On Question Time last week, Zach Polanski, the Green Party leader and erstwhile boob-whisperer , declared that there is no evidence that the wealthy leave Britain because of wealth taxes. A bold claim, and a wrong one. It’s also revealing, symptomatic of a growing belief on the populist left that Britain’s problems could be solved if only we shook the ultra-rich’s pockets a little harder.

Polanski assured the audience that a wealth tax would only fall on those with more than £10m in assets – as if this made it both morally tidy and economically painless. Unfortunately, history and basic arithmetic disagree. France tried almost exactly that, with a rate charged on individuals ranging from 0.5 per cent to 1.5 per cent on assets over €10m.

For all the rhetoric about ‘justice’, a wea

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