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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Westminster terror: Parliament hit by deadly attack

The Met Police is treating the events in Westminster as a "terrorist incident". 

A terrorist attack outside Parliament in Westminster has left four dead, plus the attacker, and injured at least 40 others. 

Police shot dead a man who attacked officers in front of the parliament building in London, after a grey 4x4 mowed down more than a dozen people on Westminster Bridge.

At least two people died on the bridge, and a number of others were seriously hurt, according to the BBC. The victims are understood to include a group of French teenagers. 

Journalists at the scene saw a police officer being stabbed outside Parliament, who was later confirmed to have died. His name was confirmed late on Wednesday night as Keith Palmer, 48.

The assailant was shot by other officers, and is also dead. The Met Police confirmed they are treating the events as a "terrorist incident". There was one assailant, whose identity is known to the police but has not yet been released. 

Theresa May gave a statement outside Number 10 after chairing a COBRA committee. "The terrorists chose to strike at the heart of our Capital City, where people of all nationalities, religions and cultures come together to celebrate the values of liberty, democracy and freedom of speech," she said.

London Mayor Sadiq Khan has tweeted his thanks for the "tremendous bravery" of the emergency services. 

Labour leader Jeremy Corbyn also released a short statement. He said: "Reports suggest the ongoing incident in Westminster this afternoon is extremely serious. Our thoughts are with the victims of this horrific attack, their families and friends. The police and security staff have taken swift action to ensure the safety of the public, MPs and staff, and we are grateful to them."

After the incident this afternoon, journalists shared footage of injured people in the street, and pictures of a car which crashed into the railings outside Big Ben. After the shots rang out, Parliament was placed under lockdown, with the main rooms including the Commons Chamber and the tearoom sealed off. The streets around Parliament were also cordoned off and Westminster Tube station was closed. 

Those caught up in the incident include visitors to Parliament, such as schoolchildren, who spent the afternoon trapped alongside politicians and political journalists. Hours after the incident, the security services began evacuating MPs and others trapped inside Parliament in small groups. 

The MP Richard Benyon tweeted: "We are locked in Chamber of House of Commons." Shadow education secretary Angela Rayner tweeted: "I'm inside Parliament and me and my staff are safe."

The MP Jo Stevens was one of the first to confirm reports that a police officer had been attacked. She tweeted: "We've just been told a police officer here has been stabbed & the assailant shot."

George Eaton, the New Statesman politics editor, was in the building. He has written about his experience here:

From the window of the parliamentary Press Gallery, I have just seen police shoot a man who charged at officers while carrying what appeared to be a knife. A large crowd was seen fleeing the man before he entered the parliamentary estate. After several officers evaded him he was swiftly shot by armed police. Ministers have been evacuated and journalists ordered to remain at their desks.   

According to The Telegraph, foreign minister Tobias Ellwood, a former soldier, tried to resucitate the police officer who later died. Meanwhile another MP, Mary Creagh, who was going into Westminster to vote, managed to persuade the Westminster tube staff to shut down the station and prevent tourists from wandering on to the scene of the attack. 

A helicopter, ambulances and paramedics soon crowded the scene. There were reports of many badly injured victims. However, one woman was pulled from the River Thames alive.

MPs trapped inside the building shared messages of sympathy for the victims on Westminster Bridge, and in defence of democracy. The Labour MP Jon Trickett has tweeted that "democracy will not be intimidated". MPs in the Chamber stood up to witness the removal of the mace, the symbol of Parliamentary democracy, which symbolises that Parliament is adjourned. 

Brendan Cox, the widower of the late, murdered MP Jo Cox, has tweeted: "Whoever has attacked our parliament for whatever motive will not succeed in dividing us. All of my thoughts with those injured."

Hillary Benn, the Labour MP, has released a video from inside Parliament conveying a message from MPs to the families of the victims.

Former Prime Minister David Cameron has also expressed his sympathy. 

While many MPs praised the security services, they also seemed stunned by the surreal scenes inside Parliament, where counter-terrorism police led evacuations. 

Those trapped inside Parliament included 40 children visiting on a school trip, and a group of boxers, according to the Press Association's Laura Harding. The teachers tried to distract the children by leading them in song and giving them lessons about Parliament. 

In Scotland, the debate over whether to have a second independence referendum initially continued, despite the news, amid bolstered security. After pressure from Labour leader Kezia Dugdale, the session was later suspended. First Minister Nicola Sturgeon tweeted that her "thoughts are with everyone in and around Westminster". The Welsh Assembly also suspended proceedings. 

A spokesman for New Scotland Yard, the police headquarters, said: "There is an ongoing investigation led by the counter-terrorism command and we would ask anybody who has images or film of the incident to pass it onto police. We know there are a number of casualties, including police officers, but at this stage we cannot confirm numbers or the nature of these injuries."

Three students from a high school from Concarneau, Britanny, were among the people hurt on the bridge, according to French local newspaper Le Telegramme (translated by my colleague Pauline). They were walking when the car hit them, and are understood to be in a critical condition. 

The French Prime Minister Bernard Cazeneuve has also tweeted his solidarity with the UK and the victims, saying: "Solidarity with our British friends, terribly hit, our full support to the French high schoolers who are hurt, to their families and schoolmates."

Julia Rampen is the editor of The Staggers, The New Statesman's online rolling politics blog. She was previously deputy editor at Mirror Money Online and has worked as a financial journalist for several trade magazines.