Labor unions, religious groups and socialist politicians played a strong role, of course. But it was largely the aristocracy and billionaires who eventually pushed for political and economic changes that spread the wealth more evenly throughout the population and created the modern middle class.

Mr. Johnson says that for Rockefeller, it was a case of economic survival and stability.

“If you take a hard-headed political view, I think he felt that a stable society was in the best interests of business. He lived through some unstable times, like the panic of 1893 and labor strikes. There’s a calculation involved, where you can take the enlightened view and say “we don’t have to have it all. It would be much better to create a stable prosperous society where as many people benefit as possible, rather than having it all for yourself. Otherwise, the whole thing collapses.”