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When depressions come they are because NOBODY see's them coming.
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Well, I would say that generally stock market crashes by themselves don't cause depressions, however they can be the catalyst. Or in the case of the Great Depression, the stock market crash did some decent damage but the true damage was caused by other policy errors such as trade barrier increases, monetary policy too tight, huge new regulations, spending and taxes, etc. (they somehow did every bad policy you can think of at once).
Believe me, if we had increased our tarrifs from 2% to 50% a few years ago we would be in a Depression right now too. Instead, the NASDAQ crashed 70% and caused a mild recession that we have already bounced back from. If anything the government + fed successfully did the exact correct policies to counteract the effect (namely go into a deficit + loose monetary policy + not mess up other things too much) in the short term.
One might successfully be able to argue the gov + fed focused *too much* on the short term-- ie they should have done nothing, and our recession would have lasted another year with another million job losses-- but in turn it would have wrung out some of the debt excesses thus possibly preventing a future depression which is much worse. Ie lose 1 million extra jobs now rather than risk a 10 million job loss 5 years from now.
Unfortunetly politicians, feds, + people as a whole I'd say are just too impatient-- it is always "are we skyrocketing now?" "are we skyrocketing now?"
Even though even with an extra 2 million job losses our unemployment is WAY lower than Europe and even better than our 20-year average that is still not enough for many people

In fixing one "problem" people create other worse ones. It's almost like when people take one medicine and it causes a bad side effect, so they add a 2nd medicine to fix the side effect and a 3rd to fix the 2nd's side effect and next thing you know they are taking 20 pills and they would be better off not taking any medicine.
Same situation here

Our citizens demand fast responses and so the government makes decisions focus on the short term. Hense they successfuly steered us out of the NASDAQ damage but aren't focusing on long-term problems like our 2% savings rate and Social Security.
I won't complain too much because going 1 for 2 is better than what govt + fed usually bat at.. normally they screw up the long-term + short-term, at least this time they got one right
-Taric