r > g has been the historical norm since the industrial revolution, with the notable exception of the period ~1910-1970Actually, the data says quite a bit more than that. I'm looking at the chart shown in Krugman's review (Figure 1, a bit more than halfway down the page):
http://www.nybooks.com/articles/archives/2014/may/08/thomas-...
First, this chart says that r > g has been the historical norm since antiquity, not just since the industrial revolution. The first data point on the chart is for years "0 - 1000". I'm assuming that Piketty's book gives some explanation of how he figured out the rate of return on capital during the Dark Ages. :)
Second, the chart says that the difference between r and g decreased all through the period studied, up to the "1913 - 1950" data point, when r < g became true for the first time. Then, up through the "1950 - 2012" data point, r and g both rose sharply, but r < g was still true, and the difference between the two was more or less the same.
Third, the chart says that, for the rest of the 21st century, not only will r > g be true again, but r will continue to rise while g will fall sharply--which has never happened before in history. So I think it's wrong to claim that Piketty's prediction for the 21st century is just a prediction of things "returning to the historical norm". It's not--it's a prediction of a drastic change from the historical norm.