"We start laughing around 3.5 to 4 months old; Scientists believe it is a way to build relationships, not necessarily as a response to something funny."
This makes a lot of sense if you stop to think about laughter in relation to babies. We acquire the ability to laugh long before we acquire the capacity to speak words. Therefore, laughter serves as a means of communication and a bonding activity between a baby and its parent or, in cases like ours, its sibling. A parent gets a baby to laugh and then continues to do the behavior that illicits the laughter. In turn, the baby carries on laughing at the antics of the parent almost as a means of positive reinforcement.
The laughter of babies and what it might portray about the workings of babies’ brains is being studied by Birkbeck Babylab, a London based research group. The most popular post on their Baby Laughter project blog is the discussion of "should we tickle babies". When it comes to tickling, something to consider is that the idea of tickling is not necessarily a pleasant experience for everyone. In fact, many adults do not enjoy being tickled and because babies cannot defend themselves, perhaps they shouldn't be tickled. One theory about why we are ticklish is so that we are sensitive to bugs and small parasites that might crawl on our skin while we are asleep. A pretty creepy thought, I must say, but it certainly does help to explain the reason we wiggle and squirm away from the tickling. The researchers believe that babies, for the most part, do enjoy being tickled and actually probably see tickling as a game they can play with their favorite people. The researchers go on to suggest that babies can tell you by crying or squirming away when they have had enough. So, as long as we pay attention to their cues, tickling can be a fun way to trigger a baby's laughter and reinforce that bond between parent and child.
Rigby sure does seem to enjoy it and, as I mentioned in his four month update, I find it to be one of the cutest things I've ever witnessed!