Ferguson -- Laurence Tisch professor of history at Harvard University and author of a number of historical works, including a history of money -- made the remark in response to a question at an Altegris Strategic Investment Conference in Carlsbad, Calif., The Boston Globe reported.
He had been asked about one of Keynes' most famous remarks talking about long-run investment strategies: "In the long run, we are all dead."
Ferguson responded that Keynes, presumed to be a homosexual, did not have children and was therefore presumably not interested in the "long run" effects of the economic policies he advocated.
Keynes was married, although he is widely believed to have had at least one gay affair with the writer Lytton Strachey, the report said.
"This was doubly stupid," Ferguson said in his apology Saturday. "First, it is obvious that people who do not have children also care about future generations. Second, I had forgotten that Keynes's wife Lydia miscarried."
Ferguson was an adviser to the Republican presidential campaign in 2008. He has been criticized for defending colonialism and for his support for the Iraq War.
Keynes, who pushed for government intervention in the economy and stimulus spending during economic downturns, is reviled by many conservatives.