Small changes to the tone of posts fed to users of X can increase feelings of political polarisation as much in a week as would have historically taken at least three years, research has found.

A groundbreaking experiment to gauge the potency of Elon Musk’s social platform to increase political division found that when posts expressing anti-democratic attitudes and partisan animosity were boosted, even barely perceptibly, in the feeds of Democrat and Republican supporters there was a large change in their unfavourable feelings towards the other side.

The degree of increased division – known as “affective polarisation” – achieved in one week by the changes the academics made to X users’ feeds was as great as would have on average taken three years between 1978 and 2020.

Most of the more than 1,000 users who took part in the experiment during the 2024 US presidential election did not notice that the tone of their feed had been changed.

The campaign was marked by divisive viral posts on X, including a fake image of Kamala Harris cosying up to Jeffrey Epstein at a gala and an AI-generated image posted by Musk of Kamala Harris dressed as a communist dictator that had 84m views.

Repeated exposure to posts expressing antidemocratic attitudes and partisan animosity “significantly influences” users’ feelings of polarisation and boosts sadness and anger, they found.

Musk bought Twitter in 2022, rebranded it X and introduced the “for you” feed, which instead of only showing posts relating to accounts users actively follow, uprates content calculated to maximise engagement.

The extent to which more antidemocratic posts make users feel greater animosity towards political opponents “demonstrates the power of the algorithm”, said Martin Saveski, assistant professor at the University of Washington information school, who, with colleagues at the universities of Stanford, Johns Hopkins, and Northeastern, produced the study published in the journal Science.

“The change in their feed was barely perceptible, yet they reported a significant difference in how they felt about other people,” added Tiziano Piccardi, assistant professor at the Johns Hopkins University computer science department and co-author of the research. “Based on US trends, that shift corresponds to roughly three years of polarisation.”

The study also found that relatively subtle changes to the content of users’ feeds can significantly reduce political animosity among Republicans and Democrats, suggesting X had the power to increase political harmony if Musk chose to use it in that way.

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“What’s exciting about these results is that there is something that the platforms can do to reduce polarisation,” said Saveski. “It’s a new approach they could take in designing their algorithms.”

X was approached for comment.

Eight in 10 American adults say that not only can Republicans and Democrats not agree on policies and plans, but they cannot agree on basic facts, according to Pew research. More than half of people in the UK believe the differences in people’s political views are so divisive it is dangerous for society, recent polling by Ipsos found.

The changes in political polarisation resulting from exposure to X posts were measured using a novel approach. First, the academics used AI to analyse posts in X’s “for you” feed in real time. Then the system showed more divisive posts to one cohort and fewer divisive posts to another, a power normally the sole preserve of X. Divisive posts included those that showed support for undemocratic practices, partisan violence, opposition to bipartisan consensus and biased evaluations of politicised facts.

After a week of reading these subtly altered feeds, the researchers asked users to rank how warm or cold, favourable or unfavourable they felt towards their political opponents. The changes in “affective polarisation” were ranked at more than two degrees on a 0 to 100 degree “feeling thermometer”. This was the same amount of increased polarisation that typically occurred in the US in the four decades to 2020. Feeding users fewer posts with antidemocratic attitudes and partisan animosity decreased political division by a similar amount.

Social media platforms have long been accused of encouraging divisive content to boost user engagement and therefore advertising revenues. But the research found that while there was a slight reduction in overall engagement in terms of time spent on the platform and numbers of posts viewed when divisive content was down-ranked, those users tended to “like” or repost more often.

“The success of this method shows that it can be integrated into social media AI to mitigate harmful personal and societal consequences,” the authors wrote. “At the same time, our engagement analyses indicate a practical trade-off: interventions that down-rank [antidemocratic and partisan content] may reduce short-term engagement volume, posing challenges for engagement-driven business models and supporting the hypothesis that content that provokes strong reactions generates more engagement.”

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