Elections 7 July 2017 Could by-elections cost the Conservatives their parliamentary majority? A terrifying number of people have asked, so I answered. Photo: Getty Sign UpGet the New Statesman\'s Morning Call email. Sign-up Where there’s death, there’s hope. Certainly that’s the quiet joke between Labour politicians and their aides about the Conservatives’ wafer-thin parliamentary majority. Lose seven by-elections and the government would be unable to see off a vote of no confidence, as its alliance with the Democratic Unionist Party would no longer be enough to outnumber the parties of the left and centre. (Though there are a variety of reasons the Liberal Democrats might abstain, prolonging the government’s life.) The possibility that the government could be defeated through a series of by-election losses is now the subject of more entries for the “You Ask Us” section of our podcast than any other. Could the Conservatives be forced into an election they don’t want thanks to by-elections? There are two answers to this question. The first is “Yes, obviously”. Labour needs only a one-point swing from the Conservatives to take 15 seats from them – not quite enough to be the largest party, but, as I’ve explained before, a hung parliament is much more friendly territory for Labour than the Conservatives, as there are many more possible buddies for Jeremy Corbyn than Theresa May. Labour would be able to pass at least some of its agenda and to get budgets through without too much difficulty. As oppositions tend to do a lot better in by-elections than at the subsequent general election – to take the last two times Britain changed governments, the swing from Conservative to Labour in by-elections during the 1992 to 1997 parliament was 30 per cent, the swing at the election 10.2 per cent, the swing for the Conservatives in the 2005 to 2010 parliament was 12 per cent, the swing at the election 5 per cent – the “danger zone” for the Conservatives is a lot bigger than Labour’s poll lead would suggest. (The party now has a small average lead of two points, enough to gain more than 30 seats and be the largest party in another hung parliament). Let’s say that Labour leads the Conservatives 47 to 35 per cent, a five-point swing from Tory to Labour on 2017 and enough to take power with a parliamentary majority. You'd expect that Labour would have a good hope of winning any seat with a Tory majority below 12,000 at a by-election – putting just 162 seats outside of Labour's grasp. So it’s not even a case of Labour needing to wait for by-elections in seats they might hope to take at a general election. All but the safest Conservative seats should be vulnerable. But the bad news is that sufficient numbers of by-elections, let alone sufficient numbers in that outer core of Conservative seats, are unlikely to arise. Why? Because by-elections are happening less, and by-elections in Conservative seats even less so. MPs, like everyone else, are getting healthier and living longer. (The average age has also dropped a bit, but has remained at around 50 since 1979.) You can see this very visibly in the number of by-elections in the ten parliaments from 1945 to 1979, in which 323 were held, and the nine parliaments from 1979 to the present day, in which just 115 have been held. In 1992-7, the last time that the Conservatives had seven by-elections in a parliament, life expectancy was 15 years lower than it is today. 50 is not as old as it was. (For the purposes of this piece, I have discounted the four MPs in this period killed in acts of terror: Airey Neave, Ian Gow, Anthony Berry, and Jo Cox.) The average MP, regardless of party, is three times less likely to die in office than they were from 1945 to 1979 – but what has remained unchanged even as the overall number of by-elections has declined is the gap between the number of by-elections in Labour-held seats and Conservative ones. In both raw terms (more Labour MPs have died in office than Conservative) and percentage terms (of the total number of Labour MPs, a higher proportion have died in office than the number of Conservative MPs). As improvements in health have decreased the mortality rate, the number of Conservatives dying in office has fallen further still. Of the 24 occasions when a seat fell vacant due to an MP's death since 2005, just one, that of Eric Forth in 2006, was of a Conservative MP, with Labour MPs the biggest, both in overall numbers and as a percentage of the party’s MPs. Why do Conservative MPs tend not to die in office? Among other things, the average Conservative MP is younger, by about five years, than the average Labour MP. They are less likely to have worked in a manual occupation or to have grown up in poverty. There are far fewer (15 per cent) above the retirement age than there are in Labour (30 per cent). And the effects are clear – on average, the death rate among Labour MPs is more than double that of Conservative MPs. By-elections aren’t only caused by deaths but those that aren’t tend to be in safer seats, and the fear of a Labour government is such in Conservative circles that the pressure to stay in your post rather than risk letting Labour in is likely to be high. Which isn’t to say that there couldn’t be enough by-elections in Conservative seats to turf the party out of office. But it is to explain why their narrow majority could prove more enduring than we expect. › Defend Europe's plan to block migrants is just the far right setting sail on the Med Stephen Bush is political editor of the New Statesman. His daily briefing, Morning Call, provides a quick and essential guide to domestic and global politics. Subscribe For more great writing from our award-winning journalists subscribe for just £1 per month!