Investors often welcome falling commodities prices because the lower costs will have benefits across the economy. But traders have also been looking for gains in commodities because that could signal resources are becoming more scarce as demand improves.
Commodities producers fell Monday. Aluminum maker Alcoa and Freeport-McMoRan Copper & Gold slid. Alcoa fell 78 cents, or 6.5%, to $11.21, while Freeport-McMoRan fell $3.37, or 5.8%, to $55.14.
In corporate news, Goldman Sachs lowered its rating on Wal-Mart Stores to "Neutral" from "Buy," seeing few catalysts that could push the stock higher. The retailer fell $1.38, or 2.8%, to $48.46.
Nick Kalivas, vice president of financial research at MF Global, said traders are cautious ahead of quarterly earnings reports this week from Best Buy, FedEx and BlackBerry maker Research in Motion, all of which are important in their industries.
"It might keep us sideways or lower if we can't get some good news from some of these numbers," he said.
Investors are also getting more specifics on Washington's plans for the biggest overhaul of the financial system since the Great Depression. Among the proposals, the Federal Reserve would have authority over the largest financial institutions whose failure could jeopardize the entire financial system.
Treasury Secretary Timothy Geithner and President Obama's chief economic adviser, Lawrence Summers, outlined the White House proposal in an article Monday in The Washington Post.
Trading volume remained light Monday, as it has been for weeks. That indicates fewer traders are standing behind the market's moves. Volume does tend to slow in the summer as traders take vacations, but thin volume could indicate there is less conviction behind the market's moves.
About eight stocks fell for every one that rose on the New York Stock Exchange, where volume came to a light 4.55 billion shares, up from Friday's 4.39 billion.
In other trading, the Russell 2000 index of smaller companies fell 15.00, or 2.9%, to 511.83.