Speculators now account for half of all traders in the main U.S. oil market, and their growing presence coincided with this decade's historic rise in the price of crude, according to a new Rice University study. The study does not try to prove that speculators caused the price spike, as many politicians and consumer advocates believe. But the authors note that prices rose steadily along with the number of speculative investors, and fell with them as well. Seven years ago, speculators accounted for 20 percent of oil traders on the New York Mercantile Exchange. That number jumped to 55 percent by the time oil prices reached their all-time peak above $145 per barrel last summer. Now oil costs $72, and speculative investors account for half the traders. The government limits the number of oil contracts that each speculator can hold. But under the Commodity Futures Modernization Act [passed in 2000], trades on electronic exchanges or overseas markets don't count toward those limits. The study uses data from the Commodity Futures Trading Commission. Speculators are defined as traders who use oil strictly as a financial investment, those who will never take delivery of a tanker-full of crude. This confirms what we and others have said for some time," said Tyson Slocum, director of the energy program at the Public Citizen watchdog group. "The good thing from the oil price run-up of 2008 is it has forced Congress to realize there's a problem in these markets, and the answer is re-regulation." The financial industry opposes tightening the regulations.