This is one of those ‘how long is a piece of string questions?’
Babies are born, most of them anyway, at least in part ticklish. For example the ‘Babinski Sign’ (extending the big toe in response to having the sole of the foot tickled) is a sign of brain damage in adults, but a sign of a healthy nervous system in a baby. Checking for the Babinski Sign is typically part of the checks done just after birth. The other places that we typically ‘attack’ when we tickle someone develop later. Anecdotal evidence suggests that most babies start laughing around 3 months and seem to start reacting in ways their parents regard as ticklish around 4 months old. Incidentally, baby rats can be heard ‘laughing’ (at frequencies above human hearing) when tickled – so perhaps kids are laughing inwardly before they can vocalise it!
Here’s another fact for you. A recent study found that most people don’t even enjoy being tickled and normally laugh because of the interaction rather than the feeling.
So a possible reason for babies not being ticklish from birth is that they don’t have enough social awareness yet. Like a good joke, part of the hilarity is knowing what is about to happen; likewise, tickling is funny when you have the social awareness to understand what’s about to happen – you’ll scream, laugh and probably hit them as you twitch like a fish out of water. The parents see signs they ‘regard as ticklish’ – and there is a difference between their understanding of tickling and the child’s. The concept of tickling is learnt (much like everything else we take for granted).
One fact we know for sure is that a laughing baby will (metaphorically) make the heart of most observers melt.
Answer by Lewis Pike and Matt Powell
Question from Shaun Ellis via Facebook