Toddlers as young as 19 months are able to distinguish jokes from mistakes - a skill that lays the groundwork for their later ability to recognise lies and false beliefs. That's according to Elena Hoicka and Merideth Gattis, who tested a large group of children aged between 19 and 36 months.
Children were asked to copy actions made by the researcher - for example stirring a spoon in a cup, or combing their hair. Next, the researcher performed a range of joke actions (e.g. putting a boot on their hand), which they did laughing, and mistakes (e.g. putting a lid on a sugar jar so that it was not quite in place), after which they said "oops!".
All the children, from 19 months upwards, copied the joke actions, but corrected the mistakes - a sign, the researchers said, that they were able to tell the difference between a mistake and a joke.
After this, things got trickier. The researchers performed actions that could either be interpreted as a mistake or a joke: for example, putting a hat on so that it covered their eyes, or brushing their teeth with the wrong end of the brush. Half the time the researchers laughed afterwards, the rest of the time they said "oops!" The idea is that the ambiguous nature of the actions meant that, to know if a joke or mistake had occurred, the children had to be able to interpret the researcher's vocal response.
This time an age-difference emerged. The proportion of occasions that the 19 to 24-month-olds copied or corrected these actions did not vary according to whether the researcher laughed or said "oops!". By contrast, the children aged 25 months and upwards, corrected more when the researcher said "oops!" and copied more when they performed the action laughing - a sign, the researchers said, that children of this age are able to distinguish humorous intent from mistakes.
Elena Hoicka and Merideth Gattis said this means that the ability to recognise humorous intent comes after the ability to recognise jokes, but before the ability to recognise pretense and lies. "We propose that humour understanding is an important step toward understanding that human actions can be intentional not just when actions are right, but even when they are wrong," they concluded.
HOICKA, E., GATTIS, M. (2008). Do the wrong thing: How toddlers tell a joke from a mistake. Cognitive Development, 23(1), 180-190. DOI: 10.1016/j.cogdev.2007.06.001