The genuine fury from the left at people three inches closer to the political center reflects a turbocharged tribalism. Freud called this “the narcissism of small differences”; the legal scholar Cass Sunstein calls it “group polarization,” where “deliberation tends to move groups, and the individuals who compose them, toward a more extreme point.” In his 2019 book Conformity, Sunstein noted that “confident people are both more influential … and more prone to polarization.” One consequence of group polarization, he found, was that those who held a minority position, or had useful information that ran counter to the prevailing trend, stayed silent or were ignored. Their groups therefore made worse decisions.

The Twitter Primary drives its members to extremes, while chilling the speech of outsiders. An excess of certainty leads activists to bad decisions and misapprehensions. Spend enough time on Twitter and you could believe that Corbyn “won the argument” in December, despite losing the general election. The postmortem on Labour’s defeat risks being hampered by a pervasive sense on social media that the party didn’t really lose, not really: Well, everyone I know voted for Corbyn. Activists may intellectually concede the reality of the Conservatives’ 80-seat majority, but it doesn’t feel like the Tories won. And that means there is less reason for them to support a change in tactics.