Expensive pictures are primarily what economists call positional goods — things that are valuable largely because other people can’t have them. The painting on the wall, or the sculpture in the garden, is intended to say as much about its owner’s bank balance as about his taste. With most kit a higher price reduces demand. But art, sports cars and fine wine invert the laws of economics. When the good that is really being purchased is evidence that the buyer has forked out a bundle, price spikes cause demand to boom.