Since then, that polarization has grown even stronger. The reasons for that are unclear. “I suspect that part of it has to do with the rise of constant 24-hour news,” Mr. Westwood said, “and also the shift that we’ve unfortunately gone through in which elections are more or less now a permanent state of affairs.”

To find out more about the consequences of that polarization, Mr. Westwood, along with Shanto Iyengar, a Stanford professor who studies political communication, embarked on a series of experiments. They found something quite shocking: Not only did party identity turn out to affect people’s behavior and decision making broadly, even on apolitical subjects, but according to their data it also had more influence on the way Americans behaved than race did.

That is a sea change in the role of partisanship in public life, Mr. Westwood said.

“Partisanship, for a long period of time, wasn’t viewed as part of who we are,” he said. “It wasn’t core to our identity. It was just an ancillary trait. But in the modern era we view party identity as something akin to gender, ethnicity or race — the core traits that we use to describe ourselves to others.”

That has made the personal political. “Politics has become so important that people select relationships on that basis,” Mr. Iyengar said. For instance, it has become quite rare for Democrats to marry Republicans, according to the same Westwood/Iyengar paper, which cited a finding in a 2009 survey of married couples that only 9 percent consisted of Democrat-Republican pairs. And it has become more rare for children to have a different party affiliation from their parents.

But it has also made the political personal. Today, political parties are no longer just the people who are supposed to govern the way you want. They are a team to support, and a tribe to feel a part of. And the public’s view of politics is becoming more and more zero-sum: It’s about helping their team win, and making sure the other team loses.

How partisan bias fuels fake news

Partisan tribalism makes people more inclined to seek out and believe stories that justify their pre-existing partisan biases, whether or not they are true.

“If I’m a rabid Trump voter and I don’t know much about public affairs, and I see something about some scandal about Hillary Clinton’s aides being involved in an assassination attempt, or that story about the pope endorsing Trump, then I’d be inclined to believe it,” Mr. Iyengar said. “This is reinforcing my beliefs about the value of a Trump candidacy.”