Unlike mutual funds, ETFs create or redeem shares in large blocks at the request of big institutional investors, known as authorized participants. Small investors must purchase or sell shares on an exchange, where they trade throughout the day like stocks. U.S. mutual funds are priced once a day, after the close of regular trading.

The creation of ETFs to invest in gold has given investors a way to benefit from rising gold prices, without having to directly own or trade the precious metal. Research firm Morningstar Inc. tracks about two dozen gold mutual funds in the U.S., all of which invest in gold-mining stocks or indexes composed of companies engaged in exploring and mining the precious metal. The stocks and indexes don't necessarily move in tandem with the price of the underlying metal.

"The market volatility has scared investors away from stocks," Paul Justice, an analyst with Morningstar in Chicago, said in an interview. "No mutual fund can offer what the gold ETF can, but the S&P 500 ETF has many competitors."

GDP Revisions

The S&P 500 index has tumbled 18 percent from this year's peak on April 29. The index returned an average of 1.9 percent a year including dividends in the decade ended Aug. 12, souring investors who counted on equities to resume their historic long- term gains of about 10 percent a year.

Morgan Stanley cut its forecast on Aug. 18 for 2011 global expansion, citing an "insufficient" response to Europe's debt troubles, weakened confidence and the prospect of fiscal tightening. The bank lowered its growth prediction to 3.9 percent from a previous forecast of 4.2 percent.

Citigroup Inc. cut its growth forecasts for the world's largest economy and trimmed its earnings estimates for the S&P 500. The U.S. economy may expand less than previously forecast in 2011 and 2012 because of potential "political paralysis" and fiscal-tightening measures, Citigroup analysts wrote in a report dated Aug. 18.

Consistent Gains

The bank cut its 2011 gross domestic product growth forecast to 1.6 percent from 1.7 percent and lowered its 2012 GDP growth estimate to 2.1 percent from 2.7 percent. It cut its 2011 and 2012 per-share earnings estimates for the S&P 500 to $97 and $101 respectively.

SPDR Gold Trust has climbed every year since inception, including a 30 percent gain in 2011. The gold ETF has more than doubled in price since the collapse of Lehman Brothers Holdings Inc. in September 2008 roiled financial markets, while the SPDR S&P 500 ETF has fallen 4.7 percent including dividends.