Is more choice better? Ten years ago the answer seemed obvious: Yes. Now the conventional wisdom is the opposite: lots of choice makes people less likely to choose anything, and less happy when they do choose.

The most famous supporting evidence is an experiment conducted by two psychologists, Mark Lepper and Sheena Iyengar. They set up a jam-tasting stall in a posh supermarket in California. Sometimes they offered six varieties of jam, at other times 24; jam tasters were then offered a voucher to buy jam at a discount.

The bigger display attracted more customers but very few of them actually bought jam. The display that offered less choice made many more sales – in fact, only 3 per cent of jam tasters at the 24-flavour stand used their discount voucher, versus 30 per cent at the six-flavour stand. This is an astonishingly strong effect – and utterly counter to mainstream economic theory.