"After analysing data on both prices and individual holdings from America’s Commodity Futures Trading Commission, the OECD study found that there was “no convincing evidence that positions held by index traders…impact market returns”; indeed, the OECD reckons that larger positions led to lower market volatility (although data issues mean the finding is more convincing for agricultural products than for energy markets)."
http://en.wikipedia.org/wiki/S%26P_GSCI
quotes this article:
http://www.economist.com/node/16432870?subjectid=2512631&...
"After analysing data on both prices and individual holdings from America’s Commodity Futures Trading Commission, the OECD study found that there was “no convincing evidence that positions held by index traders…impact market returns”; indeed, the OECD reckons that larger positions led to lower market volatility (although data issues mean the finding is more convincing for agricultural products than for energy markets)."