Is it just a coincidence that five presidents in a row now will be deemed illegitimate by the opposition? Or could the reasons lie not only in the disparate particulars of each election—a 5–4 Supreme Court ruling, ambiguous laws about citizenship, Russian mischief, a pandemic-inspired haul of absentee ballots—but in a broader truth about our political culture today?

By now, everyone knows that American politics since the 1990s have polarized. Trump’s election made polarization topic A after 2016, but the trend has been evident since the advent of “red” and “blue” states as tools of analysis following the 2000 Florida recount. For more than two decades, Americans have been sorting themselves into communities of the like-minded, online and in real life. Liberals have left the Republican Party and conservatives have abandoned the Democrats, rendering the parties more ideologically homogeneous. Ticket-splitting—casting votes for candidates of different parties for different offices—which was on the rise for the second half of the 20th century, has declined in the 21st. The number of senators willing to confirm a Supreme Court nominee chosen by a president of the other party has also plummeted. Social media and partisan media have created information silos that reinforce partisan tendencies.

One idea developed by political scientists, but not extensively discussed by journalists, is that of “negative partisanship” or “affective polarization” (or, sometimes, “negative affective polarization”). In English, this means not that the parties have necessarily become more extreme in their politics, but that Americans in one political camp regard the opposing camp (hence, “negative”) as increasingly extreme, and with growing hostility (hence, “affective”). When voters exhibit negative affective polarization, they are more inclined not just to disagree with members of the other party or camp (polarization), but also to consider their opponents immoral, hateful, or dishonest—or illegitimate. This spills over into personal relations too. In one much-cited finding, Americans used to object to the prospect of their children marrying outside their race, but cared little if their children wedded someone of a different political party. Today, the reverse holds: Few Americans see anything wrong with interracial unions, but marrying across party lines invites protest.