Perverse monetary policy was the greatest cause of the Great Depression. But five non-monetary missteps were important in making the Depression great, and the same missteps damaged the global economy as well. While many are thinking about the Depression, few seem concerned about replicating these Foolish Five today:

Giving in to protectionism. In Herbert Hoover’s time, Senator Smoot and Rep. W.C. Hawley proposed a tariff that was to raise effective duties by as much as half. More than a thousand economists signed an open letter warning that the duties would “raise the cost of living and injure the great majority of our citizens.”

But Hoover’s Republican Party didn’t much care. In its 1928 platform, the GOP had pledged to “reaffirm our belief in the protective tariff.” Ambivalent, Hoover signed the bill. An irate Canada and many other nations retaliated. At a time when America was begging for foreign markets, it lost them. The selfish signal discouraged an already unstable Europe.

Today, international trade claims a sizable share of our economy. Bilateral free-trade agreements with Colombia or Panama are good insurance — cheap steps that might prevent an expensive loss, that of the Western Hemisphere to Venezuela’s Hugo Chavez.

Yet again, one party — the Democrats, this time — is cavalier. House Speaker Nancy Pelosi is blocking passage of these bilateral agreements. And another ambivalent politician — Senator Obama — has sent mixed messages to Canada about just how much he wants to roll back the North American Free Trade Agreement.

Blaming the messenger. Punishing the stock market for the 1929 crash was popular in Washington in the early 1930s. Lawmakers attacked the practice of short selling; Senate Banking Committee counsel Ferdinand Pecora hauled J.P. Morgan and other Wall Streeters in for hearings. By 1934, Congress was creating the Securities and Exchange Commission. The Roosevelt administration also prosecuted business leaders, including former Treasury Secretary Andrew Mellon and utilities magnate Samuel Insull. The new regulatory culture cut crime and protected investors. But the arbitrary nature of the assault petrified Wall Streeters.

Today, too, a “Blame the Street” mood prevails. SEC Chairman Chris Cox has criticized “naked shorts,” an attack with a legitimate anti-fraud component. But targeting short selling also generates uncertainty. The investigations of Bear Stearns and Freddie Mac are just the beginning; more prosecutions are likely. Like the Sarbanes-Oxley Act, which followed Enron’s accounting meltdown, this cleanup will send companies and jobs abroad.

Increasing taxes in a downturn. Hoover more than doubled income tax rates, taking the top marginal rate to 63% from 25%. FDR hiked the top rate to 90%. Perhaps worse, Roosevelt’s Treasury crafted taxes to punish business, including an undistributed profits tax and an excess profits tax, that ultimately sucked cash from a capital-starved economy.

Today, Democrats are planning tax increases that make Bill Clinton’s hike look mild. The proposals start with lifting the cap on Social Security payroll taxes — an effective increase in the top marginal tax rate of 6.2%, or for some 12.4%, all by itself. Add in the promised repeal of the Bush tax cuts and you have an additional 4.6% increase. Effective top rates approach 50%. There are also proposed increases for dividends and capital gains. Taken together, these will make the U.S. economy sluggish and more like that of Europe.

Assuming bigger government will bring back growth. There’s a sense today that Washington has retreated too much from daily lives. Wall Streeters mutter that “the system” (the financial markets) doesn’t work anymore. In the 1930s, people didn’t just mutter that — they believed it. Public-sector expansion seemed the only way to sustain America’s promise. New Deal programs did much to alleviate the pain month to month — many found dignity in six months of work at the Works Progress Administration, the Public Works Administration, or the Civilian Conservation Corps. But economics is a competition for scarce capital. Such state solutions tended to suppress the creation of long-term private-sector jobs, as did the aggressive Wagner Act for organized labor. The National Recovery Administration, the New Deal’s centerpiece, favored large businesses at the expense of small fry. The new Tennessee Valley Authority and Roosevelt’s repressive Public Utility Holding Company Act combined to crowd out private utilities that hoped to light up the South. As for Wall Street, those New Yorker magazine cartoons were accurate: Wall Streeters retreated into their martinis and country houses rather than rebuild. This yielded the “Depression within the Depression” of 1937.

Ignoring the cost of inconsistency. FDR spoke of “bold persistent experimentation.” Mr. Obama speaks of “change.” Both can do damage. What’s more, the list of experiments is always finite. Our bailouts look reassuring, but even Washington cannot rescue the entire economy. And foreign investors wonder where Washington will stop. Already concerned about the inconsistent dollar policy, China is now troubled by the inconsistent rescues.

The proximate danger today is a repeat of the 1970s, not the 1930s. But if lawmakers don’t remember the old missteps, they might find that their new recovery legislation imperils our recovery.

Miss Shlaes, the author of “The Forgotten Man: A New History of the Great Depression,” is a senior fellow at the Council on Foreign Relations. © 2008 The Washington Post.