“It remains unclear whether anti-establishment voters, in this election or others, tend to choose the most anti-establishment platform, the most outside-the-system leader or the opposite of whoever is in power”

Few systems in the world are more complex than the one between a person’s ears, particularly when he or she is choosing how to vote. It is both a political and social act, governed by a host of factors that are broadly known but interact in profoundly complicated ways.

It does not always seem that way. When issues and candidates fit a broadly similar mold year after year, voting tends to follow predictable patterns. But populism has introduced a new, or at least newly powerful, set of factors, forcing anyone who works in or studies politics to throw out long-held models.

For one, anti-establishment sentiment is scrambling parties and their agendas.

Britain’s two leading parties each ran against the establishment from within it, Mrs. May by presenting herself as the champion of Brexit, and Mr. Corbyn by rejecting Labour’s decades of centrism.

It remains unclear whether anti-establishment voters, in this election or others, tend to choose the most anti-establishment platform, the most outside-the-system leader or the opposite of whoever is in power.

Another factor: Parties are weakening, but polarization is strengthening. Voters increasingly see themselves as voting against the party or person they dislike, rather than for one they do like.

Sometimes that leads people to abandon mainstream parties in droves, as happened in the Dutch election.

Other times, polarization leads voters to support one mainstream party because that is the best way to oppose another. In Britain, Labour and the Conservatives have grown more dominant than they have been in years.

These changes have been exacerbated by tectonic shifts in demographic coalitions. Working-class whites are shifting from left- to right-wing parties, driven by a backlash against economic and demographic change. Voters are becoming likelier to split by age and education. Such changes are forcing parties to drastically alter their platforms and strategies.

But scholars are only beginning to understand these new coalitions, which are motivated by crosscutting issues.

Max Fisher and Amanda Taub, “Why Populism Is So Difficult to Predict”, The New York Times (11 June 2017), Y16.