most stock market indices actually do look quite a bit like the real economy, with a big fall and little recovery. However, the top 5 mega-cap stocks have held up the major American stock market indices like Atlas holding up the world, and reached levels of concentration not seen in nearly four decades.

It's notable that this has not been the case internationally. MSCI equal-weighted international indices (whether it's emerging markets, or the full all-country ex-USA index) have performed about the same as their market-cap weighted versions year-to-date as of the end of April.

So, this has not really been a case of a performance differential between U.S. stocks vs. international stocks broadly. It has been a case of the top five U.S. stocks outperforming just about everything else, including the rest of the American stock market.

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