The bit of that recent Piketty magnum opus that had economists scratching their heads was his demand for a wealth tax. For it's a standard commonplace within the subject that you really don't want to tax capital. Doing so makes the future a great deal poorer than it could be. Just like that old windows tax made the future a lot darker than it needed to be. There's an opportunity to explain why in some numbers being attributed to the likely effect of Red Ed's mansion tax:

Mansion tax could wipe an average of 5pc off homes worth more than £2m should Labour win the next general election and make the proposals a reality.

The plans touted by the shadow chancellor, Ed Balls, could mean a 10pc drop for properties valued at £10m or more, and an 8pc fall for homes valued above £5m according to a new report from Savills.

The property group has also estimated a 6pc decline for those homes worth more than £3m.

People who own homes with a price tag above the £2m threshold could see their property value fall 4pc following Mr Balls' announcement that they will face a monthly levy of £250, a sum which gets progressively larger for more expensive properties.

£250 a month on a £2 million property is 0.15%. This drops the capital value of the asset under discussion by 5% or so. But the sort of wealth tax being demanded by Piketty is 1-2% annually, ten times larger than this tax. Now no, straight line predictions aren't all that good, there's changes in elasticity to consider, but it would be reasonable enough to think that a ten times the tax would have ten times the effect as a first stab at a guess.

So, in Piketty's desired world we'll be taxing wealth, or capital (they're rather the same thing) at 1.5% a year and thus the value of that capital, those bonds, stock and shares, that are being taxed will fall 50%. And that's why wealth taxes don't work very well. For consider the effect upon investment of a fall of 50% in the value of having made a successful investment.

It's still just as difficult to come up with a good business idea. It's still just as difficult to make that idea work, still just as expensive to make it do so. But the payoff from being one of the one in five that do manage to get something real going has just halved. Obviously, fewer people are going to make the effort and take the risks. Meaning that the future will be poorer by the lack of the effects of those new businesses that never were started.

Wealth taxes don't work very well for the simple reason that they make all our children poorer than they would have been even if they do make our children more equal. It's not a good bargain, not a good trade off.