Causes of the Great Depression
Second, there are the monetarists, who believe that the Great Depression started as an ordinary recession, but that significant policy mistakes by monetary authorities (especially the Federal Reserve) caused a shrinking of the money supply which greatly exacerbated the economic situation, causing a recession to descend into the Great Depression. Related to this explanation are those who point to debt deflation causing those who borrow to owe ever more in real terms.
Causes of the Great Recession
Excessive consumer housing debt was in turn caused by the mortgage-backed security, credit default swap, and collateralized debt obligation sub-sectors of the finance industry, which were offering irrationally low interest rates and irrationally high levels of approval to subprime mortgage consumers because they were calculating aggregate risk using gaussian copula formulas that strictly assumed the independence of individual component mortgages, when in fact the credit-worthiness almost every new subprime mortgage was highly correlated with that of any other because of linkages through consumer spending levels which fell sharply when property values began to fall during the initial wave of mortgage defaults. Debt consumers were acting in their rational self-interest, because they were unable to audit the finance industry's opaque faulty risk pricing methodology.