NEW YORK ( TheStreet) -- In the last few weeks the idea of maintaining a diversified portfolio has taken a beating, especially where emerging markets are concerned.Since mid-February the iShares Emerging Market Index Fund ( EEM) is down 12%, vs. a 6% gain for the SPDR S&P 500 ( SPY). That lag has intensified since the high for the S&P 500 in early May. Is this reason enough for investors to abandon emerging markets in their portfolios? The first question to ask is: What is the purpose of investing in emerging markets? Most people will answer that emerging markets help investors diversify their portfolios away from domestic stocks and capture promising long-term growth. People looking for superior diversification are buying an asset class they expect to add value to their long-term investment results, but no asset class can always outperform another asset class. Over the last 10 years, EEM is up 186%, vs. 65% for the SPY. Although value has clearly been added over that time, the last 10 years include several periods of meaningful performance lags. Some may remember the Asian Contagion from 1997 in which the Vanguard Emerging Markets Stock Index Fund ( VEIEX) dropped 33% in four months. In the next four months, VEIEX then went up 81%. EEM did not exist then, but they are both proxies for the same thing. Periods like the present, when emerging markets lag, have happened many times in the past and will happen again in the future, but there is still long-term potential value. Whatever sense of concern there is now about the future of emerging markets, it was far greater in 1997. The Asian Contagion was not the end of emerging markets, and neither is the current market downturn. Investors looking to capture favorable long-term growth prospects must realize that growth in all countries is subject to economic cycles. The timing of cycles in different countries will sometimes be different than those of the U.S., but when any economy enters a period of slower growth or contraction, then that is when the corresponding stock markets are likely to turn down. That doesn't invalidate a country's long-term growth prospects. If a country has resources to export, a young aspirational population and an ascendant middle class when its economy is booming, it will still have those positive attributes during a slowdown.