If the Internet is truly tearing the nation apart, though, it’s hard to see that in the data. Plugged-in millennials aren't the ones who seem to be getting more polarized, according to a new Stanford study. In fact, it's the opposite: Over the past 20 years, political acrimony spiked among older Americans — the same people who are least likely to use the Internet.

“It’s a very simple idea,” said Stanford economics professor Matthew Gentzkow, who co-authored the paper with colleagues Levi Boxell and Jesse Shapiro. “If the driver of increasing political polarization is social media and filter bubbles and all that, then the trend should be especially pronounced for younger people. Instead, polarization has gone up more for groups that don’t go online.”

The following chart from their research shows that between 1996 to 2012, the greatest surges in political polarization occurred among Americans aged 65 and older. Though Internet use has risen among that group, most of them still do not get their political news online, and only a tiny fraction are on social-media sites such as Facebook and Twitter. They were mostly insulated from the rise of partisan online media.

The researchers considered several measures of polarization, which is a tricky idea to capture in a single survey question. They examined historical opinion data on how people felt about voters from the opposite party, for instance, and whether people’s views were consistently partisan or if they held a mix of perspectives from both sides of the aisle. The surveys also contained information about each participant’s age and demographic background, which the researchers referenced against data from Pew about which groups were most likely to go online.

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Younger Americans have always been more polarized, and that this pattern hasn't changed much the past two decades. It's older Americans who are catching up, becoming more and more suspicious of people from the opposite party.

The study doesn’t necessarily rule out the effects of the Internet — but it challenges the simple theory that blames social media for the bitterness of modern politics. Polarization was increasing in America well before the Internet, Gentzkow said, and while the trend has continued, it doesn’t seem to be accelerating, according to their data.

So if it's not the Internet that has Americans at each other's throats, what is it?

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A more likely culprit might be cable news, which is something that older American do consume. Studies find that Fox News, a right-leaning channel founded in 1996, had a measurable effect on voting patterns. Places that got Fox News in time for the 2000 election increased their support for George W. Bush by about half a percentage point. And in subsequent elections, places where Fox News was easier to find on the channel lineup had higher levels of Republican voting, according to economists Gregory Martin and Ali Yurukoglu.

The economists suggest that the power of Fox News has actually increased over time, in part because the network has drifted further to the right in the past decade. This chart from their paper, which analyzed cable transcripts for partisan phrases, shows that Fox News became increasingly conservative at the same time that rival channel MSNBC became increasingly liberal. The economists argue that increasing media polarization might explain two-thirds of the rise in political polarization among Americans in recent years.

Yet, cable news is still not a satisfying culprit because the media doesn’t operate in a vacuum. Television responds to audience ratings, and partisan networks such as Fox News and MSNBC exist to satisfy a thirst for opinionated news. Their more radical cousins — online publications such as Breitbart on the right, and Raw Story on the left — became the subject of much consternation in 2016, but these outlets are wildly popular for a reason. Limiting the influence of such sites with, say, fake news filters in Facebook, wouldn’t fix the underlying problem of demand.

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Besides, the trend of rising partisanship predates even the partisan cable networks. As Boxell, Gentzkow and Shapiro show, political polarization has been increasing since at least the 1970s. Researchers still disagree on the underlying mix of causes, but here’s a pattern that stands out: As political scientists Shanto Iyengar, Gaurav Sood and Yphtach Lelkes pointed out in 2012, Americans haven't shifted their political opinions as much as they have changed their attitudes toward members of the opposing party. In other words, the tone of U.S. politics has gotten more vicious.

One widely cited example: In 1960, only about 5 percent of Americans said they would disapprove their children married someone of the opposite party. But in 2010, 40 percent of parents said they would be “upset” at such a marriage.

This sharp turn in public opinion may be a sign of how party divisions in America now reflect deeper societal fissures. In the early 1960s, Republicans and Democrats were more mixed up, both geographically and socially. "No matter what you were, there were people in the other party who looked like you and had the same cultural values and believed more or less the same thing as you," Stanford professor Mo Fiorina told Vox recently.

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But in the wake of the civil rights movement, party membership increasingly segregated along ideological and social differences. The South, for instance, switched from being most Democratic to most Republican. “Previously, the boundaries of the parties did not line up perfectly with ideologies," Gentzkow said. "You had these conservative Democrats in the South, for instance, and you had moderate Republicans elsewhere.” Today, both sides are much more isolated.

So, it may be that U.S. politics turned nasty because our political and personal identities merged. Through the lens of that theory, the most recent election, where candidates emphasized rancorous divisions between race and class, is the culmination of a decades-long process of social sorting.

Of course, the Internet has contributed to the problem. Yet, long before Facebook news bubbles and Twitter algorithm tweaks, people lived in bubbles formed by their neighbors and their families and their co-workers, bubbles shaped by race and socioeconomic status.

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It’s easy to blame the media because the Web has made it easier to see examples of extreme partisanship. But 20 years ago, you probably wouldn’t have been complaining about that old Facebook friend who’s always sharing fake news. Chances are, you wouldn’t have kept in touch with them at all.

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