It was, according to accounts filtering out of the White House, an extraordinary scene. Hank Paulson, the U.S. treasury secretary and a man with a personal fortune estimated at $700m (£380m), had got down on one knee before the most powerful woman in Congress, Nancy Pelosi, and begged her to save his plan to rescue Wall Street.

    The Guardian, September 26, 20081

The financial crisis of 2008 was a complex event that took most economists and market participants by surprise. Since then, there have been many attempts to arrive at a narrative to explain the crisis, but none has proven definitive. For example, a Congressionally-chartered ten-member Financial Crisis Inquiry Commission produced three separate narratives, one supported by the members appointed by the Democrats, one supported by four members appointed by the Republicans, and a third written by the fifth Republican member, Peter Wallison.2

It is important to appreciate that the financial system is complex, not merely complicated. A complicated system, such as a smartphone, has a fixed structure, so it behaves in ways that are predictable and controllable. A complex system has an evolving structure, so it can evolve in ways that no one anticipates. We will never have a proven understanding of what caused the financial crisis, just as we will never have a proven understanding of what caused the first World War.

There can be no single, definitive narrative of the crisis. This entry can cover only a small subset of the issues raised by the episode.

Metaphorically, we may think of the crisis as a fire. It started in the housing market, spread to the sub-prime mortgage market, then engulfed the entire mortgage securities market and, finally, swept through the inter-bank lending market and the market for asset-backed commercial paper.

Home sales began to slow in the latter part of 2006. This soon created problems for the sector of the mortgage market devoted to making risky loans, with several major lenders—including the largest, New Century Financial—declaring bankruptcy early in 2007. At the time, the problem was referred to as the “sub-prime mortgage crisis,” confined to a few marginal institutions.

But by the spring of 2008, trouble was apparent at some Wall Street investment banks that underwrote securities backed by sub-prime mortgages. On March 16, commercial bank JP Morgan Chase acquired one of these firms, Bear Stearns, with help from loan guarantees provided by the Federal Reserve, the central bank of the United States.

Trouble then began to surface at all the major institutions in the mortgage securities market. By late summer, many investors had lost confidence in Freddie Mac and Fannie Mae, and the interest rates that lenders demanded from them were higher than what they could pay and still remain afloat. On September 7, the U.S. Treasury took these two GSEs into “conservatorship.”

Finally, the crisis hit the short-term inter-bank collateralized lending markets, in which all of the world’s major financial institutions participate. This phase began after government officials’ unsuccessful attempts to arrange a merger of investment bank Lehman Brothers, which declared bankruptcy on September 15. This bankruptcy caused the Reserve Primary money market fund, which held a lot of short-term Lehman securities, to mark down the value of its shares below the standard value of one dollar each. That created jitters in all short-term lending markets, including the inter-bank lending market and the market for asset-backed commercial paper in general, and caused stress among major European banks.

The freeze-up in the interbank lending market was too much for leading public officials to bear. Under intense pressure to act, Treasury Secretary Henry Paulson proposed a $700 billion financial rescue program. Congress initially voted it down, leading to heavy losses in the stock market and causing Secretary Paulson to plead for its passage. On a second vote, the measure, known as the Troubled Assets Relief Program (TARP), was approved.

In hindsight, within each sector affected by the crisis, we can find moral hazard, cognitive failures, and policy failures. Moral hazard (in insurance company terminology) arises when individuals and firms face incentives to profit from taking risks without having to bear responsibility in the event of losses. Cognitive failures arise when individuals and firms base decisions on faulty assumptions about potential scenarios. Policy failures arise when regulators reinforce rather than counteract the moral hazard and cognitive failures of market participants.

The Housing Sector

From roughly 1990 to the middle of 2006, the housing market was characterized by the following:

  • an environment of low interest rates, both in nominal and real (inflation-adjusted) terms. Low nominal rates create low monthly payments for borrowers. Low real rates raise the value of all durable assets, including housing.
  • prices for houses rising as fast as or faster than the overall price level
  • an increase in the share of households owning rather than renting
  • loosening of mortgage underwriting standards, allowing households with weaker credit histories to qualify for mortgages.
  • lower minimum requirements for down payments. A standard requirement of at least ten percent was reduced to three percent and, in some cases, zero. This resulted in a large increase in the share of home purchases made with down payments of five percent or less.
  • an increase in the use of new types of mortgages with “negative amortization,” meaning that the outstanding principal balance rises over time.
  • an increase in consumers’ borrowing against their houses to finance spending, using home equity loans, second mortgages, and refinancing of existing mortgages with new loans for larger amounts.
  • an increase in the proportion of mortgages going to people who were not planning to live in the homes that they purchased. Instead, they were buying them to speculate. 3

These phenomena produced an increase in mortgage debt that far outpaced the rise in income over the same period. The trends accelerated in the three years just prior to the downturn in the second half of 2006.

The rise in mortgage debt relative to income was not a problem as long as home prices were rising. A borrower having difficulty finding the cash to make a mortgage payment on a house that had appreciated in value could either borrow more with the house as collateral or sell the house to pay off the debt.

But when house prices stopped rising late in 2006, households that had taken on too much debt began to default. This set in motion a reverse cycle: house foreclosures increased the supply of homes for sale; meanwhile, lenders became wary of extending credit, and this reduced demand. Prices fell further, leading to more defaults and spurring lenders to tighten credit still further.

During the boom, some people were speculating in non-owner-occupied homes, while others were buying their own homes with little or no money down. And other households were, in the vernacular of the time, “using their houses as ATMs,” taking on additional mortgage debt in order to finance consumption.

In most states in the United States, once a mortgage lender forecloses on a property, the borrower is not responsible for repayment, even if the house cannot be sold for enough to cover the loan. This creates moral hazard, particularly for property speculators, who can enjoy all of the profits if house prices rise but can stick lenders with some of the losses if prices fall.

One can see cognitive failure in the way that owners of houses expected home prices to keep rising at a ten percent rate indefinitely, even though overall inflation was less than half that amount.4Also, many house owners seemed unaware of the risks of mortgages with “negative amortization.”

Policy failure played a big role in the housing sector. All of the trends listed above were supported by public policy. Because they wanted to see increased home ownership, politicians urged lenders to loosen credit standards. With the Community Reinvestment Act for banks and Affordable Housing Goals for Freddie Mac and Fannie Mae, they spurred traditional mortgage lenders to increase their lending to minority and low-income borrowers. When the crisis hit, politicians blamed lenders for borrowers’ inability to repay, and political pressure exacerbated the credit tightening that subsequently took place

The Sub-prime Mortgage Sector

Until the late 1990s, few lenders were willing to give mortgages to borrowers with problematic credit histories. But sub-prime mortgage lenders emerged and grew rapidly in the decade leading up to the crisis. This growth was fueled by financial innovations, including the use of credit scoring to finely grade mortgage borrowers, and the use of structured mortgage securities (discussed in the next section) to make the sub-prime sector attractive to investors with a low tolerance for risk. Above all, it was fueled by rising home prices, which created a history of low default rates.

There was moral hazard in the sub-prime mortgage sector because the lenders were not holding on to the loans and, therefore, not exposing themselves to default risk. Instead, they packaged the mortgages into securities and sold them to investors, with the securities market allocating the risk.

Because they sold loans in the secondary market, profits at sub-prime lenders were driven by volume, regardless of the likelihood of default. Turning down a borrower meant getting no revenue. Approving a borrower meant earning a fee. These incentives were passed through to the staff responsible for finding potential borrowers and underwriting loans, so that personnel were compensated based on “production,” meaning the new loans they originated.

Although in theory the sub-prime lenders were passing on to others the risks that were embedded in the loans they were making, they were among the first institutions to go bankrupt during the financial crisis. This shows that there was cognitive failure in the management at these companies, as they did not foresee the house price slowdown or its impact on their firms.

Cognitive failure also played a role in the rise of mortgages that were underwritten without verification of the borrowers’ income, employment, or assets. Historical data showed that credit scores were sufficient for assessing borrower risk and that additional verification contributed little predictive value. However, it turned out that once lenders were willing to forgo these documents, they attracted a different set of borrowers, whose propensity to default was higher than their credit scores otherwise indicated.

There was policy failure in that abuses in the sub-prime mortgage sector were allowed to continue. Ironically, while the safety and soundness of Freddie Mac and Fannie Mae were regulated under the Department of Housing and Urban Development, which had an institutional mission to expand home ownership, consumer protection with regard to mortgages was regulated by the Federal Reserve Board, whose primary institutional missions were monetary policy and bank safety. Though mortgage lenders were setting up borrowers to fail, the Federal Reserve made little or no effort to intervene. Even those policy makers who were concerned about practices in the sub-prime sector believed that, on balance, sub-prime mortgage lending was helping a previously under-served set of households to attain home ownership.5

Mortagage Securities

A mortgage security consists of a pool of mortgage loans, the payments on which are passed through to pension funds, insurance companies, or other institutional investors looking for reliable returns with little risk. The market for mortgage securities was created by two government agencies, known as Ginnie Mae and Freddie Mac, established in 1968 and 1970, respectively.

Mortgage securitization expanded in the 1980s, when Fannie Mae, which previously had used debt to finance its mortgage purchases, began issuing its own mortgage-backed securities. At the same time, Freddie Mac was sold to shareholders, who encouraged Freddie to grow its market share. But even though Freddie and Fannie were shareholder-owned, investors treated their securities as if they were government-backed. This was known as an implicit government guarantee.

Attempts to create a market for private-label mortgage securities (PLMS) without any form of government guarantee were largely unsuccessful until the late 1990s. The innovations that finally got the PLMS market going were credit scoring and the collateralized debt obligation (CDO).

Before credit scoring was used in the mortgage market, there was no quantifiable difference between any two borrowers who were approved for loans. With credit scoring, the Wall Street firms assembling pools of mortgages could distinguish between a borrower with a very good score (750, as measured by the popular FICO system) and one with a more doubtful score (650).

Using CDOs, Wall Street firms were able to provide major institutional investors with insulation from default risk by concentrating that risk in other sub-securities (“tranches”) that were sold to investors who were more tolerant of risk. In fact, these basic CDOs were enhanced by other exotic mechanisms, such as credit default swaps, that reallocated mortgage default risk to institutions in which hardly any observer expected to find it, including AIG Insurance.

There was moral hazard in the mortgage securities market, as Freddie Mac and Fannie Mae sought profits and growth on behalf of shareholders, but investors in their securities expected (correctly, as it turned out) that the government would protect them against losses. Years before the crisis, critics grumbled that the mortgage giants exemplified privatized profits and socialized risks.6

There was cognitive failure in the assessment of default risk. Assembling CDOs and other exotic instruments required sophisticated statistical modeling. The most important driver of expectations for mortgage defaults is the path for house prices, and the steep, broad-based decline in home prices that took place in 2006-2009 was outside the range that some modelers allowed for.

Another source of cognitive failure is the “suits/geeks” divide. In many firms, the financial engineers (“geeks) understood the risks of mortgage-related securities fairly well, but their conclusions did not make their way to the senior management level (“suits”).

There was policy failure on the part of bank regulators. Their previous adverse experience was with the Savings and Loan Crisis, in which firms that originated and retained mortgages went bankrupt in large numbers. This caused bank regulators to believe that mortgage securitization, which took risk off the books of depository institutions, would be safer for the financial system. For the purpose of assessing capital requirements for banks, regulators assigned a weight of 100 percent to mortgages originated and held by the bank, but assigned a weight of only 20 percent to the bank’s holdings of mortgage securities issued by Freddie Mac, Fannie Mae, or Ginnie Mae. This meant that banks needed to hold much more capital to hold mortgages than to hold mortgage-related securities; that naturally steered them toward the latter.

In 2001, regulators broadened the low-risk umbrella to include AAA-rated and AA-rated tranches of private-label CDOs. This ruling helped to generate a flood of PLMS, many of them backed by sub-prime mortgage loans.7

By using bond ratings as a key determinant of capital requirements, the regulators effectively put the bond rating agencies at the center of the process of creating private-label CDOs. The rating agencies immediately became subject to both moral hazard and cognitive failure. The moral hazard came from the fact that the rating agencies were paid by the issuers of securities, who wanted the most generous ratings possible, rather than being paid by the regulators, who needed more rigorous ratings. The cognitive failure came from the fact that that models that the rating agencies used gave too little weight to potential scenarios of broad-based declines in house prices. Moreover, the banks that bought the securities were happy to see them rated AAA because the high ratings made the securities eligible for lower capital requirements on the part of the banks. Both sides, therefore, buyers and sellers, had bad incentives.

There was policy failure on the part of Congress. Officials in both the Clinton and Bush Administrations were unhappy with the risk that Freddie Mac and Fannie Mae represented to taxpayers. But Congress balked at any attempt to tighten regulation of the safety and soundness of those firms.8

The Inter-bank Lending Market

There are a number of mechanisms through which financial institutions make short-term loans to one another. In the United States, banks use the Federal Funds market to manage short-term fluctuations in reserves. Internationally, banks lend in what is known as the LIBOR market.

One of the least known and most important markets is for “repo,” which is short for “repurchase agreement.” As first developed, the repo market was used by government bond dealers to finance inventories of securities, just as an automobile dealer might finance an inventory of cars. A money-market fund might lend money for one day or one week to a bond dealer, with the loan collateralized by a low-risk long-term security.

In the years leading up to the crisis, some dealers were financing low-risk mortgage-related securities in the repo market. But when some of these securities turned out to be subject to price declines that took them out of the “low-risk” category, participants in the repo market began to worry about all repo collateral. Repo lending offers very low profit margins, and if an investor has to be very discriminating about the collateral backing a repo loan, it can seem preferable to back out of repo lending altogether. This, indeed, is what happened, in what economist Gary Gorton and others called a “run on repo.”9

Another element of institutional panic was “collateral calls” involving derivative financial instruments. Derivatives, such as credit default swaps, are like side bets. The buyer of a credit default swap is betting that a particular debt instrument will default. The seller of a credit default swap is betting the opposite.

In the case of mortgage-related securities, the probability of default seemed low prior to the crisis. Sometimes, buyers of credit default swaps were merely satisfying the technical requirements to record the underlying securities as AAA-rated. They could do this if they obtained a credit default swap from an institution that was itself AAA-rated. AIG was an insurance company that saw an opportunity to take advantage of its AAA rating to sell credit default swaps on mortgage-related securities. AIG collected fees, and its Financial Products division calculated that the probability of default was essentially zero. The fees earned on each transaction were low, but the overall profit was high because of the enormous volume. AIG’s credit default swaps were a major element in the expansion of shadow banking by non-bank financial institutions during the run-up to the crisis.

Late in 2005, AIG abruptly stopped writing credit default swaps, in part because its own rating had been downgraded below AAA earlier in the year for unrelated reasons. By the time AIG stopped selling credit default swaps on mortgage-related securities, it had outstanding obligations on $80 billion of underlying securities and was earning $1 billion a year in fees.10

Because AIG no longer had its AAA rating and because the underlying mortgage securities, while not in default, were increasingly shaky, provisions in the contracts that AIG had written allowed the buyers of credit default swaps to require AIG to provide protection in the form of low-risk securities posted as collateral. These “collateral calls” were like a margin call that a stock broker will make on an investor who has borrowed money to buy stock that subsequently declines in value. In effect, collateral calls were a run on AIG’s shadow bank.

These collateral calls were made when the crisis in the inter-bank lending market was near its height in the summer of 2008 and banks were hoarding low-risk securities. In fact, the shortage of low-risk securities may have motivated some of the collateral calls, as institutions like Deutsche Bank and Goldman Sachs sought ways to ease their own liquidity problems. In any event, AIG could not raise enough short-term funds to meet its collateral calls without trying to dump long-term securities into a market that had little depth to absorb them. It turned to Federal authorities for a bailout, which was arranged and creatively backed by the Federal Reserve, but at the cost of reducing the value of shares in AIG.

With repos and derivatives, there was moral hazard in that the traders and executives of the narrow units that engaged in exotic transactions were able to claim large bonuses on the basis of short-term profits. But the adverse long-term consequences were spread to the rest of the firm and, ultimately, to taxpayers.

There was cognitive failure in that the collateral calls were an unanticipated risk of the derivatives business. The financial engineers focused on the (remote) chances of default on the underlying securities, not on the intermediate stress that might emerge from collateral calls.

There was policy failure when Congress passed the Commodity Futures Modernization Act. This legislation specified that derivatives would not be regulated by either of the agencies with the staff most qualified to understand them. Rather than require oversight by the Securities and Exchange Commission or the Commodity Futures Trading Commission (which regulated market-traded derivatives), Congress decreed that the regulator responsible for overseeing each firm would evaluate its derivative position. The logic was that a bank that was using derivatives to hedge other transactions should have its derivative position evaluated in a larger context. But, as it happened, the insurance and bank regulators who ended up with this responsibility were not equipped to see the dangers at firms such as AIG.

There was also policy failure in that officials approved of securitization that transferred risk out of the regulated banking sector. While Federal Reserve Officials were praising the risk management of commercial banks,11risk was accumulating in the shadow banking sector (non-bank institutions in the financial system), including AIG insurance, money market funds, Wall Street firms such as Bear Stearns and Lehman Brothers, and major foreign banks. When problems in the shadow banking sector contributed to the freeze in inter-bank lending and in the market for asset-backed commercial paper, policy makers felt compelled to extend bailouts to satisfy the needs of these non-bank institutions for liquid assets.


In terms of the fire metaphor suggested earlier, in hindsight, we can see that the markets for housing, sub-prime mortgages, mortgage-related securities, and inter-bank lending were all highly flammable just prior to the crisis. Moral hazard, cognitive failures, and policy failures all contributed the combustible mix.

The crisis also reflects a failure of the economics profession. A few economists, most notably Robert Shiller,12warned that the housing market was inflated, as indicated by ratios of prices to rents that were high by historical standards. Also, when risk-based capital regulation was proposed in the wake of the Savings and Loan Crisis and the Latin American debt crisis, a group of economists known as the Shadow Regulatory Committee warned that these regulations could be manipulated. They recommended, instead, greater use of senior subordinated debt at regulated financial institutions.13Many economists warned about the incentives for risk-taking at Freddie Mac and Fannie Mae.14

But even these economists failed to anticipate the 2008 crisis, in large part because economists did not take note of the complex mortgage-related securities and derivative instruments that had been developed. Economists have a strong preference for parsimonious models, and they look at financial markets through a lens that includes only a few types of simple assets, such as government bonds and corporate stock. This approach ignores even the repo market, which has been important in the financial system for over 40 years, and, of course, it omits CDOs, credit default swaps and other, more recent innovations.

Financial intermediaries do not produce tangible output that can be measured and counted. Instead, they provide intangible benefits that economists have never clearly articulated. The economics profession has a long way to go to catch up with modern finance.

About the Author

Arnold Kling was an economist with the Federal Reserve Board and with the Federal Home Loan Mortgage Corporation before launching one of the first Web-based businesses in 1994.  His most recent books areSpecialization and Trade and The Three Languages of Politics. He earned his Ph.D. in economics from the Massachusetts Institute of Technology.


“A desperate plea – then race for a deal before ‘sucker goes down’” The Guardian, September 26, 2008.


The report and dissents of the Financial Crisis Inquiry Commission can be found at

See Stefania Albanesi, Giacomo De Giorgi, and Jaromir Nosal 2017, “Credit Growth and the Financial Crisis: A New Narrative” NBER working paper no. 23740.


Karl E. Case and Robert J. Shiller 2003, “Is there a Bubble in the Housing Market?” Cowles Foundation Paper 1089


Edward M. Gramlich 2004, “Subprime Mortgage Lending: Benefits, Costs, and Challenges,” Federal Reserve Board speeches.


For example, in 1999, Treasury Secretary Lawrence Summers said in a speech, “Debates about systemic risk should also now include government-sponsored enterprises.” See Bethany McLean and Joe Nocera 2010, All the Devils are Here: The Hidden History of the Financial Crisis Portfolio/Penguin Press. The authors write that Federal Reserve Chairman Alan Greenspan was also, like Summers, disturbed by the moral hazard inherent in the GSEs.


Jeffrey Friedman and Wladimir Kraus 2013, Engineering the Financial Crisis: Systemic Risk and the Failure of Regulation, University of Pennsylvania Press.


See McLean and Nocera, All the Devils are Here


Gary Gorton, Toomas Laarits, and Andrew Metrick 2017, “The Run on Repo and the Fed’s Response,” Stanford working paper.


Talking Points Memo 2009, “The Rise and Fall of AIG’s Financial Products Unit”


Chairman Ben S. Bernanke 2006, “Modern Risk Management and Banking Supervision,” Federal Reserve Board speeches.


National Public Radio 2005, “Yale Professor Predicts Housing ’Bubble’ Will Burst”


Shadow Financial Regulatory Committee 2001, “The Basel Committee’s Revised Capital Accord Proposal”

See the discussion in Viral V. Acharya, Matthew Richardson, Stijn Van Nieuwerburgh and Lawrence J. White 2011, Guaranteed to Fail: Fannie Mae, Freddie Mac, and the Debacle of Mortgage Finance, Princeton University Press.