Below is a chart showing the performance of the S&P 500 tracking SPY ETF along with the performance of the emerging markets ETF (EEM) since the bull market began back in March 2009. As shown, after nearly doubling the performance of the S&P 500 early on during the current bull, emerging markets are now lagging. As of this morning, SPY was up 107.1% since 3/9/09, while EEM was up 93.9%.
The weakness for emerging markets really began last summer when sovereign debt concerns popped up. After regaining the lead on SPY earlier this year, EEM once again faltered this spring and hasn't been able to catch up since.