A frequently observed trend in American politics is the rise of self-identified independents. In Gallup’s most-recent estimation, 42 percent of Americans say that they’re independent and not members of either political party. The last time more people identified as a member of a party than as independents was in 2012.

It’s a weird trend in a moment of spiking partisanship, but it’s easy to see how the two could overlap. As partisans become more partisan, it drives some people away from parties entirely. Those independents aren’t then independent in the sense that they vote for members of each party; most independents vote consistently with one party or the other. In the vernacular, they’re leaners — they lean to the Democrats or the Republicans.

In Gallup’s latest poll, 7 percent of independents don’t lean to one party or the other.

So the question is: Why are these independents independents instead of simply being members of the party with which they usually vote?

To answer that, we turn to data from Pew Research.

Partisans are members of their parties for two primary reasons, according to a poll Pew conducted earlier this year. Three-quarters are members of their parties because — surprise! — they support their party’s positions. Among both Democrats and Republicans, the percentage saying that is up from 2016. But another big reason is that partisans see the policies of the other party as harmful to the country.

In 2016, Republicans were more likely to say that they were members of their party because of the Democrats’ harmful policies than they were Republicans because of Republican policies.

Among leaners, though, opposition to the other party’s policies are a much stronger motivator.

In other words, independents who lean Democratic aren’t thrilled with Democratic policies — but they strongly dislike Republican ones.

Pew also asked the question directly: Why don’t leaners simply join the party with which they tend to vote? Among Democratic leaners, the most popular responses were a split between disagreeing with the party on key issues and being frustrated with leaders of the party. Among Republicans, frustration with leaders was a more common reply — but the percentage saying that had dropped significantly since 2016.

This trend in American culture isn’t limited to politics. We can see echoes of this in religion: Americans are increasingly likely to say they’re spiritual but not religious, even as they still putatively identify with a religion. More broadly, it mirrors the decline of confidence in institutions broadly.

One result? Both the Democratic and Republican parties are more ideologically homogeneous than at any point in recent history.

That’s not likely to woo independents back.