A previous post suggested that leverage was a contributory factor to the current crisis but Ricardo Caballero via Economists View has suggested that it was concentration of risk. Michael Lewis on AIG has indicated that it could well be concentration of risk as well:
A.I.G. F.P. was already insuring these big, diversified, AAA-rated piles of consumer loans; to get it to insure subprime mortgages was only a matter of pouring more and more of the things into the amorphous, unexamined piles. They went from being 2 percent subprime mortgages to being 95 percent subprime mortgages. And yet no one at A.I.G. said anything about it—not C.E.O. Martin Sullivan, not Joe Cassano, not Al Frost, the guy in A.I.G. F.P.’s Connecticut office in charge of selling his firm’s credit-default-swap services to the big Wall Street firms. The deals, by all accounts, were simply rubber-stamped by Cassano and then again by A.I.G. brass—and, on the theory that this was just more of the same, no one paid them special attention. It’s hard to know what Joe Cassano thought and when he thought it, but the traders inside A.I.G. F.P. are certain that neither Cassano nor the four or five people overseen directly by him, who worked in the unit that made the trades, realized how completely these piles of consumer loans had become, almost exclusively, composed of subprime mortgages.
... Toward the end of 2005, Cassano promoted Al Frost, then went looking for someone to replace him as the ambassador to Wall Street’s subprime-mortgage-bond desks. As a smart quant who understood abstruse securities, Gene Park was a likely candidate. That’s when Park decided to examine more closely the loans that A.I.G. F.P. had insured. He suspected Joe Cassano didn’t understand what he had done, but even so Park was shocked by the magnitude of the misunderstanding: these piles of consumer loans were now 95 percent U.S. subprime mortgages. Park then conducted a little survey, asking the people around A.I.G. F.P. most directly involved in insuring them how much subprime was in them. He asked Gary Gorton, a Yale professor who had helped build the model Cassano used to price the credit-default swaps. Gorton guessed that the piles were no more than 10 percent subprime. He asked a risk analyst in London, who guessed 20 percent. He asked Al Frost, who had no clue, but then, his job was to sell, not to trade. “None of them knew,” says one trader. Which sounds, in retrospect, incredible. But an entire financial system was premised on their not knowing—and paying them for their talent!
By the time Joe Cassano invited Gene Park to London for the meeting in which he would be “promoted” to the job of creating even more of these ticking time bombs, Park knew he wanted no part of it. He announced that, if he was made to take the job, he’d quit. (Had he taken it he would now be a magazine cover.)