Extremism is a word we use to describe belief systems (other, of course, than our own) that are based on violence, hatred or intolerance. In the decade after 9/11, studies of extremism tended to focus on the Salafi-jihadi ideology that inspired al-Qaeda. Following mass killings in Norway and New Zealand, the 2010s saw a new emphasis on the drivers of far-right extremism. Both these classic threats remain, often accompanied by those standard bigotries, misogyny and anti-Semitism. But over the past ten years, our shift online has brought further change, accelerating the process of radicalisation and causing extremism to metastasise into forms that threaten not only our physical safety but also our ability to distinguish truth from falsehood.

The current landscape can only be described as alarming. Dehumanising prejudices, conspiracy theories and disinformation, promoted by political actors or hostile states, leach into public discourse and call into question the boundary between the extreme and the normal. Virtual communities nourish the sick fantasies of cranks, and help them bridge