The chart below plots every other year from 1917 to 2011 by polarization, as measured by the difference between the average ideology of Democrats and Republicans in the House, and by inequality, as measured by the share of income going to the top 1 percent of wage earners.
We tend to think of our era as exceptional, both in terms of polarization and, to a lesser extent, inequality. But as the chart shows, the 2000s bear similarities to the early part of the 20th century on both measures, especially inequality.
More strikingly, the 30 or so years from 1950 through 1980 are all clustered together a the lower-left hand corner of the chart, indicating a period of remarkably low polarization and inequality. Not only that, but the two measures were quite stable during that period. On the chart, there isn't much distance separating the dot for 1953 from the dot for 1983. Contrast that with the gap between 1983 and 2003.
If anything, the chart seems to suggest that that period of low inequality and party comity may have been the outlier compared to the long-term historical trends - the former is one of the main conclusions of Piketty's book.
We can further separate out the polarization figures by party. As Tom Mann noted in The Atlantic Monday, modern political polarization is largely a Republican phenomenon. The Republican party voters, he writes, "are more skewed to their ideological pole than Democratic Party voters are to theirs."
And indeed, as the charts below show, inequality is much more closely correlated with Republican partisanship than with Democratic partisanship. Those r-squared values noted on the charts are the key - higher numbers indicate a closer relationship between the variables plotted. And that value is much higher when you're looking at the relationship between inequality and Republican partisanship, versus Democratic partisanship.
What we're really seeing, then, is a link, albeit not a causal relationship, between rising Republican partisanship and income inequality. It's important to note again that these charts tells us nothing about causality - rising inequality could be the cause of rising partisanship, or vice versa, or there may be any number of other factors influencing both independently. Political scientists certainly aren't in widespread agreement over the root causes of polarization.
Moreover, as Piketty notes, World War II and its recovery had the effect of lowering inequality. The long climb upward of inequality and partisanship after 1981 likely was a result of a number of factors - tax policy has almost certainly played a role in driving up inequality, for instance.
But the numbers do suggest that we could learn a lot by looking at partisanship and inequality as parts of a whole, rather than as separate unrelated phenomena.
Got a theory about the relationship between polarization and inequality? Let me know in the comments.