Paul Krugman is right: there is no clear or unambiguous relationship between inequality and capitalist economic growth.
If there were such a relationship, liberal thinkers could make their case that less inequality would promote more growth and everyone would benefit.
The fact is, however, there are more unequal and less unequal forms and periods of capitalist growth. As a result, there’s no general correlation between inequality and growth within capitalism.
The chart above depicts both inequality (the profit-wage ratio, in blue on the left) and economic growth (nominal GDP growth, in red on the right) for the United States. We can clearly see periods of relatively high economic growth accompanied by growing inequality (e.g., in the late 1970s) and periods of very slow economic growth also accompanied by growing inequality (e.g., since 2009). The converse is also true: high economic growth with falling inequality (in the early 1950s) and slow economic growth with falling inequality (e.g., late 1980s).
What this means, at least for me, is we need to pay attention to the particular conditions for economic growth during each period or phase of capitalist development. In general, capitalism can grow (or not) under both more unequal and less unequal conditions.
That’s not to say, however, that within any given period (more or less) growth and (more or less) inequality are not connected. A plausible story can be told that growing inequality during the 2000s fueled the financial bubble that eventually burst in 2007-08.
We also need to reverse the relationship and look at the relationship from growth to inequality. It’s pretty clear that the nature of the growth that has occurred since 2009 has created more inequality, that is, the particular form of the recovery that has been enacted since the crash of 2007-08 has enhanced corporate profits (and incomes at the very top) and made everyone else (who receive wages to get by) pay the costs.
Finally, while we’re on the topic, there is one general tendency of capitalist growth we need to point out: the role of profitability. In the end, profitability is what makes capitalism work (or not). As we can see from the chart above (which includes just the corporate profit share and growth), each period of a declining profit share is followed by a recession, after which the profit share rises (at least for a time).
So, to paraphrase the Rolling Stones: capitalists can’t always get what they want. When they don’t (leading up to the most recent financial crash), neither will anyone else. And, even when they do (as they have since 2009), there’s no guarantee anyone else will.