An infant knows its own condition better than a parent does, because the infant’s brain is connected to sensors throughout the body. Both the infant and the parent have an interest in the parent’s responding to the infant’s needs, such as by feeding it when it is hungry and cuddling it when it is cold. That gives the infant an opportunity to elicit more care than the parent wants to give. The baby can cry when it is not so cold or hungry, or withhold a smile until it gets its way. The baby need not be literally faking. Since parents should evolve to recognize sham crying, the baby’s most effective tactic might be to feel genuinely miserable, even when there is no biological need. Self-deception may begin early.

From Steven Pinker’s How the Mind Works, Harmondsworth, Middlesex: Penguin, 1998 (first published in 1997), p. 445.