Nurses leave Nigeria and come to the UK, hoping for a better career. Farmers leave Mexico to work in construction or catering in the US. Such migrants can have a profound impact on the economy, as well as the society and politics both of the country they leave and the country to which they move. Social scientists, naturally, take an interest.

But one economist, Edward Castronova of Indiana University, studies an unusual kind of migration. Unhappy with your job as a Starbucks barista? Why not become a starship captain instead? Impossible if you stay in the country of your birth, but simplicity itself if, instead, you emigrate to an online fantasy world.