Diversification

Diversification lowers the damage any one holding can do. It cannot lower risk that affects everything at once, which is why "diversified" portfolios still fall in a general crash.

There is a tension worth naming: concentration builds fortunes and diversification preserves them. Most people who become wealthy did so concentrated - in one business - and then diversified hard afterwards.

It takes less than people assume, and then it stops helping. Within one market, most of the benefit has arrived by roughly twenty to thirty holdings; a single global index fund holds thousands and does it in one line. Beyond that point, adding funds is not diversification, it is duplication - three funds tracking overlapping indices are one position with three sets of fees.

The concentration nobody counts is the one outside the portfolio. A salary from one employer, company shares from the same employer and a mortgage in the same city is the same bet placed three times, and it is usually far larger than anything in the investment account. Fixing that is worth more than owning a fifth fund.