The reason given for cutting the 50p rate of tax to 45p was avoidance. It wasn't clearly phrased as such – most of the talk was about how it had raised less money than expected, or had changed behaviour in ways that harmed growth – but that is what it was nonetheless. To many, this will seem strange. "You avoided tax, so we will make you pay less". But it is an integral part of the line of thought that lies behind the cut.
Art Laffer first made the argument that cutting tax rates could boost revenue. The reasoning is broadly that, when the marginal tax rate (the amount you pay on each extra pound earned) gets too high, people start doing things to reduce their taxable income.
The palatable version is that they work less, because an extra hour of work no longer pays as well as it did, and this is probably true; there are certainly anecdotal tales of highly paid consultants turning down work later on in the year to spend more time at home.
The less palatable version is that they avoid more tax, because spending the money and effort required to set up a limited company, be paid "overseas", or funnel your income through a Swiss bank account in the name of your dog becomes more worthwhile the more it saves you.
Both of these "behavioural changes" are factored in to the Laffer curve, the rough prediction of how much revenue will be gained at various marginal tax rates. HMRC produced three such curves, each based around a different "taxable income elasticity" (TIE), a measure of how much an individual's behaviour changes given the tax rate:
They based their analysis around a TIE of 0.45 (a figure basically plucked from thin air – HMRC admit the evidence to choose is "extremely limited", and the studies they cite range from -0.6 to 2.75), which showed a peak of revenue at around 48 per cent. Quite why this then led the Chancellor to cut the rate to three per cent below that is unclear. If he wanted to raise revenue, his own analysis is showing that he's done it wrong.
The problem is, one thing which affects the TIE is the ease with which one can avoid tax. Make tax avoidance harder, TIE goes up, and the peak revenue rate increases. In fact, given the anti-avoidance measures announced at the budget yesterday, TIE will already be higher than it was at the time of the analysis, boosting the argument for keeping the 50p rate.
There is one massive category of avoidance which can't be cited as a reason for cutting the rate, however. The HMRC's stats show that £6.6bn less income was declared in 2010-11 due to it being "forestalled" – paid the year before, so as to take advantage of the lower rate. This is avoidance on a massive scale (Richard Murphy points out that it is £1.6bn more than the estimation for all tax avoidance in 2011), yet, contra Tim Worstall, it has no bearing on the decision on whether or not to cut the rate, because it can only ever be done once.
By cutting the tax so early in its life, Osborne has ensured that we make the decision unable to know the full effect of cutting it. We can guess at how much will have been raised for the 2011-12 tax year, when forestalling was harder (although not impossible, and HMRC warn that it "continues to reduce revenues in 2011-12 and beyond"), but by the time we know for sure, it will be too late. The 45p tax will be in, and there won't be a "normal" year of the 50p rate to compare it to.