Group identity can bowl over a good idea just because the party line says so. Bad decisions get made, and no new information will change minds. You need parties to sort ideas into coherent packages, but if party activists cannot see that the other side sometimes has a point, they will stick to an unpopular policy on principle and lose elections.

A more serious problem with polarisation is the risk to democracy itself. The essential promise of democracy is that sometimes the other side has a point and voters can correct errors. The losing side accept the loss, knowing that they can try again in three years.

This transition of power is a sacred ritual, which is why it shocks to see it dismembered by Polariser-in-Chief Donald Trump. If, like Trump and his co-conspirators, you believe the other side are not just wrong, but evil, then logically they must be destroyed, not just beaten.

Josie's Post column on political polarisation is here. [Paywall]