A new study by Pew Research verifies much we already know about political extremism in America: It’s getting worse and interfering with social and economic progress. The big question is: Why?

Pew doesn’t address that question, but here’s a plausible answer: Voters are becoming angrier because living standards are falling and the middle class is shriveling. Prosperity breeds comity, but when it gets harder to get ahead, the natural inclination is for the losers to look for somebody to blame and the winners to feel more threatened. That’s been going on for nearly 30 years.

Income inequality began to worsen in the United States starting around the early 1980s. Most of Pew’s data on political polarization begins in 1994, and shows Democrats and Republicans consistently growing more distant from each other in their views and ideology. Most of the split occurred during the last 10 years, a time in which median household income dropped for the first time since the end of World War II.

Here’s a set of charts from Pew showing the increase in political polarization:

And here’s some polling data from Gallup showing how Americans answer when asked if there is “not much opportunity” or “plenty of opportunity” in America today:

The decline in economic optimism clearly coincides with an increase in polarization in the political sphere. It’s always tricky to draw cause-and-effect relationships from different sets of data, but it would hardly be surprising that a consistently tougher economy made voters more bitter, and led them to blame their difficult straits on the policies of whatever party they view as the opposition.

Many voters don’t need to be told that the middle class is under stress, yet it’s increasingly apparent that political policies may have little to do with it. A recent study by economists at the Federal Reserve Bank of Dallas found that middle-class jobs are, in fact, disappearing, as many workers can attest. Yet the primary cause, they found, was technological change, with computers, robots and other gizmos increasingly replacing human workers in a lot of routine jobs that can be done by machines. The offshoring of jobs to lower-cost countries was also a factor, but not as big a one as some people may think.

The economy has now recovered all the jobs lost during the recession that ran from 2007 to 2009, but many people now have jobs that pay less than the ones they held 5 or 10 years ago. It might be tempting to blame President Obama or Congressional Republicans for that, but the true causes probably developed over years, spanning periods in which both parties had control of the White House and Congress.

The problem with partisan hostility, of course, is that it may actually prevent policy solutions that would make everybody better off. The big tax reform effort in 1986, for instance, was a bipartisan effort credited at least in part for the booming economy of the 1990s. With numerous loopholes and complexities layered onto the tax code since then, many analysts of both parties feel a similar tax reform effort could give the economy another much-needed boost.

Yet today’s hostile political climate makes the likelihood of compromise—essential to passing most big legislation—remote. The same goes for efforts to build new infrastructure, fix a broken immigration system and pass other laws that aren’t terribly controversial but usually just die amid vicious political combat.

One trend that offers a hint of relief: More voters now consider themselves independent than say they are Republicans or Democrats. And independents tend to be moderates uncomfortable with the extremism in both parties. There’s still a middle in America, somewhere.

Rick Newman’s latest book is Rebounders: How Winners Pivot From Setback To Success. Follow him on Twitter: @rickjnewman.