People respond to incentives, so if you want something done, reach for your wallet.That's what you'd expect an economist to say, but it is a belief that infuriates many commentators.

I will concede that offering cash is not always productive. In the days when I was young, free and single, I was never tempted to try to seduce cute girls at parties by slipping them a couple of crisp twenties. (Perhaps I should have done it. It is not as if my hit rate on an unpaid basis was particularly good either.)

Yet many policy wonks believe not just that there are some things that money can't buy, but that cash incentives are counterproductive and even morally corrosive. The touchstone of this school of thought is Richard Titmuss's book The Gift Relationship, published in 1970.Titmuss's most memorable and influential claim was that the British system of voluntary blood donation led to better outcomes – healthier blood, supplied in a more timely fashion – than the American system of paying blood donors.