A month after stocks began trading again, investors have been both astonished and cheered to see the benchmark S&P 500 back where it was before Sept. 11. But it's worth noting that it's not quite the same S&P as before.

S&P Sector Performance Since Sept. 10

Source: Standard & Poor's

While the S&P 500 is Wall Street's favorite proxy for the market, it is also a portfolio of stocks. Since the terrorist attacks on the U.S., the dynamics of that portfolio have changed markedly -- and not in the ways one might have assumed, given an economy that's looking a whole lot softer all the time. That could create opportunity for defensive-minded investors.

Typically, when the outlook darkens, investors shift into areas that offer steady earnings regardless of the economic environment. Consumer staples, like the food and drink companies, health care and utilities, all find favor. Meanwhile, companies whose fortunes ebb and flow along with the economy, like consumer cyclicals, basic materials producers, industrials and techs, flag.

Or so the theory goes. In the past month, however, the movements have been willy-nilly. Sure, health care has been the best-performing sector in the S&P, gaining 3.5% through Oct. 16. But then, consumer staples, down 4.5%, has unexpectedly been the worst. Basic materials and industrial companies have been hit hard, but technology and consumer discretionary companies have performed well.

"Some of the winners seem to make sense and some don't," says J.P. Morgan strategist Tom Van Leuven. "The return to technology hardware is dubious. Another one that seems odd is retail: Sure, discount retailers can outperform in this kind of environment, but it doesn't seem like the retail group as a whole ought to do well."

In fact, the biggest contributor to the S&P 500 since trading resumed, on a capitalization basis, is

Wal-Mart

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, up 14.9%. The world's biggest retailer's emphasis on discounting, with a product mix that has shifted toward consumer staples, has got some investors convinced it's the place to be these days. Still, given that the company accounts for nearly 10% of nonauto retail sales in the U.S., it's hard to see how a stalling economy is going to help its business.

Tech's move higher seems equally odd, but, points out Stanley Nabi, managing director at Credit Suisse Asset Management, that may have more to do with a bounceback in a volatile and beaten-down sector than with investors deciding that the business outlook has suddenly improved. "That's natural," he says of the tech sector's move, "because it became woefully depressed."

Oracle

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, for instance, has been the third-biggest contributor to the S&P since Sept. 10, gaining 30.4%. But it's still down 50% on the year.

But whether the shift toward cyclical areas of the market is thought out or just a bounce, it seems investors might profit by taking the other side of the bet. Even with the

Fed's

recent rate cuts and the government's plans for fiscal spending, the U.S. business outlook for the next half-year is worse, not better, than it was before. "The economy is weakening," says Van Leuven. "And we continue to focus on sectors, groups and companies where earnings are less sensitive to the economy."

It's to investors' advantage that some of those defensive areas are cheaper now than they were a month ago.