To the surprise of (almost) no one, the French supertax on the wealthy raised only a quarter of the amount expected.

Unsurprisingly, taxes make people change their behaviour. It’s literally why we have high taxes on things like alcohol and cigarettes. Here, rich French taxpayers changed to taking dividends instead of salaries.

When forecasting how much a tax will raise, you can’t just look at the current position of taxpayers, add a tax to it, then assume that’s how much the tax will raise.

For example, 200m Mars Bars are sold in the UK every year. If you added a tax of £1 to each Mars Bars, you of course wouldn’t raise £200m. People would simply buy other chocolate bars.