John Maynard Keynes, in his famous 1936 work, “The General Theory of Employment, Interest and Money,” likened the stock market to a beauty contest that ran in the newspapers of his day. Readers were asked to pick which would be voted the prettiest face. The key to selecting the winner, Keynes argued, wasn’t choosing the face you think is the most beautiful, but rather anticipating the face that others will pick.

Such theories aside, the uncertainties in the financial world now seem to have overloaded our collective financial brains, at least judging by the markets.

Denise Shull, the founder of Trader Psyches, which consults funds and investors on neuroscience strategies, says many investors are using the 2008 panic as their new reference point. After so many people lost so much money, many investors no longer hesitate to sell at the first sign of trouble, he says.

“How your brain deals with uncertainty — when it recognizes it is in an uncertain situation — is that it tries to pull it from a bigger context,” Ms. Shull says. “The context of 2008 and not wanting to see it happen again would absolutely influence these people to hit the sell button. Then it becomes self-fulfilling for the market.”

IT has actually been relatively easy to make money on Wall Street over the last year or so. Buoyed by hopes that the Federal Reserve would be able to reignite growth through quantitative easing, its controversial bond-buying program, the American stock market shot up last August and kept on rising fairly steadily through this May.

That run gave investors, the pros and individuals alike, a false sense of security, says Roy Niederhoffer, who runs a money management firm called R.G. Niederhoffer Capital Management.

Mr. Niederhoffer has a degree in computational neuroscience from Harvard and is a big believer in the idea that neuroscience can help people invest. He says he was surprised at how complacent investors have been, given Europe’s problems, the slack American economy and, until a deal last week, the debt-ceiling showdown in Washington.

One measure of that complacency is the Chicago Board Options Exchange’s VIX index, which uses the price of stock options to measure investors’ expectations about future volatility. After spiking following the earthquake in Japan this year, the VIX drifted lower through June and remained relatively subdued until early July. It has since shot up, although it remains far below the record levels hit during the financial crisis.

Richard Sylla, a finance professor at the Stern School of Business at New York University who has written about market sell-offs, said the plunge last week had all the hallmarks of a classic panic attack. The debt-ceiling debate, concern over the American economy, Europe — the bad news just kept coming, and investors finally snapped.

“When you pile all of those things together, investors become queasy,” he says.

Mr. Niederhoffer agrees. “I think, without question, there’s been a tremendous shift in psychology, but it took a while,” he says. Even when the market began to wobble last month, many investors refused to throw in the towel.

Mr. Niederhoffer has actually had a tough time because investors have been so complacent. He tends to zig when other investors zag, known as a contrarian strategy. Last week, two of his hedge funds were up 8.6 percent and 13.5 percent.

Now he smells opportunity. “We see what’s happening as a return of the psychology of the markets in 2007 and 2008,” he said. “This feels like it’s got legs.”

If he’s right, investors had better fasten their seat belts.