(FORTUNE Magazine) – If you own an S&P 500 index fund, you're actually making a diversified international play, according to a new study by Morgan Stanley Dean Witter. The index's companies make almost a quarter of their revenue abroad and are thus exposed to gains--or losses--in several regions.

Surprised? So were we. Most know that shareholders of Nike, (global revenues: 40%) have been hit by turmoil in Southeast Asia. But today, even S&P members like Polaroid and Cisco are dependent on global markets for half their revenue.

--Julie Creswell