This primary season has wrecked the idea that momentum lasts more than a few milliseconds, and given life to another grand theory of politics: Demographics are destiny.

Buttigieg and Klobuchar, whose appeal had always skewed to white voters, exceeded their national polling averages in the very white states of Iowa and New Hampshire. Sanders, who has a high floor of support among young people, low-income voters, and Latinos, cleaned up in Nevada, which is almost one-third Latino. And Biden, whose support is highest among black voters and the elderly, dominated in South Carolina, which has the country’s fifth-highest share of black voters.

The demographic-destiny theory has some grounding in political science. A 2019 paper on the “momentum myth” from researchers at Vanderbilt University found that the characteristics of each state’s electorate, rather than some national force of momentum, explained the state-by-state outcome of the 2016 Democratic primary. Across the country, Sanders won where voters were whiter and younger, and Hillary Clinton won in states that were older with larger black populations.