadvantage in a language-based society.
Mr Christie says: "A socially sophisticated person knows when to withhold a remark that might otherwise be hurtful. There is a degree to which lies are socially cohesive."
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