Often the simplest rules are simply the best.
The top priority on the stock exchange is: Spread your risks as broadly as possible.
This also applies to investments in equity ETFs.
The practical thing about this form of investment is that investors don't have to do much to spread the risk.
If you simply invest in an ETF on the MSCI World, you can distribute your money to 1,600 companies in 23 industrial nations with a single investment.
Granted, two-thirds of that money ends up in the United States, and a large chunk of it in a handful of tech giants like Apple, Microsoft, and Amazon.
But that's the way the balance of power is on the global stock markets - and if they change, the MSCI World investor doesn't have to lift a finger to rebalance their portfolio.
Of course that's pretty boring.
Anyone who has a bit of fun investing always wants to try to be a bit smarter and, in the end, richer than everyone else, or at least the neighbor.
The ETF industry is also always coming up with something new to make even more sophisticated (and expensive) index funds on new trends or factor strategies palatable to investors.
But a look at the recent past shows that anyone who wanted to beat the entire world market with such special ETFs needed a lucky hand.