Many theories exist about what humour is and what social function it serves. The prevailing types of theories attempting to account for the existence of humour include psychological theories, the
vast majority of which consider humour-induced behaviour to be very healthy.
Babies often smiling within the first three days of their lives, especially while sleeping. However these early smiles are spontaneous and are not a response to outside stimulation. These early
smiles are referred to as "reflex smiles". Between 6 and 8 weeks, most babies begin to exhibit "social smiles", meaning the baby will smile at his/her caregivers or in response to stimuli. Most
babies laugh between 3 and 4 months.