BASHING “speculators” is a popular pastime for American politicians trying to explain high and volatile oil prices. But whether speculation has really been responsible for spiking prices is a controversial issue. In 2008 the Commodity Futures Trading Commission (CFTC) issued a report dismissing the role of speculators in last year’s startling run-up in prices. But banks, hedge funds and others who bet on oil (without a use for the stuff itself) still face limits on the positions they can take, if Gary Gensler, the new CFTC head, can show that their influence in markets does harm. On September 4th the...