Wikipedia here

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)."

Three previous submissions of this April 2011 article:

http://news.ycombinator.com/item?id=2493253

http://news.ycombinator.com/item?id=2499617

http://news.ycombinator.com/item?id=2533280

It has also been submitted in comments.