Politicians like the idea of “investing in public works” as a solution to unemployment because, unlike most forms of investment, its by-products are gratifyingly visible.

President-Elect Obama’s plan to “create 2.5 million jobs” through infrastructure improvement is obviously prompted by the images FDR conjured up of his own administration.

FDR’s economic policies were sold to the public by newsreel montages of new dams, bridges, and highways. On exactly the same principle, Hitler would be filmed driving along a new autobahn, saluted by thousands of shovel-bearing workers, supposedly the unemployed who built it. Mussolini drained the Pontine Marshes as part of a deliberate imitation of Julius Caesar, who had planned something similar before the Ides of March intervened. Even in the ancient world, politicians like Caesar understood the value of prestige projects: the Parthenon was built to remind the Athenians to vote Pericles.

Yet the images may disguise the fact that their impact on employment is usually not that great.

FDR’s Public Works Administration actually used established private contractors, not the unemployed, and was separate from mass employment programmes like his Civil Works Administration and Civilian Conservation Corps, which lacked the training and expertise for the more complicated projects. Indeed, FDR’s allies among the unions vetoed proper training by the programmes that might undercut union members.

Major construction projects, like the Hoover Dam – which is usually included in those FDR montages, but was actually begun under his predecessor: Herbert Hoover – require specialist contractors and employees who are highly skilled but relatively few in number. Hitler’s shovel carrying masses were strictly for the cameras.

Since the 1930s, technical advances and stricter legal requirements have made construction workforces even smaller and more expert.

The real lesson of the Depression is that viable long-term jobs can only be created by viable long-term businesses. FDR, like Mussolini and Hitler, paid for his programmes with increased deficits and taxes that decreased the money available for investment in those viable businesses. Mr Obama should learn from their mistakes.