Gina Mireault has an article about infant laugher in Aeon. Infants show laughter about about four months. And then they use it:
For example, infants can employ fake laughter (and fake crying!) beginning at about six months of age, and do so when being excluded or ignored, or when trying to engage a social partner. These little fake-outs show that infants are capable of simple acts of deception much earlier than scholars previously thought, but which parents knew revealed infants’ cleverness. Similarly, the psychologist Vasu Reddy of the University of Portsmouth has found that, by eight months, infants can use a specific type of humour: teasing. For example, the baby might willingly hand over the car keys she’s been allowed to play with, but whip her hand back quickly, just before allowing her dad to take possession, all the while looking at him with a cheeky grin. Reddy calls this type of teasing ‘provocative non-compliance’. She has found that eight- to 12-month-olds use other types of teasing as well, including provocative disruption, as in toppling over a tower someone else has carefully built.Teasing is the infant’s attempt to playfully provoke another person into interacting. It shows that infants understand something about others’ minds and intentions. In this example, the infant understands that she can make her father think that she will relinquish the car keys. The ability to trick others in this way suggests that infants are maturing toward a Theory of Mind, the understanding that others have minds that are separate from one’s own and that can be fooled. Psychologists have generally thought children don’t reach this milestone until about four and a half years of age. Infants’ ability to humorously tease reveals they are progressing toward a Theory of Mind much earlier than previously thought.
But, please, stop with the Theory of Mind.