By Josh Rosner

The Big Short, the screen adaptation of Michael Lewis' book on the 2008 financial crisis, has reopened the debate about what caused and exacerbated the catastrophic collapse. Some commentators continue to assert that the crisis was entirely the fault of Fannie Mae and Freddie Mac, but that claim just doesn't hold up.

Here's what happened.

In the mid-1990s, after weak legislative reform of the government-sponsored enterprises (GSEs) Fannie Mae and Freddie Mac, these firms - supported by government efforts to expand homeownership - began to subordinate their historic and effective public roles as countercyclical providers of liquidity to the mortgage market and to place profit growth ahead of their utility mission.

Still, it was not until the Wall Street banks ramped up their private-label securitization and the GSEs began losing market share in 2004 that they lowered their standards and became meaningfully exposed to questionable loans.

However, the GSEs never originated the subprime loans at the heart of the crisis and never bought the worst of those loans. If one looks at the delinquency and default levels of agency loans vs. the overall market in 2009, it's clear that Fannie and Freddie were not the chief culprits in the disaster. Fannie's inventory of loans considered seriously delinquent amounted to 3 percent of outstanding loans, and Freddie's was 2.3 percent.

It is true that the GSEs were major players in the mortgage market. However, this turned out to be a good thing. On a blended basis, serious delinquencies were just over 6 percent of the country's mortgages at the time. In essence, it was the banks that underwrote mortgages they never should have offered in the first place. That is why they were fined by the Justice Department.

Misguided actions by Congress and administrations beginning in the 1990s muddled the missions of Fannie and Freddie. By 2007, a murky mission and the GSEs' exposure to private-market arbitrage did indeed put them on unsteady footing. However, the conclusion one should draw is that Fannie and Freddie reflected a breakdown in the system but did not cause the breakdown. This was the conclusion of the National Commission on the Causes of the Financial and Economic Crisis in the United States.

What concerns me today is increasing concentration in the financial system and an implied government guarantee behind our largest banks. Eight years after "too big to fail" became part of our financial lexicon, five banks control 40 percent of the country's total deposits, and the number of other large banks (more than $10 billion in assets) has grown from 76 in 2000 to more than 100 today. Those other large banks now hold an additional 40 percent of U.S. deposits. All this has occurred as small banks (which are typically more responsive to their local customers) hold only 20 percent of deposits and have declined in number from 8,200 in 2000 to 5,900 today.

The behemoths have significant advantages over the other 3,500 banks in the United States. If we dismantled Fannie and Freddie, these banks would be the biggest beneficiaries. Some critics who insist that Fannie and Freddie caused the financial crisis overlook the myriad irresponsible acts of Wall Street and other private-sector actors and omit any mention of what could happen if we concentrated mortgages in huge banks.

The Big Short tells a story in cinematic style, so it is reasonable to pick apart various scenes and portrayals. But it is not purely a work of fiction or fantasy. Any assertion that blame for the 2008 financial crisis rests primarily with Fannie and Freddie is.

Josh Rosner is the co-author of "Reckless Endangerment: How Outsized Ambition, Greed, and Corruption Led to Economic Armageddon." He wrote this for