American investors have poured money into foreign stocks in recent years lured by the hope of outsized gains. They have been well rewarded in the past 12 months but in May markets plummeted around the world. Mutual funds investing in foreign stocks for example lost more than 8% in the two weeks ending May 25 although their previous stunning performance left them up nearly 31% for the 12 months ending on that date. The late-May plunge was especially severe in emerging markets. Is this another bubble bursting the way the tech-stock bubble collapsed several years ago? Wharton professors offer their take on the downturn and its implications for nervous investors.