As Johnson puts it, the tax base has been eroded, compared with the rest of the UK, and even when compared with other devolved regions such as Wales where taxes have remained in line with the rest of the country. But, of course, there should not be any great surprise about that.

In fact, Sturgeon and her government are getting a real-world lesson in basic economics. As the economist Arthur Laffer established more than half a century ago, with his famous Laffer Curve, when you put up taxes, behaviour changes, and so does the way that people work, and the distribution of jobs, so you won’t necessarily collect all the taxes you expect to.

That is especially true when you are taxing what is basically a region of a far larger economy. People are mobile, and so is work.

At the margin, jobs will drift south of the border where taxes are lower. Or people won’t bother increasing their hours, or putting in more overtime, because they don’t think it is worth the hassle anymore.

Sturgeon should have already known that. There is plenty of evidence from around the world that jobs and wealth shift around a country when tax rates are set by region.

In Switzerland, where cantons have significant tax-setting powers, the finance industry has shifted to Zug, the lowest tax region, compared with traditional bases such as Zurich or Geneva.

In the United States, where states have significant powers to set their own tax rates, there has already been a major shift out of high-tax California to Texas and Florida, where taxes are a lot lower.

In October, for example, Tesla officially moved its headquarters from Palo Alto, California, to Austin, Texas, joining other giant tech companies such as Oracle and Hewlett-Packard.

Texas of course levies no personal income taxes, although everyone still has to pay federal taxes, while California has some of the highest rates in the US.

Again and again, the same pattern repeats itself. When a region or country puts up taxes, jobs and investment drift away, and the amount of money collected is far less than expected.

True, the impact in Scotland is not yet especially dramatic. One percentage point in tax rates, even for someone earning over £150,000, is not a huge sum of money.

The extra that someone on an average salary has to pay won’t make it suddenly worthwhile to move from Edinburgh to Manchester or Glasgow to Leeds.

And yet, we can see the marginal impact already. Traditionally Scotland used to be one of the wealthiest parts of the UK, second only to London and the South-East.

But its higher taxes are gradually destroying that. In truth, the Tartan tax has been a dismal failure, and one that is likely to do increasing damage to the Scottish economy the longer it is kept in place.