Most stockmarket returns come from a tiny fraction of shares

Does this statistic consider cap gains only?

Or did the author take dividend into account?

I'm not surprised that cap gains are concentrated in a few stocks, but overal wealth creation? I have my doubts.

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TLDR: very few stocks have outsized returns over a relatively short period of time (usually early in the life of the companies). Most stocks (>50%) in the S&P500 (which suggests the DOW would be worse) are just outright bad investments.

That’s not what the article concludes with. The conclusion is that it’s difficult to time stock trades at the right time, and even indexing the US market is vulnerable to bubbles, so one should diversify by investing in European markets as well.

Honestly I found it difficult to discern any coherent point from the article at all. At the end they just say "diversify".

But they also say that a lot of stocks are poor investments, so taking "diversify" to the extreme will dilute/eliminate any profits you make. So, in order to provide useful information, they really need to be a lot more specific than just "diversify".