ONE of the best tax-avoidance tactics in the late Roman Empire was to sell yourself into slavery. You didn’t really have to work as somebody’s slave, of course — it was more like rock star Hotblack Desiato being “dead for a year for tax reasons” in Douglas Adams’s wondrous confection The Hitch-Hiker’s Guide to the Galaxy — but with the legal status of slave, you were exempt from taxation.

Nowadays the legal manipulations used to avoid taxation are less dramatic, but they are spectacularly effective. James Henry, former chief economist at business consultancy McKinsey and a member of the board of directors of Tax Justice Network, has just published a report, ‘The Price of Offshore Revisited’, that estimates the amount of wealth hidden in tax havens by the super-rich at a minimum of $21tn.

It might be as much as $32 trillion, he adds, but greater precision is impossible when the whole point of holding money overseas is to keep it secret. Henry came up with this range of numbers by sifting through data from the Bank for International Settlements, the International Monetary Fund, the World Bank, and private sector analysts — and it does not even include yachts, mansions, art works and other forms of wealth held overseas.

It doesn’t matter. The point is that it’s a very large amount of money: equal to the annual Gross Domestic Product of both the United States and Japan. Some of it is the laundered proceeds of crime, and much of it is money stolen from national budgets by corrupt national elites (an estimated $306bn from Nigeria, $798bn from Russia, $1,189bn from China), but most is deposited by the respectable super-rich of the West.

Henry’s report, published in The Observer last weekend, calculates that almost half of the minimum estimate of $21tn is owned by just 92,000 people, some of whom pay no tax at all. A number of very small places (Liechtenstein, Cayman Islands, Jersey) and a few larger countries like Switzerland make a good living by providing these secret tax shelters, and work very hard to protect their clients from exposure.

Back home, the ‘high net-worth individuals’ also enjoy the services of “a highly paid, industrious bevy of professional enablers in the private banking, legal, accounting, and investment industries,” said Henry. We always sort of knew about it; now we know the scale.

Information of this sort is dangerous. It annoys those who merely work for a salary or an hourly wage, and whose taxes have to fill the gap created by the defection of the super-rich. It might even destabilise the established social order. But the British government, at least, knows how to deal with that sort of thing.

Less than 48 hours after Henry’s revelations, British politician David Gauke, one of the Treasury ministers, went public with the assertion that the lower orders cheat on their taxes just as much as the rich. “Getting a discount with your plumber by paying cash in hand is something that is a big cost to the Revenue and means others must pay more in tax,” he said.

Well, yes. Paying cash to a tradesman to get a discount (knowing that he will then not report this income to the tax authorities) is something that many people may have done.