Among the recurring themes in the global capital markets in recent years is the ongoing preeminence of the US stock market as a performance leader. When measured in terms of relative returns for the major asset classes, it’s been game, set, match. That’s good news for investors who’ve maintained healthy allocations to US equities in the last several years–and a source of frustration for everyone else. The supremacy of American stocks will end at some point, but for the moment there’s no sign that a changing of the guard is at hand.

The S&P 500 yesterday (July 20) was more or less flat, although the benchmark briefly traded above its all-time closing high on Monday. “While I wouldn’t be shocked if we move to new highs, the market may hesitate here,” Mark Luschini, chief investment strategist at Janney Capital Management, tells Bloomberg. “The earnings that we’ve gotten so far have been better than expected. M&A activity remains alive and well, so that’s a positive.”

Whatever happens in the days and weeks to come, there’s no doubt that the US equity performance has left the competition (broadly defined) in the dust. That’s old news, of course, but the depth and duration is striking and explains quite a lot about the divergence of results in portfolio strategies. Simply holding a healthy allocation to the S&P 500 (or its equivalent) has been a winning trade in no uncertain terms.

Consider how the S&P 500 ETF (SPY) has performed in relative terms against the ETF proxies that comprise an investable version of the Global Market Index (GMI), an unmanaged, market-value-weighted mix of all the major asset classes. The graph below shows the relative performance for SPY vs. these funds. Because some of the ETFs have a limited history, the starting date for the analysis begins on Apr. 4, 2012 in order to show relative returns for all the funds. The basic theme is clear: the overwhelming dominance of US equities.

The flat blue line shows the Vanguard Total Stock Market (VTI) ETF, which more or less tracks SPY due to a similar portfolio that’s driven by large US stocks. Otherwise, the slow but steady outperformance of SPY is conspicuous, as indicating by downward sloping lines. The degree of SPY’s outperformance varies and even reverses at times, depending on the asset class. But there’s no mistaking the broad movement in favor of US equities in recent years.

The S&P 500’s reign is a reflection of the comparatively upbeat US macro profile…. but the trend is also planting the seeds of its own destruction. Mean reversion, in other words, will win in the end. The challenge, of course, is deciding when the comeuppance begins. The answer, as usual, is that no one really knows in real time.

The truth will out… eventually. In the meantime, paring a US equity allocation that’s grown fat through the years has merit. That doesn’t mean that SPY and its counterparts won’t climb further. But periodic rebalancing never goes out of style… especially when one piece of a portfolio has been burning so brightly relative to everything else.