In his famous letter to Berkshire Hathaway shareholders in 1988, Warren Buffett declared: “When we own portions of outstanding businesses with outstanding managements, our favorite holding period is forever.” Compare this statement, apparently from a kindred spirit: “As time goes on, I get more and more convinced that the right method in investment is to put fairly large sums into enterprises which one thinks one knows something about and in the management of which one thoroughly believes.”

Earlier, in 1984, Buffett had explained his view on diversification by approvingly quoting the theatre impresario Billy Rose: “If you have a harem of 40 women, you never get to know any of them very well.” Here's that kindred spirit again: “It is a mistake to think that one limits one's risk by spreading too much between enterprises about which one knows little and has no reason for special confidence.”

In each case the second statement is from a man who is as quotable as the Sage of Omaha himself: John Maynard Keynes, writing to a colleague in 1934. The affinity between Buffett and Keynes isn't a new discovery but a newly published study of Keynes's investments for King's College, Cambridge allows us to see whether Keynes took his own advice, and whether the results paid off.