Actually, it's not exactly lonely at the top this time. US stocks are neck-and-neck with US REITs among the major asset classes in terms of relative year-to-date changes based on an equally weighted ETF-based portfolio of the major asset classes (excluding cash) through April 10. Both markets are at the highest range of their allocations this year for an unmanaged, equally weighted portfolio with a start date of December 31, 2012.
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But for every winner, there is a loser in a relative horse race, and the burden currently falls heavily on foreign stocks and foreign government bonds in developed markets (in US dollar terms), along with commodities broadly defined.