With news that the Democratic Unionist Party’s 10 MPs are considering voting down the budget over Brexit divisions, the usual arguments are being rolled out about the ‘tail wagging the dog’ during coalitions.

For two out of three elections in a row, we see power-brokering under Parliament’s supposedly majoritarian voting system. For those opposed to switching to a proportional voting system for Westminster as a ‘recipe for power-sharing’, this is quite the conundrum.

Following the 2017 election, the DUP agreed to a confidence and supply agreement in exchange for over £1bn in extra spending over two years, with money allocated to go towards infrastructure, health, education, deprivation and ultra-fast broadband in Northern Ireland.

It means the DUP agree to support the government in votes of confidence (a vote to prove that the government has a majority and should be the government), as well as in votes of supply (the budget). They are free to vote as they please on other matters, such as the recent Agriculture Bill, where the DUP abstained.

While fewer than half of voters voted for them, together the two parties have a working majority of 13 MPs. This is because the convoluted way we elect MPs helps some parties, but hinders others.

Whether you back their take on Brexit or not, the DUP are able to exert so much pressure on these negotiations but because they are over-represented in Parliament.

That’s because a general election in the UK is actually 650 unrelated mini-elections. To win a seat a candidate doesn’t need to win the support of the majority of voters in their constituency, just one more than whoever comes second.

So if supporters of a party are spread out around the country, they can get no MPs at all, even if they number in the hundreds of thousands.

The outcome is a parliament that doesn’t properly represent the country, as millions of voters effectively elect nobody at all.

It’s why the Green Party (with 526,000 supporters spread-out across England and Wales) only have one MP, and the DUP (with 292,000 supporters condensed in the north-east of Northern Ireland) have ten MPs. Likewise, the Conservatives received 42% of the vote and got 49% of the seats.

The long and short of it is that Westminster’s voting system prioritises the geographical concentration of votes over most other factors.

The supposed benefit of First Past the Post is that it places a single party with a workable majority in government. A quick look at the last three general elections would quickly knock that notion on the head, yet parties still campaign on the basis that they will be in government on their own.

So, rather than discuss who their likely partners might be during the public campaign so voters can make an informed decision, they put together a deal after the election.

Westminster’s voting system actively increases the likelihood of deals that include budget increases for specific geographic areas and parties that the vast majority could never vote for, by hugely over-representing parties who only stand in specific geographic areas. Rather than properly representing political viewpoints themselves – from green politics and liberalism to single-issue parties.

With the whole country represented in parliament – under a proportional voting system – there are more potential partners to work with. If a minor party makes an unreasonably large demand on the major party, the major party can shop around for new partners.

It is only in Westminster – with a broken voting system artificially reducing the pool of potential partners and over-representing geographic parties – that the ‘tail can wag the dog’.