Brad DeLong does the necessary on Niall Ferguson; no need for me to pile on. But I think there’s more to be said about Depression-era debt. To get the full picture, you need to go all the way back to 1929.

If you were ignorant of basic facts about the Depression — or if you didn’t know that movements in a ratio can reflect changes in the denominator as well as the numerator — you might think that it’s possible to summarize fiscal policy by looking at the federal debt-GDP ratio, which looks like this from 1929-41:

Clearly, then, Herbert Hoover was a wild deficit spender, while FDR was much more cautious. Right?

OK, we know that’s wrong. Here’s what nominal debt, the numerator in the debt ratio, looks like:

So Hoover ran up very little debt — only about 6 percent of 1929 GDP. FDR, on the other hand, ran up a lot of debt, about 47 percent of 1933 GDP. But Hoover presided over a shrinking, deflationary economy, while FDR presided over a rapidly growing (from a low base) economy with rising prices.

I’ve been careful to use the term “presided over”: you don’t want to attribute all the differences in the two sub-eras to policy, let alone fiscal policy. Nonetheless, the fact that virtually all the deterioration in the US debt position from 1929 to 1939 took place under the tight-fisted Hoover rather than under FDR is an object lesson in the crucial importance of growth in dealing with debt. And the Hoover experience also provides a nice illustration of self-defeating austerity — not only didn’t austerity produce economic recovery, it didn’t even improve the fiscal position.

It’s too bad that people who don’t understand any of that seem to have the upper hand in policy.