What makes a sackable offence for a politician these days? It’s a mystery we’ve discussed on the New Statesman podcast a number of times in recent weeks, due to the apparent double standards in Theresa May’s hiring and firing policy.

The latest example is the departure of First Secretary Damian Green, who was forced to resign from cabinet due to “misleading” statements about police claims that porn was discovered on his office computer.

In his resignation letter, Green admits his behaviour broke the ministerial code – nothing to do with alleged inappropriate advances towards Tory writer Kate Maltby, which he refutes, or pornography on a parliamentary computer – but because he falsely denied knowledge of the pornography’s existence. (He denies downloading or watching porn on Commons computers himself.)

So if the technical reason for his departure is making inaccurate comments, why is the Brexit Secretary David Davis still in a job?

In front of the Brexit select committee at the beginning of December, Davis was forced to admit that he had not made any impact assessments for Brexit. This contradicted his previous assurances both to the Commons in February and the Foreign Affairs select committee last year that his department was analysing the impact of leaving the EU on “50 sectors”.

It was this assurance that led to Parliament passing a motion to call for ministers to release these papers.

MPs accused him of “blatant lying” and having “misled” the House on this issue.

So why hasn’t he been sacked?

Last month, there was the case of former International Development Secretary Priti Patel, who had to resign after breaking the ministerial code in early November by having undisclosed meetings with Israeli officials, including the Prime Minister Benjamin Netanyahu, when on holiday.

She made misleading claims that the Foreign Office knew in advance of her visits (it didn’t), and her actions – in the words of her resignation letter – “fell below the standards of transparency and openness that I have promoted and advocated”.

Again, for failing to be open and transparent, a cabinet minister had to resign. So why not Davis for doing the same? And why not other cabinet members for other potential breaches of the ministerial code?

The Foreign Secretary Boris Johnson’s freelance Brexit politicking (remember his 4,000-word article instructing the Prime Minister on his preferred approach to negotiations?) surely stretches the principle of collective responsibility beyond recognition. Or if not for that, shouldn’t his credibility as a minister have run out when his careless remark during a select committee hearing has potentially condemned a British woman to five more years in an Iranian prison?

And then there was former Defence Secretary Michael Fallon, who resigned a few days before Patel because he had “fallen below the high standards that we require of the Armed Forces that I have the honour to represent” – thought to be a reference to allegations of sexual misconduct flying around Westminster. But while Fallon resigned, Green stayed in his role while under investigation.

What’s going on here? Is it simply that what makes a sackable offence depends on who’s doing the sacking? It may be that Theresa May is so weak that she can only get away with the least politically-contentious sackings; as Stephen Bush points out, there are cabinet members who Theresa May would like to get rid of for political purposes – like Philip Hammond and Boris Johnson – but can’t.

And it feels like ministers are allowed to get away with far more than they did even a few years ago, for example when immigration minister Mark Harper stood down in 2014 over the illegal status of his cleaner.

But with the help of research by Liam McLoughlin, a PhD politics researcher at the University of Salford who has been looking into this subject, it looks like historically resignations due to scandal aren’t that common. He found that from 1945-97, 34 per cent of all scandals resulted in a ministerial resignation (87 per cent of which were sex scandals, and 50 per cent financial). And in terms of MPs rather than ministers, only eight (out of 51 resignations) have stood down and triggered a by-election due to general scandal from 1979-2016.

“While resigning mid-term is a fantastic way to generate attention as a protest, resigning due to a scandal will leave you with that mark,” says McLoughlin. “It’s the nuclear option.”

In fact, politicians are more likely to carry out a “soft-resignation” – simply deciding not to stand at the next election – than quit (a record 149 MPs chose not to stand in 2010, compared with the former record in 1945 that saw 128 MPs not seeking re-election).

Last month, I found that cases of MPs resigning have been rare throughout history, suggesting more about our system than the position of any one prime minister. Could this be the same with ministers?

“There is a system of discipline in place with risk and reward,” finds McLoughlin. “Be good, and you can play on the field – become a minister. Be bad, and we will take it away and you’ll be benched… This allows some to resign as a minister due to a scandal, but still come back in some fashion.”

The recent rehabilitation of Liam Fox is one example, but Peter Mandelson’s multiple resurrections in the New Labour years suggests this isn’t unique to May’s situation.

So it looks like errant ministers have generally been getting away with it for most of history – and are still doing so today.