including actual research of individual oil field production throughout the world. Terrorist and insurgent groups have increasingly targeted oil and gas installations, and succeeded in stopping a substantial volume of exports during the 2003–2008 height of the American occupation of Iraq. Such attacks are sometimes perpetrated by militias in regions where oil wealth has produced few tangible benefits for the local citizenry, as is the case in the Niger Delta. Many factors have resulted in possible and/or actual concerns about the reduced supply of oil. The post-9/11 war on terror, labor strikes, hurricane threats to oil platforms, fires and terrorist threats at refineries, and other short-lived problems are not solely responsible for the higher prices. Such problems do push prices higher temporarily, but have not historically been fundamental to long-term price increases. Investment/speculation demand Investment demand for oil occurs when investors purchase futures contracts to buy a commodity at a set price for future delivery. "Speculators are not buying any actual crude. ... When [the] contracts mature, they either settle them with a cash payment or sell them on to genuine consumers." Several claims have been made implicating financial speculation as a major cause of the price increases. In May 2008 the transport chief for Germany's Social Democrats estimated that 25 percent of the rise to $135 a barrel had nothing to do with underlying supply and demand. Testimony was given to a U.S. Senate committee in May indicating that "demand shock" from "institutional investors" had increased by over the previous five years, almost as much as the increased physical demand from China (). The influence of institutional investors, such as sovereign wealth funds, was also discussed in June 2008, when Lehman Brothers suggested that price increases were related to increases in exposure to commodities by such investors. It claimed that "for every $100 million in new inflows, the price of West Texas Intermediate, the U.S. benchmark, increased by 1.6%." Also in May 2008, an article in The Economist pointed out that oil futures transactions on the New York Mercantile Exchange (NYMEX), nearly mirrored the price of oil increases for a several-year period; however, the article conceded that the increased investment might be following rising prices, rather than causing them, and that the nickel market value had halved in the year between May 2007 and May 2008 despite significant speculative interest. It also reminded readers that "Investment can flood into