Partisan reflexes often get blamed for whatever is currently going wrong in Washington. But partisanship works in part because it is such a helpful mental shortcut. Take the repeal of the Affordable Care Act, or Obamacare, by the House of Representatives, says Mr Bartels. Repealing the law is bound to have substantial effects on people’s lives, but measuring those effects against a background of changing social conditions, health-insurance markets and medical technology would be tricky even for a health economist, let alone for a moderately well-informed voter. “So why not accept the judgment of people you trust?”

This kind of groupthink is so powerful that it shapes the way people see the world around them. Right after the election, and more than two months before Mr Trump took office, Republicans told pollsters that their personal finances were in much better shape than they had been the week before the ballot. Democrats said the opposite. The question had nothing to do with politics, and yet the answers given were somehow conditioned by the election. To understand how this can happen, compare and contrast two congressional districts: the one that is heading Republican fastest, and its Democratic counterpart. Conveniently, they are less than 200 miles apart.