4. Emerging market stocks via Vanguard Emerging Markets (VWO). The export dependency of developing nations is being sideswiped by Japanese currency debasement as well as limited demand from the euro-zone. Economic growth is waning in China as well as Brazil. Meanwhile, years of underperformance has tested even the most ardent buy-n-holders. Outflows from emerging market equities have been enormous. Granted, P/E ratios of 10 may be at a 30% discount to U.S. stocks. Nevertheless, technical traders often smell blood in the water when assets trade below 200-day trendlines and 50-day moving averages cross below 200-day moving averages. 5. U.S. Real Estate Investment Trusts via Vanguard REIT (VNQ). Between the start of the year and early May, few asset classes soared as high as the dividend-paying real estate investment trust. And why not? Businesses and individuals were clamoring for property due to the lowest mortgage rates on record. That warm fuzzy feeling changed the minute the Fed hinted that it might slow its bond-buying program. Less bond buying, higher mortgage rates. Higher mortgages, less refinancing activity and less mortgage applications. Granted, VNQ catapulted on a lack of fear, but it has sold off on a profit-taking abundance of fear. Holding above a long-term 200-day moving average may be key to the viability of the asset class over the next 6 months. Vanguard recently introduced an ETF for international bonds, Vanguard Total International Bond ( BNDX). It is not possible to discuss the asset class of international bonds with this vehicle. That said, we can note that international bonds were hit far worse by the Fed's discussion of tapering than domestic securities. ETFs like WisdomTree Emerging Market Local ( ELD), SPDR International Treasury Bond ( BWX) as well as iShares International Treasury ( IGOV) all registered losses between 5% and 10% over the last six months. Vanguard does not have a precious metals ETF either. Maybe that's a good thing for the fund family. After all, Vanguard Precious Metals ( VGPMX) is down about 35% year-to-date thanks in large part to the extreme bearish price movement for gold and silver. In sum, the only asset class that has not stared into the face of adversity is U.S. stocks. Still, avoiding a summertime slump may require positive corporate guidance in the upcoming earnings season, stable or falling bond yields as well as eurozone debt crisis containment. How tall is that order? Follow @etfexpertThis article was written by an independent contributor, separate from TheStreet's regular news coverage.