 It's definitely the case that, as Disa says, laughter is a very important way that humans indicate that they are playing. So when children are playing, they will laugh a lot. And it's true that as an adult, we have more times in the day when we can't be in that mode. One of the problems is sort of how you actually measure this, because one of the worst things you can do to find out about how often people laugh is to ask them.