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.

Photo: Getty
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Like it or hate it, it doesn't matter: Brexit is happening, and we've got to make a success of it

It's time to stop complaining and start campaigning, says Stella Creasy.

A shortage of Marmite, arguments over exporting jam and angry Belgians. And that’s just this month.  As the Canadian trade deal stalls, and the government decides which cottage industry its will pick next as saviour for the nation, the British people are still no clearer getting an answer to what Brexit actually means. And they are also no clearer as to how they can have a say in how that question is answered.

To date there have been three stages to Brexit. The first was ideological: an ever-rising euroscepticism, rooted in a feeling that the costs the compromises working with others require were not comparable to the benefits. It oozed out, almost unnoticed, from its dormant home deep in the Labour left and the Tory right, stoked by Ukip to devastating effect.

The second stage was the campaign of that referendum itself: a focus on immigration over-riding a wider debate about free trade, and underpinned by the tempting and vague claim that, in an unstable, unfair world, control could be taken back. With any deal dependent on the agreement of twenty eight other countries, it has already proved a hollow victory.

For the last few months, these consequences of these two stages have dominated discussion, generating heat, but not light about what happens next. Neither has anything helped to bring back together those who feel their lives are increasingly at the mercy of a political and economic elite and those who fear Britain is retreating from being a world leader to a back water.

Little wonder the analogy most commonly and easily reached for by commentators has been that of a divorce. They speculate our coming separation from our EU partners is going to be messy, combative and rancorous. Trash talk from some - including those in charge of negotiating -  further feeds this perception. That’s why it is time for all sides to push onto Brexit part three: the practical stage. How and when is it actually going to happen?

A more constructive framework to use than marriage is one of a changing business, rather than a changing relationship. Whatever the solid economic benefits of EU membership, the British people decided the social and democratic costs had become too great. So now we must adapt.

Brexit should be as much about innovating in what we make and create as it is about seeking to renew our trading deals with the world. New products must be sought alongside new markets. This doesn’t have to mean cutting corners or cutting jobs, but it does mean being prepared to learn new skills and invest in helping those in industries that are struggling to make this leap to move on. The UK has an incredible and varied set of services and products to offer the world, but will need to focus on what we do well and uniquely here to thrive. This is easier said than done, but can also offer hope. Specialising and skilling up also means we can resist those who want us to jettison hard-won environmental and social protections as an alternative. 

Most accept such a transition will take time. But what is contested is that it will require openness. However, handing the public a done deal - however well mediated - will do little to address the division within our country. Ensuring the best deal in a way that can garner the public support it needs to work requires strong feedback channels. That is why transparency about the government's plans for Brexit is so important. Of course, a balance needs to be struck with the need to protect negotiating positions, but scrutiny by parliament- and by extension the public- will be vital. With so many differing factors at stake and choices to be made, MPs have to be able and willing to bring their constituents into the discussion not just about what Brexit actually entails, but also what kind of country Britain will be during and after the result - and their role in making it happen. 

Those who want to claim the engagement of parliament and the public undermines the referendum result are still in stages one and two of this debate, looking for someone to blame for past injustices, not building a better future for all. Our Marmite may be safe for the moment, but Brexit can’t remain a love it or hate it phenomenon. It’s time for everyone to get practical.