Critics such as economist Paul Krugman and U.S. Treasury Secretary Timothy Geithner have argued that the regulatory framework did not keep pace with financial innovation, such as the increasing importance of the shadow banking system, derivatives and off-balance sheet financing. A recent OECD study suggest that bank regulation based on the Basel accords encourage unconventional business practices and contributed to or even reinforced the financial crisis. In other cases, laws were changed or enforcement weakened in parts of the financial system. Key examples include: Jimmy Carter's Depository Institutions Deregulation and Monetary Control Act of 1980 (DIDMCA) phased out a number of restrictions on banks' financial practices, broadened their lending powers, allowed credit unions and savings and loans to offer checkable deposits, and raised the deposit insurance limit from $40,000 to $100,000 (thereby potentially lessening depositor scrutiny of lenders' risk management policies.) In October 1982, U.S. President Ronald Reagan signed into law the Garn--St. Germain Depository Institutions Act, which provided for adjustable-rate mortgage loans, began the process of banking deregulation, and contributed to the savings and loan crisis of the late 1980s/early 1990s. In November 1999, U.S. President Bill Clinton signed into law the Gramm--Leach--Bliley Act, which repealed part of the Glass--Steagall Act of 1933. This repeal has been criticized for reducing the separation between commercial banks (which traditionally had fiscally conservative policies) and investment banks (which had a more risk-taking culture). However, the vast majority of failures were at institutions that were created by Glass-Steagall. In 2004, the U.S. Securities and Exchange Commission relaxed the net capital rule, which enabled investment banks to substantially increase the level of debt they were taking on, fueling the growth in mortgage-backed securities supporting subprime mortgages. The SEC has conceded that self-regulation of investment banks contributed to the crisis. Financial institutions in the shadow banking system are not subject to the same regulation as depository banks, allowing them to assume additional debt obligations relative to their financial cushion or capital base. This was the case despite the Long-Term Capital Management debacle in 1998, where a highly leveraged shadow institution failed with systemic implications. Regulators and accounting standard-setters allowed depository banks such as Citigroup to move significant amounts of assets and liabilities off-balance sheet into complex legal entities called structured investment vehicles, masking the weakness of the capital base of the firm or degree of leverage or risk taken. One news agency estimated that the top four U.S. banks will have to return between $500 billion and $1 trillion to their balance sheets during 2009. This increased uncertainty during the crisis regarding the financial position of the major banks.[96] Off-balance sheet entities were also used by Enron as part of the scandal that brought down that company in 2001. As early as 1997, Federal Reserve chairman Alan Greenspan fought to keep the derivatives market unregulated. With the advice of the President's Working Group on Financial Markets, the U.S. Congress and President allowed the self-regulation of the over-the-counter derivatives market when they enacted the Commodity Futures Modernization Act of 2000. Derivatives such as credit default swaps (CDS) can be used to hedge or speculate against particular credit risks. The volume of CDS outstanding increased 100-fold from 1998 to 2008, with estimates of the debt covered by CDS contracts, as of November 2008, ranging from US$33 to $47 trillion. Total over-the-counter (OTC) derivative notional value rose to $683 trillion by June 2008. Warren Buffett famously referred to derivatives as "financial weapons of mass destruction" in early 2003.