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?”.