The dynamics that fuel the Big Sort accelerated in the second half of the 20th century, coinciding with a massive increase in education. Between 1960 and 2008, for instance, the proportion of women with bachelor’s degrees nearly quintupled. The dramatic rise in educational attainment has a couple of unexpected side effects. For one, research shows that higher education has a polarizing effect on people: Highly educated liberals become more liberal, while highly educated conservatives grow more conservative. Second, people with college degrees enjoy greater freedoms, including social and geographic mobility. During the 1980s and 1990s, 45 percent of college-educated Americans moved to a new state within five years of graduation, compared with only 19 percent of their counterparts who had only a high-school diploma.

Meanwhile, evolutionary forces are pulling these more mobile, like-minded individuals together, because our political orientations play a key role in our choice of a mate. In society as a whole, spouses tend to resemble one another—at least a bit more than they would if coupling occurred at random—on most biometric and social traits. These traits include everything from skin color to earlobe size to income to major personality dimensions like Extraversion. Most of these statistical relationships are quite weak. But one of the strongest of all correlations between spouses by far is between their political orientations (0.65, to be precise). Spouses tend to have similar attitudes on moral issues like school prayer and abortion not because they converge over time, but rather because “birds of a feather flock together.” Biologists call this assortative mating.

Political scientists Peter Hatemi, Rose McDermott, and Casey Klofstad have fed actual assortment rates from the 1980s into a computer simulation of American society. Their simulation took into account the fact that political orientation is a moderately heritable trait. When they ran the program, the population’s left-right curve widened substantially in just the first five generations. In the next 10 generations, the curve expanded only a bit more and then reached equilibrium. At this point, the percentage of political extremists in the population had increased from 4.5 to 11.2 percent, while moderates had dropped by 17 percent. In other words, the “birds of a feather” had bred a more polarized nation.

This slow dance of ideological assortment and reproduction is already underway on the ground, and it’s likely contributing to our unprecedented political polarization. When this fracturing began to accelerate in the 1980s, only a quarter of the electorate voted for a president and a legislator in different parties. By 2012 the proportion of voters who split their ballot had plummeted to 11 percent. And polarization of voting in the House of Representatives has now reached record levels, surpassing even 19th-century highs from the post-Civil War era.