The same, she realized, was true of partisan identity. Everyone has multiple identities: racial, religious, professional, ideological and more. But while those multiple identities might once have pushed people in different partisan directions — think of the conservative Democrats of old in the South or all the liberal Republicans in the Northeast — today it’s more common to line up behind one party. A white conservative who lives in a rural area and is an evangelical Christian is likely to feel that the Republican Party is the best representative of all of those separate identities, for instance. An African-American liberal who lives in a city and works in a professional job is likely to feel the same way about the Democratic Party.

Can this explain why American politics have become so polarized over the last several decades? Starting in 1980, the National Election Study, a long-running survey that tracks Americans’ political preferences, showed that Republicans and Democrats were growing apart: Each reported increasingly negative opinions of the opposing party. And other data showed that polarization was seeping into nonpolitical arenas, making Republicans and Democrats less likely to marry or be friends.

Ms. Mason decided to make that the focus of her doctoral thesis, and found much to support her hypothesis: Americans’ overlapping political identities were driving extreme polarization.

When multiple identities line up together, all pushing people toward the same party, partisan identity becomes a kind of umbrella for many different characteristics that people feel are important to them. That magnifies people’s attachment to their team.

And that, in turn, raises the stakes of conflict with the opposing “team,” Ms. Mason found. In every electoral contest or partisan disagreement, she explained, people now feel that they are fighting for many elements of who they are: their racial identity, professional identity, religious identity, even geographical identity.

“The way I think of it is, imagine that the World Series also affected the N.C.A.A. and the Super Bowl and every other team you care about,” she said. “So as our identities line up with party identity, politics becomes more and more consequential.”

That may have been the key to Mr. Trump’s success in the 2016 election, she believes. “With Trump, if you can point to one brilliant thing he did, it’s that he as a politician, kind of for the first time, said ‘we’re losers.’ ” Social psychology research has shown that the best way to get people to defend their identity is to threaten it. By saying “we don’t win anymore — we’re losers — and I’m going to make us win again,” Ms. Mason said, Mr. Trump’s pitch to voters both created the sense of threat and promised a defense: a winning political strategy for the age of identity politics.