White hair. Pink nose. Tail. The mouse is three months old. He’s been in a cage for four weeks in a period of enforced solitude. But today he will get a visitor.

A new mouse enters his cage, “our” mouse sizes him up — there’s “an initial pattern of exploratory activity”, as the researchers running this trial will put it. Then suddenly our mouse makes a startling move. He stands on his back legs, rattles his tail and aggressively bites the intruder, wrestling him to the ground.

The ensuing fight — brutal, violent and prompted simply by the introduction of another mouse — is videotaped by the researchers. They have seen this play out before. In almost all cases, the longer a mouse is isolated, the more aggressive it is to the newcomer.

So mice, once isolated, turn on each other. But is this truth about mice true too of men? Could loneliness not only be damaging our mental and physical health but also be making the world a more aggressive, angry place? And if so, what are the implications for a cohesive society and democracy?

With less than six weeks to go until the US presidential election, understanding the relationship between loneliness and the ballot box has never been more urgent. An attendee at a Trump campaign rally in Michigan this month © Bloomberg

Loneliness will inevitably increase as we head towards a second wave of Covid-19 and the return of many curbs on social interaction. This will impact not just the old, whose feelings of isolation were invoked by Boris Johnson, the UK prime minister, in a speech announcing new coronavirus rules this week. Early studies show that as many as one in four UK adults felt lonely during lockdown, with young people aged 18-29 and low-income workers the loneliest.

It was Hannah Arendt — one of the titans of 20th-century thought — who first wrote about the link between loneliness and the politics of intolerance. A young Jew, Arendt fled Germany in 1933. After the war, she devoted herself to making sense of why it was the country had descended into barbarism. In 1951 she published The Origins of Totalitarianism. It’s a wide-ranging book, encompassing the rise of anti-Semitism, the role of propaganda, and imperialism’s fusion of racism and bureaucracy. But at the end, she turns to what appears to be a surprising factor: loneliness.

Arendt writes that for those characterised by “isolation and lack of normal social relationships . . . it is through surrendering their individual selves to ideology that [they] rediscover their purpose and self-respect”. Loneliness, or “the experience of not belonging to the world at all”, is, Arendt writes, “the essence of totalitarian government . . . the preparation of its executioners and victims”.