American politics jumps from one polarised moment to the next. The Senate splits almost entirely on party lines over Donald Trump’s impeachment, and then self-proclaimed “socialist” Bernie Sanders emerges as Trump’s likely opponent in November. 

It’s a good moment to publish a book called Why We’re Polarized. “I joke that the Senate performed a live interpretive dance of the book,” says the author Ezra Klein, founder of explanatory news site Vox, on the phone from San Francisco.

What is driving polarisation? And where does it end? US politics didn’t use to be like this. Until the 1960s, the two main parties were jumbles of disparate belief systems: Democrats combined northern liberals and southern segregationists, while Republicans weren’t particularly conservative.