Treason used to be a word associated with spies or assassins, one reserved for crimes against the state of the utmost severity. Yet to take just two recent examples, here is an article in the Sun describing how “Treacherous Theresa” has surrendered our freedom. “May's name will rank alongside those of the worst eels in Western history — and she deserves it”. Cross the Atlantic, and here is a presenter at Fox News calling for “the traitorous, treasonous group that accused Donald Trump” to be locked up. “True justice” she calls it.

It seems that the word treason is now being used to describe the actions of a prime minister the writer disagrees with, or to describe a legal inquiry that successfully prosecuted a number of individuals once close to the US president. How did this escalation of language happen, and does it matter? To understand both questions we need to start with what links these two examples: they are both from media outlets owned by Rupert Murdoch.

As a detailed analysis of the Murdoch dynasty by Jonathan Mahler and Jim Rutenberg of the New York Times shows, Rupert Murdoch created and, with the help of his sons, runs a supremely successful media empire. Media businesses, in particular, are subject to regulations and part of Murdoch’s success has been bypassing them. As Mahler and Rutenberg write: “Murdoch’s news empire is a monument to decades’ worth of transactional relationships with elected officials.” These are not always right-wing politicians, as Murdoch’s support for Tony Blair showed, but they tend to be, reflecting the News Corp. head’s own views and business interests.

Murdoch is not part of a long-standing establishment but rather the opposite. In that sense, he is a particularly influential example of what we could call the neoliberal elite, which Aeron Davis describes so well in his book Reckless opportunists: Elites at the end of the Establishment. But why would someone like Murdoch, and the UK’s other press barons, be happy with people employed by their media organisations using inflammatory language such as “treasonous”?

The standard response of many people in the media is that it sells newspapers. Newspapers or radio stations or TV channels like Fox are merely expressing the views of their readers. There is no doubt that is partly true, but the reality is that this is a two-way relationship. The media reflects the views of those that read or watch its content, but it also shapes those views. The excuse that titles simply reflect their audience’s opinions cannot be used to absolve them of responsibility for what is said or written.

There is now overwhelming academic evidence that the media can have a potentially powerful influence on what those who consume it think and do. A particularly interesting and powerful recent study by two economists looked at US cable channels, which remain the main source of news on political campaigns, even in the digital age. They isolate viewers who view these channels simply because of their place in the channel ranking, rather than because of their political preferences, in order to look at how influential the channel was.

They found that the existence of Fox News boosted the Republican vote share in 2000 by about 0.5 per cent, which fits with another study that used a different method to isolate the influence of Fox. However, the growing viewership and increasingly right-wing stance of Fox swelled its impact on the Republican vote share in 2008 to a huge 6 per cent, a far greater influence than that of any other channel.

An equally revealing finding is that the political stance of Fox is far to the right of where it should be to maximise viewers. In other words, Fox is broadcasting material that maximises its ability to shift its audience to the right, rather than to maximise its profits.

Unfortunately, there are as yet no studies of Trump’s election, but it seems very likely that the influence of Fox was crucial in his 2016 victory over Hillary Clinton. During the Republican primaries, Fox had a more critical view of Trump, perhaps because Murdoch did not think he was up to the job. Mahler and Rutenberg found three sources who reported Murdoch saying “he’s a [expletive] idiot” about Trump, although Murdoch’s spokesman denies this. It was, ironically, other broadcasters who gave Trump significantly more coverage than his opponents because he was “good TV”. Reporters then talked favourably about Trump, simply because he was gaining vote share.

After it became clear Trump would win the Republican nomination, Murdoch saw his chance to form a close relationship with a prospective US president. That influence is now so strong that one recent article in the New Yorker was entitled “The Making of the Fox News White House” (h/t @rupertww).

Would this level of influence also apply to the UK press? There is every reason to think so. For example, this study found that when Murdoch’s Sun switched support to Labour, it increased the party’s vote in 1997 by 2 per cent. That was not enough to influence the result, but when the Sun switched back to the Conservatives in 2010, it had a similar impact in the opposite direction, which was enough to influence that result. Newspapers influence attitudes towards austerity, and the best predictor of attitudes on immigration is newspaper readership. I note other studies with similar findings here.

There is no doubt that both Trump and Brexit reflect deep, underlying causes. What the media is able to do is help direct those causes in particular ways. To again quote Mahler and Rutenberg: “The Murdoch empire did not cause this [populist] wave. But more than any single media company, it enabled it, promoted it and profited from it.” Given the narrowness of Trump’s victory and the Brexit majority, it is extremely likely that Fox News and the Brexit press were respectively the difference between defeat and victory.

Once we accept that the media can have an influence on mainstream politics, it would be very surprising if it did not also influence the political fringe. We should be shocked at soldiers using a photograph of the leader of Her Majesty’s Opposition for target practice, but we cannot simply attribute this to soldiers expressing their personal views about Corbyn’s attitude to Nato and his past associations. What, in soldiers’ eyes, legitimises such behavior is the relentless demonisation of the Labour leader in the press. The press both reflects and influences.

In a more serious case than the British army’s target practice, Corbyn was the intended target of the man responsible for the terrorist attack at Finsbury Park mosque. A Labour MP, Jo Cox, was murdered during the Brexit campaign, while a member of a far-right organisation plotted to kill one of her colleagues. Many MPs have received credible death threats. According to Britain’s counter-terrorism chief, the man responsible for the Finsbury Park attack was “driven to an act of terror by far-right messaging he found mostly on mainstream media”. As Guardian columnist Gary Younge writes, the threat from far-right terrorism is growing alarmingly and while “the violence may come from the fringes, the encouragement comes from the centre.”

If you think the idea of terrorists being inspired by the mainstream media is fanciful, just listen to the extract from Fox I linked to in the first paragraph above. Of course this is an unintended effect of the extreme language the partisan media uses.

Whether the rise of far-right parties and groups is an unintended consequence is less clear, particularly when the BBC chooses to broadcast an interview with a far-right leader straight after the murder of 50 people in New Zealand. There is academic evidence that media coverage of far-right groups such as Ukip increases support for them and, as I have already noted, this is partly why Trump became the Republican presidential candidate.

But the main reason the partisan media now use such language is to “fire up the base”, who in turn will influence politicians in the way media owners want. This route of influence is well-established in the US, which is why David Frum, a former speechwriter to George W. Bush, says “Republicans originally thought that Fox worked for us. And now we’re discovering we work for Fox.” We are now seeing it happen over Brexit, as Conservative MPs who oppose a no-deal Brexit are targeted for deselection and would-be leaders play to a base which is heavily influenced by the partisan press it reads.

There is one important difference between the UK and US, however. The US retains a widely-read independent press that can discuss the influence of the media. In the UK, independent broadcasters would find that more difficult and, in any case, they mostly do not try.

UK journalists tend not to talk about the partisan press as a key political player, perhaps in part because they would be talking about colleagues who work for that press. The myth that the media just reflects and does not influence is too convenient for many, so the media remains the elephant in the room in discussions about politics and political extremism in the UK.