Given this history, and the fact that recent years have seen a huge flood of speculative money entering the commodity markets—assets in commodity indexes, by some calculations, increased twentyfold between 2003 and the spring of this year—it’s not unreasonable to wonder if there might be something nefarious behind the sharp run-up in oil prices. But there’s little convincing evidence that the oil market is being significantly manipulated. Whatever chicanery is occurring—and we can assume there is some—has only a marginal effect on prices at the pump.

Congress is not, though, just attacking illegal market manipulation; it’s also taking aim at perfectly legal speculation, namely the buying and selling of futures contracts, which are effectively bets that oil prices will go up (or down). Futures contracts can be used by oil sellers (like OPEC ) or oil buyers (like the airlines) to hedge their risks by agreeing to sell or buy oil in the future at a set price. Speculators, by contrast, mostly use futures contracts to gamble on oil prices, and have no interest in buying or selling real barrels of oil. These gambles can be tremendously lucrative, but they don’t directly determine the real (or “spot”) price of oil. That’s set by the people who are buying and selling actual barrels of petroleum. Although speculators could directly distort oil prices by turning their futures contracts into oil and then taking it off the market to drive up prices, a look at oil inventories shows no sign that this is happening.

If speculators aren’t at fault, why have oil prices spiked so high? Fundamental reasons aren’t hard to find. Between 2000 and 2007, world demand for petroleum rose by nearly nine million barrels a day, but OPEC has been consistently unable, or unwilling, to significantly increase supply, and production by non-OPEC members has risen by just four million barrels a day. The prospect of military action against Iran, which would disrupt global supply, seems greater than it did a few years ago. And the plunging value of the dollar has meant that the cost of oil has jumped more in the U.S. in the past year than it has in countries with healthier currencies.

But there’s also something else at work, which the oil guru Daniel Yergin calls a “shortage psychology.” The price of oil—more than that of many other commodities—isn’t based solely on current supply and demand. It’s also based on people’s expectations about future supply and demand, because those expectations determine whether it makes sense for oil producers to sell their oil now or leave it in the ground and sell it later. Currently, the market is assuming that oil will become scarcer, and that global demand will keep rising, especially in rapidly developing countries like China and India. As a result, producers are asking very high prices to pump their oil.