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[Hyun Song Shin] most accurately portrayed the state of the global economy.
'I'd like to tell you about the Millennium Bridge in London,' he began…'The bridge was opened by the queen on a sunny day in June,' Shin continued. 'The press was there in force, and many thousands of people turned up to savor the occasion. However, within moments of the bridge's opening, it began to shake violently.' The day it opened, the Millennium Bridge was closed. The engineers were initially mystified about what had gone wrong. Of course it would be a problem if a platoon of soldiers marched in lockstep across the bridge, creating sufficiently powerful vertical vibration to produce a swaying effect. The nearby Albert Bridge, built more than a century earlier, even features a sign directing marching soldiers to break step rather than stay together when crossing. But that's not what happened at the Millennium Bridge. 'What is the probability that a thousand people walking at random will end up walking exactly in step, and remain in lockstep thereafter?' Shin asked. 'It is tempting to say, 'Close to Zero' '
But that's exactly what happened. The bridge's designers had failed to account for how people react to their environment. When the bridge moved slightly under the feet of those opening-day pedestrians, each individual naturally adjusted his or her stance for balance, just a little bit—but at the same time and in the same direction as every other individual. That created enough lateral force to turn a slight movement into a significant one. 'In other words,' said Shin, 'the wobble of the bridge feeds on itself. The wobble will continue and get stronger even though the initial shock—say, a small gust of wind—had long passed…Stress testing on the computer that looks only at storms, earthquakes, and heavy loads on the bridge would regard the events on the opening day as a 'perfect storm.' But this is a perfect storm that is guaranteed to come every day.'
In financial markets, as on the Millennium Bridge, each individual player—every bank and hedge fund and individual investor—reacts to what is happening around him or her in concert with other individuals. When the ground shifts under the world's investors, they all shift their stance. And when they all shift their stance in the same direction at the same time, it just reinforces the initial movement. Suddenly, the whole system is wobbling violently.
Ben Bernanke, Mervyn King, Jean-Claude Trichet, and the other men and women at Jackson Hole listened politely and then went to their coffee break.
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