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: Smith, selfies and pushy sons

All the best gossip from party conference, including why Dennis Skinner is now the MP for Selfie Central.

Owen Smith discovered the hard way at the Labour party conference in Liverpool that one moment you’re a contender and the next you’re a nobody. The party booked a luxurious suite at the plush Pullman Hotel for Candidate Smith before the leadership result. He was required to return the key card the day after Jeremy Corbyn’s second coming. On the upside, Smith no longer had to watch his defeat replayed endlessly on the apartment’s giant  flat-screen TV.

The Labour back-room boffin Patrick Heneghan, the party’s executive director of elections, had good cause to be startled when a TV crew pounced on him to demand an interview. The human submarine rarely surfaces in public and anonymity is his calling card. It turns out that the bespectacled Heneghan was mistaken for Owen Smith – a risky likeness when vengeful Corbynistas are on rampage. There’s no evidence of Smith being mistaken for Heneghan, though. Yet.

Members of Labour’s governing National Executive Committee are discovering new passions to pass the time during interminable meetings, as the Mods and the Corbs battle over each line of every decision. The shadow cabinet attack dog Jon “Sparkle” Ashworth, son of a casino croupier and a bunny girl, whiles away the hours by reading the poetry of Walt Whitman and W B Yeats on his iPad. Sparkle has learned that, to echo Whitman, to be with those he likes is enough.

I discovered Theresa May’s bit of rough – the grizzled Tory chairman, Patrick McLoughlin, a former Derbyshire coal miner – does his gardening in steel-toecapped wellies stamped “NCB” from his time down the pit thirty years ago. He’ll need his industrial footwear in Birmingham to kick around Tories revolting over grammar schools and Brexit.

Another ex-miner, Dennis Skinner, was the MP for Selfie Central in Liverpool, where a snap with the Beast of Bolsover was a popular memento. Alas, no cameras captured him in the Commons library demonstrating the contorted technique of speed-walkers. His father once inquired, “Why tha’ waddling tha’ bloody arse?” in Skinner’s younger days, when he’d top 7mph. Observers didn’t dare.

The Northern Poorhouse minister Andrew Percy moans that he’s been allocated a broom cupboard masquerading as an office in the old part of parliament. My snout claims that Precious Percy grumbled: “It’s so small, my human rights are violated.” Funny how the only “rights” many Tories shout about are their own.

The son of a very prominent Labour figure was caught trying to smuggle friends without passes into the secure conference zone in Liverpool. “Don’t you know who I am?” The cop didn’t, but he does now.

Kevin Maguire is the associate editor (politics) of the Daily Mirror

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 29 September 2016 issue of the New Statesman, May’s new Tories