In his nuggety little book What is Populism?, the Princeton academic Jan-Werner Müller suggests that populists reveal themselves when they try to deny the legitimacy of their political opponents or some of the citizenry: “What matters is populists’ anti-pluralism. They always exclude others … they claim to be the only legitimate representatives of the people and hence all others are at least morally excluded; and, less obviously … those who do not conform to the populists’ symbolic construction of the ‘real people’ are also shut out.” It is up to other politicians to hold the democratic line, Müller says. His analysis came to mind last week when Theresa May offered a kind of pep talk on democracy. She was in Manchester (birthplace of the Guardian) to mark 100 years since some women gained the vote.

For some it is becoming harder to disagree without also demeaning opposing viewpoints in the process Theresa May

The same day, the Oxford Internet Institute (OII) released the latest of its valuable papers, which, in effect, chart how populism is spreading through social media. Americans seem increasingly divided. Each side – but especially the Trump support group, on this evidence at least – is more entrenched with others of like mind, and prone to share similar sources of information, lots of it unreliable.

Also last week, a UK parliamentary committee enquiring into fake news trekked to Washington DC in the hope that representatives of Twitter, Facebook and Google, three of the most consequential entities in current political life, would talk more frankly about the challenges they face on behalf of everyone who would hold the line for democracy. The result seemed mixed.

May’s speech touched on the fight for suffrage, listed gains and emphasised the continuing democratic importance of free speech, a free and sustainable press, and tolerance and decency in public life. She said: “While there is much to celebrate, I worry that our public debate today is coarsening. That for some it is becoming harder to disagree without also demeaning opposing viewpoints in the process. I believe that all of us – individuals, governments, and media old and new – must accept our responsibility to help sustain a genuinely pluralistic public debate.”

Fake news sharing in US is a rightwing thing, says study Read more

The OII paper mapped the sharing of junk news among 13,477 Twitter users and 47,719 public Facebook pages between October 2017 and January 2018. It characterised junk news partly as “misleading, deceptive or incorrect information purporting to be real news about politics, economics or culture”. Users of the two social media platforms who were identified by the researchers as “Trump support group” and “hard conservative group” posted and shared junk news most. Those categorised as Democratic party supporters and others further to the US left also shared preferred information sources, but to a lesser extent, and appreciably less of the content came from what the researchers called the “junk news sites”, a list of more than 90 domains. On Twitter, the Trump support group shared 95% of the junk news sites, and accounted for 55% of the junk news traffic in the entire sample, despite comprising just 14% of those surveyed. “On Facebook,” the OII said, “the hard conservative group shares 91% of the junk news sites on the watch list, and accounted for 58% of junk news traffic in the sample.”

For all groups studied, the averages of junk news shared are 54% on Twitter and 33% on Facebook. This research, and other work like it, makes vivid a challenge that neither mainstream politicians nor new media giants seem fully to have faced yet: how will we hold the democratic line?

• Postscript added 19 February 2018: On 16 February 2018 an indictment was brought by US special counsel Robert Mueller against Russian individuals and organisations as part of his investigation into Russian interference in the 2016 US presidential election. Indirectly, it indicates the value of research like the OII studies in learning how to defend the integrity of democratic processes. Paragraph 45 of the indictment states: “Defendants and their co-conspirators also used false US personas to communicate with unwitting members, volunteers and supporters of the Trump Campaign involved in local community outreach, as well as grassroots groups that supported then-candidate Trump. These individuals and entities at times distributed the [Russian] organisation’s materials through their own accounts via retweets, reposts and similar means. Defendants and their co-conspirators then monitored the propagation of content through such participants.”

• Paul Chadwick is the Guardian’s readers’ editor