Here’s an everyday event in Donald Trump’s America. Two people run into each other in their neighbourhood, or virtually on Facebook, and instantly start discussing the president. If they are liberals, one might say, “Did you see that tweet?!” and the other will tap his forehead meaningfully. If the two support Trump, they might share a grumble about lying media.

These people are participating in the political polarisation that has riven the US and, to a much lesser degree, Brexit Britain. But they are also signalling something else to each other, namely: “You and I belong to the same tribe. We have a shared identity, and something to talk about.” In other words, they are doing something that is usually considered positive: they are forging a new kind of community. Everyone rightly laments polarisation, but what’s often overlooked is that it’s creating a novel sense of belonging, and identity, in societies that were getting scarily atomised.

Many people in western countries have been struggling to define who they are, and what tribe they belong to. Fifty years ago, most people found identity through their family, church, neighbourhood and (if male) their job and trade union.