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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Commons Confidential: Fearing the Wigan warrior

An electoral clash, select committee elections as speed dating, and Ed Miliband’s political convalescence.

Members of Labour’s disconsolate majority, sitting in tight knots in the tearoom as the MP with the best maths skills calculates who will survive and who will die, based on the latest bad poll, observe that Jeremy Corbyn has never been so loyal to the party leadership. The past 13 months, one told me, have been the Islington rebel’s longest spell without voting against Labour. The MP was contradicted by a colleague who argued that, in voting against Trident renewal, Corbyn had defied party policy. There is Labour chatter that an early general election would be a mercy killing if it put the party out of its misery and removed Corbyn next year. In 2020, it is judged, defeat will be inevitable.

The next London mayoral contest is scheduled for the same date as a 2020 election: 7 May. Sadiq Khan’s people whisper that when they mentioned the clash to ministers, they were assured it won’t happen. They are uncertain whether this indicates that the mayoral contest will be moved, or that there will be an early general election. Intriguing.

An unguarded retort from the peer Jim O’Neill seems to confirm that a dispute over the so-called Northern Powerhouse triggered his walkout from the Treasury last month. O’Neill, a fanboy of George Osborne and a former Goldman Sachs chief economist, gave no reason when he quit Theresa May’s government and resigned the Tory whip in the Lords. He joined the dots publicly when the Resolution Foundation’s director, Torsten Bell, queried the northern project. “Are you related to the PM?” shot back the Mancunian O’Neill. It’s the way he tells ’em.

Talk has quietened in Westminster Labour ranks of a formal challenge to Corbyn since this year’s attempt backfired, but the Tories fear Lisa Nandy, should the leader fall under a solar-powered ecotruck selling recycled organic knitwear.

The Wigan warrior is enjoying favourable reviews for her forensic examination of the troubled inquiry into historic child sex abuse. After Nandy put May on the spot, the Tory three-piece suit Alec Shelbrooke was overheard muttering: “I hope she never runs for leader.” Anna Soubry and Nicky Morgan, the Thelma and Louise of Tory opposition to Mayhem, were observed nodding in agreement.

Select committee elections are like speed dating. “Who are you?” inquired Labour’s Kevan Jones (Granite Central)of a stranger seeking his vote. She explained that she was Victoria Borwick, the Tory MP for Kensington, but that didn’t help. “This is the first time you’ve spoken to me,” Jones continued, “so the answer’s no.” The aloof Borwick lost, by the way.

Ed Miliband is joining Labour’s relaunched Tribune Group of MPs to continue his political convalescence. Next stop: the shadow cabinet?

Kevin Maguire is Associate Editor (Politics) on the Daily Mirror and author of our Commons Confidential column on the high politics and low life in Westminster. An award-winning journalist, he is in frequent demand on television and radio and co-authored a book on great parliamentary scandals. He was formerly Chief Reporter on the Guardian and Labour Correspondent on the Daily Telegraph.

This article first appeared in the 27 October 2016 issue of the New Statesman, American Rage