A while back, in May, 2006 I covered the Kondratieff wave and it seems to be more to the point as time goes by. Below is a link to a history lesson that's well worth reading.
The Kondratieff Wave
This gentleman's theories were published in 1925 way before the Great Depression. Here is a quote from the link, dealing with "The Autumn" just before "The Winter," labeled Depression.
Excesses of an unpopular war, along with fiscal liberalism, cause popular reaction toward stability or normalcy. A mood of isolationism permeates . The plateau period generally lasts seven to ten years and is characterized by selective industry growth, development of new ideas ( both technological and social ) and a strong feelings of affluence, terminating in a feeling of euphoria. The inflated price structure from the primary recession, along with the desire for consumption, produces a rapid increase in debt. Eventually, wealth consumption expands beyond all practical limits, and economy slips into a severe and protracted depression.
These cycles tend to be about 60 to 70 years long. If you think about it, everyone that was about 30 years old during the last depression is no longer with us. The group memory of the past depression is gone and most of the financial shenanigans going on, are "new" in our mind's eye.
The point about perceiving a depression, is that its only visible in your rear view mirror. The investment trusts that collapsed in the 1930's seem very similar to the index funds and derivatives of today.
As a post note: don't link a Depression automatically with deflation, hyperinflation is also an option. The Federal Reserve (with today's powers) and absurd national debts were not part of the mix way back then.