It takes seven years to know your way around Parliament. That’s what I was told when I arrived in the Commons press gallery seven years ago, but I am still none the wiser about how to get from the Snake Pit to the North Curtain Corridor, and have only recently discovered the location of the Yellow Submarine. As a building, the Palace of Westminster is a confusing, contradictory rabbit warren of underground corridors, secret briefing rooms at the top of towers and rooms with strange names. The very fabric of the building is dysfunctional, with pieces of masonry falling onto cars, and mice creeping through kitchens.

Winston Churchill famously said that ‘we shape our buildings, and afterwards our buildings shape us’ — which perhaps explains why the inhabitants of one of the best-known buildings in the world lead such dysfunctional lives. Marriages disintegrate within years of an MP entering the House of Commons; addictions are easy to develop and just as easy to hide; mental illness is so prevalent that Parliament has had to set up a special treatment service.

Research I carried out for my new book found that of the 666 MPs elected between 2010 and 2015 (some in by-elections), 12 per cent got divorced while serving in Parliament. Of the 307 Conservatives, 32 saw their marriages end, as did 9 per cent of Labour MPs. For the Lib Dems, it was 12 per cent, and of the six SNP MPs elected in 2010, four split up with their spouses while serving. The SNP’s marital troubles were particularly complicated by the fact that two of its MPs — Angus MacNeil and Stewart Hosie — had been having affairs with the same female journalist, though not at the same time.

This isn’t particularly out of step with the general population, but what is significant is how many people cite parliamentary life as the cause of their split — and how quickly after getting elected their marriages tend to collapse. After just three years in the job, 25 of the Tory MPs first elected in 2010 were heading for the divorce court.

Most are surprised by quite how damaging parliamentary life is, even for those who thought that things were going well with their other half. But if your family is in the constituency and you are in Westminster half the week, then the time apart is inevitably going to place a strain on your relationship. One MP whose marriage broke down within a couple of years of him getting elected says: ‘I thought my marriage was rock-solid. But I had so many people warning me that you should absolutely not do this unless your spouse is 100 per cent supportive. I thought, well, mine is about 90 per cent supportive, when in reality she was 50 per cent and that went quickly down to 30.’

Many affairs between married MPs start on a Monday night, when they are all cooped up in Parliament waiting for late votes. Then there are the parties, where quite middle-aged characters can still end up surrounded by adoring young researchers, press officers and think-tank staffers. One spouse of an MP told me she wouldn’t be surprised if her husband did cheat on her at some point, given the number of ‘groupies who fling themselves at him, even when I’m present’. Another remarked: ‘Some stop when they find out that he’s married. Others don’t care at all. There are a lot of women in Westminster who are not my sisters.’

Former Ilford North MP Lee Scott had already seen the toll Parliament had taken on his friends when he entered the Commons in 2006: ‘I had a lot of friends who were MPs and I had seen that in 60 per cent of cases their marriages had broken up. You sort of thought to yourself, “Why is that?” And then you realise that you’re with a lot of people and you can be quite lonely. Voting late, sitting in your office on your own.’

That loneliness manifests itself in other ways too. Colleagues of the late Charles Kennedy suspected that his alcoholism grew out of being a lonely and unusually young MP, while the husband of Fiona Jones, the Labour MP who died of liver failure aged just 49, said that ‘nobody cared if she drank’ in the House of Commons. In 2012, the Commons doctor Ira Madan told a staff meeting that she was concerned about the proportion of MPs she had seen with alcohol–related problems.

Labour MP Liam Byrne, whose father had a drinking problem, points out that many of his colleagues were also children of alcoholics, which makes them far more likely to develop an addiction themselves. Indeed, politics itself is addictive, with MPs locked into a cycle of dopamine hits from gaining power and praise. A surprising number come from broken or dysfunctional homes, and some admit that their drive comes partly from trying to show an absent or abusive parent that they were in fact worth loving.

Perhaps these sad backstories also explain why so many MPs keep reading abuse on social media. A common characteristic of mental ill-health is seeking confirmation from others that you really are as useless as you personally feel. And there’s a ready supply of anonymous trolls on Twitter who’ll offer just that.

Tory MP Charles Walker, who has spoken openly about his obsessive compulsive disorder, believes it isn’t so much that Parliament makes people unwell as that it exploits existing weaknesses. ‘If you are predisposed to having a weakness or a condition, Parliament will expose it,’ he says. ‘If you have a predisposition to alcoholism, Parliament will accelerate it. If your marriage is weak, it might have failed in ten years, but Parliament will ensure it falls apart in five. If you have an underlying disposition to a mental illness, which may never have developed, Parliament will ensure that it does.’

MPs do choose to put themselves through these ordeals. But their children do not. One MP recalls his son’s primary school teacher saying that when she asked him if he wanted to be an MP like his daddy, he told her, ‘No, I could never do that to my family.’ The son might, like many political children, grow up and find he has changed his mind about wanting to enter the Commons. But he put his finger on something that many gifted people who choose to avoid going into Parliament cite as a major factor: they don’t want their family to suffer.

Given what I’ve seen, I think they’re right to worry. But what a loss to democracy that normal, well-balanced people stay away not because the job itself is hard, but because the life it demands is almost impossible to live.