Perhaps the most important social feature of laughter is how contagious it is. Just listening to someone laugh is funny. To test this, try keeping a straight face while watching this video of a man tickling a gorilla. You can even catch laughter from yourself. Start with a forced laugh and if you keep it up you will soon find yourself laughing for real.
What these observations show is that laughter is both fundamentally social, and rooted deep within our brains, part and parcel of ancient brain structures. We laugh because we feel like it, because our brains make us, and because we want to fit in socially. All these things are true. But biologists distinguish at least four fundamental types of answer you can give to explain behaviour: "why did it evolve?"; "how did it evolve?"; "How does it develop across the lifespan?" and "how does it work?".
This column has given some answers to the first question (laughter evolved for social interaction) and the last question (laughter is controlled by evolutionary ancient brain centres that control breathing and speech), but even with the beginnings of answers to these two questions, the other two are far from being answered. Each time we get closer to an answer for a fundamental question, it deepens our appreciation of the challenge remaining to answer the others.
Thank you to Andrew Martin for suggesting the topic. If you have your own suggestions please send them to firstname.lastname@example.org