[Timothy Noah is senior writer for Slate.]

Liberal politicians and activists have long argued that the federal government caused the Great Divergence. And by "federal government," they generally mean Republicans, who have controlled the White House for 20 of the past 30 years, after all. A few outliers even argue that for Republicans, creating income inequality was a conscious and deliberate policy goal.

Until recently, the consensus among academics—even most liberal ones—was quite different. Economists argued that the Great Divergence was the result not of Washington policymaking but of larger "exogenous" (external) and "secular" (long-term) forces. In June, the Congressional Budget Office calculated that spending by the federal government made up 23 percent of U.S. gross domestic product, after averaging 18.5 percent during the previous four decades. But even with federal spending at this unusually high level (necessitated by a severe recession), Washington's nut remains less than one-quarter the size of the economy. Most of that nut is automatic "entitlement" spending over which Washington policymakers seldom exert much control. Brad DeLong, a liberal economist at Berkeley, expressed the prevailing view in 2006: "[T]he shifts in income inequality seem to me to be too big to be associated with anything the government does or did."...

Finding ways to increase social equality is an important goal...But the academic consensus that underlay [Long's] argument...has lately started to crumble....