A “Dear Economist” correspondent once asked me why people post clips of classic comedies on YouTube, or go to the trouble of writing online reviews, given that there seems to be nothing in it for them. A textbook economics model would say that people would not, in fact, post online reviews or contribute to YouTube. And my answer, in brief, was that they don’t. Far more people read books than write reviews of them, and far more watch YouTube videos than post them. As a broad defence of rational economic man, my answer wasn’t too bad; but as a way of understanding online volunteering, it was useless.

Economic theory is not entirely helpful, either. “Public good provision” is the economists’ name for installing a solar hot water system for the sake of the planet, or endowing a library, or contributing a paragraph to Wikipedia. The trouble is not that economics has no explanation for such contributions, but that it has too many. Perhaps people are pure altruists, motivated by the joy of others. Perhaps they enjoy the process of contributing, whether or not it actually produces something of value. Or perhaps they enjoy the good reputation that comes with being acknowledged as a Doer of Good Deeds.

Until now, most of our understanding of the question has come from laboratory experiments. Given the importance of social context, these experiments may well be giving precise answers to the wrong question.