During the late 18th and the 19th century such money consisted almost entirely of redeemable notes issued by commercial banks; and while complete legal freedom of entry into the paper currency business was rare, so were outright monopolies. Economist Milton Friedman's views on the matter of currency monopoly offer a particularly interesting case study. Despite having been an unflinching champion of classical liberalism and free markets, he at first shared the common view concerning the necessity of official currency monopolies. Concerning the behavior of the money supply, it has long been understood, by monetarists in particular, that freedom of note issue eliminates the problem of undesirable money stock changes stemming from changes in the public's preferred ratio of currency to bank deposits. More recent empirical research has tended to reinforce earlier work treating the growth of banking as a crucial determinant of economic growth, in part by quantifying what were once merely qualitative conclusions.