The Institute for Fiscal Studies’ claims that the policy will raise money do not take into account the unintended consequences – economic models rarely do, says James Price

I often tell people that the best way to make God laugh is to tell Him your plans. This is at least in part because the most immutable law in politics is, obviously, the law of unintended consequences. Rarely is this more evident than in various forecasts and predictions. Some things are black swan events, sure, others (like socialists putting up taxes), are dead certainties. But everything else is much less knowable than we like to admit.

Those working in the City know this better than most, and contingencies are built into models, but in Westminster, where being numerate is a much-less common skill, politicians throw about hypothetical billions of pounds in spending with the cast-iron certainty of a Love Island contestant describing Essex as a continent. Despite most economic figures being way off, and revised up or down long after the fact, organisations speaking with certainty about numbers are a life raft to those who are buffeted by the stormy waters of unpredictability.

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