Child benefit withdrawal will make it better to work less for families with over seven children

And come 2076, that will be the case for every family in the nation.

The child tax credit withdrawal, taking effect on Monday, will lead to marginal tax rates of over 100 per cent on families with more than eight children earning between £50,000 and £60,000.

The IFS explains how the marginal rates are calculated:

Affected taxpayers will pay back one per cent of their family’s Child Benefit for every £100 by which taxable income exceeds £50,000. One per cent of Child Benefit is £10.56 per year for a 1-child family, and an additional £6.97 per child for larger families. Hence the marginal tax rate between £50,000 and £60,000 is increased by about 11 percentage points for the first child and by an additional 7 percentage points for each subsequent one. So, for example, while about 320,000 people will find that their marginal income tax rate increases to more than 50%, about 40,000 of them - those with three or more children - will find that it jumps to at least 65%.

They offer a chart with the rate calculated up to four children:

By seven children, the marginal rate rises to 99.35 per cent, and by eight, it breaks 100 per cent (106.32 per cent, to be exact). This means that any individual with a family of eight kids earning between £50,000 and £60,000 would be better off if they reduced their salary back down to £50,000. In fact, for that individual, they would have to earn £61,105 before their post-tax income was the same as it was at £50,000.

It's unclear whether any families actually exist matching that criterion - rather wonderfully, my back-of-the-envelope maths (which assumes that the exponential decrease in the number of families of each size continues: e.g., there are 1/8th the number of families with three or more kids as there are with two or more, so I'm assuming that there are correspondingly 1/8th the number of families with four or more as there are with three or more, and so on) suggests that there may be exactly one – but even if there are none at the moment, there's no reason why there won't be one in the future. Families with eight children do, after all, exist.

In fact, as time goes on, this problem will get worse. The IFS points out that child benefit is uprated with inflation, while tax bands aren't. Currently, each extra child after the first increases your "marginal tax rate" by around seven per cent, but suppose child benefit is uprated by two per cent a year. In that case, the marginal tax would exceed one hundred per cent for families with seven children next year; for six children in the year 2020; for five children in the year 2028; and, eventually, for families with just one child – i.e., every family – in the year 2076.

Hopefully the law will be changed before then, of course. But as a rule of thumb, laws which become ridiculously damaging unless you actively intervene ought not be signed in the first place. Oops.

Children. Photograph: Getty Images

Alex Hern is a technology reporter for the Guardian. He was formerly staff writer at the New Statesman. You should follow Alex on Twitter.

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To stop Jeremy Corbyn, I am giving my second preference to Andy Burnham

The big question is whether Andy Burnham or Yvette Cooper will face Jeremy in the final round of this election.

Voting is now underway in the Labour leadership election. There can be no doubt that Jeremy Corbyn is the frontrunner, but the race isn't over yet.

I know from conversations across the country that many voters still haven't made up their mind.

Some are drawn to Jeremy's promises of a new Jerusalem and endless spending, but worried that these endless promises, with no credibility, will only serve to lose us the next general election.

Others are certain that a Jeremy victory is really a win for Cameron and Osborne, but don't know who is the best alternative to vote for.

I am supporting Liz Kendall and will give her my first preference. But polling data is brutally clear: the big question is whether Andy Burnham or Yvette Cooper will face Jeremy in the final round of this election.

Andy can win. He can draw together support from across the party, motivated by his history of loyalty to the Labour movement, his passionate appeal for unity in fighting the Tories, and the findings of every poll of the general public in this campaign that he is best placed candidate to win the next general election.

Yvette, in contrast, would lose to Jeremy Corbyn and lose heavily. Evidence from data collected by all the campaigns – except (apparently) Yvette's own – shows this. All publicly available polling shows the same. If Andy drops out of the race, a large part of the broad coalition he attracts will vote for Jeremy. If Yvette is knocked out, her support firmly swings behind Andy.

We will all have our views about the different candidates, but the real choice for our country is between a Labour government and the ongoing rightwing agenda of the Tories.

I am in politics to make a real difference to the lives of my constituents. We are all in the Labour movement to get behind the beliefs that unite all in our party.

In the crucial choice we are making right now, I have no doubt that a vote for Jeremy would be the wrong choice – throwing away the next election, and with it hope for the next decade.

A vote for Yvette gets the same result – her defeat by Jeremy, and Jeremy's defeat to Cameron and Osborne.

In the crucial choice between Yvette and Andy, Andy will get my second preference so we can have the best hope of keeping the fight for our party alive, and the best hope for the future of our country too.

Tom Blenkinsop is the Labour MP for Middlesbrough South and East Cleveland