Economics is all about realising gains from trade, but sometimes our qualms get in the way. For example, most of us have a spare kidney that – since kidneys tend to fail in pairs – is not terribly useful to us. A few – over 6,300 on the UK waiting list – could have their lives transformed if they could get hold of a compatible kidney. The few have so much to gain; the rest of us, not that much to lose. Donating a kidney is not risk-free: it kills three out of 10,000 donors, which is three times the death rate from pregnancy. The logic of economics is that there is a cash price at which both donor and recipient would walk away smiling.

Of course, it isn’t legal to complete this transaction – unless you happen to live in Iran, where recipients are allowed to provide cash compensation to donors. From a utilitarian cost-benefit viewpoint, the prohibition doesn’t make much sense. But then, neither did the outrage over the Dutch Big Donor Show, in which three desperately ill patients competed for the kidney of a terminally ill donor. In the end, of course, the show turned out to be a hoax; the patients were in on the stunt and the donor was an actress. At the awful news that none of these sick people would receive a cure, the world breathed a puzzling sigh of relief.