Arnold Kling points to an article in which famous economist Joseph Stiglitz lays out a theory for a common structure for the causes of the Great Depression and what Stiglitz calls the current Long Slump.
Here is what Stiglitz has to say:
At the beginning of the Depression, more than a fifth of all Americans worked on farms. Between 1929 and 1932, these people saw their incomes cut by somewhere between one-third and two-thirds, compounding problems that farmers had faced for years. Agriculture had been a victim of its own success. In 1900, it took a large portion of the U.S. population to produce enough food for the country as a whole. Then came a revolution in agriculture that would gain pace throughout the century—better seeds, better fertilizer, better farming practices, along with widespread mechanization. Today, 2 percent of Americans produce more food than we can consume.
What this transition meant, however, is that jobs and livelihoods on the farm were being destroyed. Because of accelerating productivity, output was increasing faster than demand, and prices fell sharply. It was this, more than anything else, that led to rapidly declining incomes. Farmers then (like workers now) borrowed heavily to sustain living standards and production. Because neither the farmers nor their bankers anticipated the steepness of the price declines, a credit crunch quickly ensued. Farmers simply couldn’t pay back what they owed. The financial sector was swept into the vortex of declining farm incomes.
He then goes on to describe how this problem propagated through the rest of the economy.
It’s interesting that in the first paragraph Stiglitz specifies that “more than a fifth” of all Americans worked on farms at the beginning of the depression, and that “2 percent” do today, but makes the non-numerical statement that “a large portion” of the population did so in 1900. I assume this would tend to lead most casual readers to think that most workers were on farms at that time. In fact, about 34 percent of the American labor force was in agriculture in 1900, and about 21 percent in 1930.
The proportion of Americans working on farms has been in continuous decline since at least 1800, when about three-quarters of the labor force was in agriculture. The decade with the biggest reduction in this proportion appears to have been the 1840s, when the percentage of the workforce in farming went from 67.2 percent to 59.7 percent (a reduction of 7.5 points). The rate of reduction from 1900 to 1960 appears to have been between four and five percentage points per decade. As far as I can tell, this was roughly the rate of reduction from about 1860–1960.
The Great Depression occurred around the middle of a century-long, steady decline in the percentage of the labor force on farms. How could this decline have been the special cause of a spectacular economic collapse that occurred in one of these ten decades, but in none of those before or after?