I saw an article in the Telegraph that very young children experiment with different forms of deception (via Boing Boing) to get what they want at an age as young as six months or so. When they are this young, they obviously cannot lie with words, but they can do so non-verbally: "Dr Reddy said: 'Fake crying is one of the earliest forms of deception to emerge, and infants use it to get attention even though nothing is wrong. You can tell, as they will then pause while they wait to hear if their mother is responding, before crying again.'"
Ok, pretty interesting stuff. It goes on into more detail of a similar nature, all fairly interesting. But one sentence stuck out to me: "Until now, psychologists had thought the developing brains were not capable of the difficult art of lying until four years old." Excuse me? Has no developmental psychologist ever been around a 2 or 3 year old before? The little devils lie like the dickens, and it is blatantly obvious that they do so. Leave some 2 and 3 year olds to play, wait till a fight starts, then ask "what happened?" and you will hear a profusion of deception. Now developmental psychology is not my expertise, so it could be a newspaper misrepresentation, but gosh golly I hope developmental psychology is further advanced than believing 3 year olds are incapable of deception.