Concentration in the Index Era

The index fund got sold as the closest thing to a free lunch investing has. Instant diversification, near-zero cost, own the whole market and stop worrying about it. For a long stretch that was a fair description. It's less fair now, and most of the people holding one haven't noticed that the thing they own has quietly turned into the opposite of what they were promised.

When a handful of enormous companies make up a third or more of a market-cap index, the fund you bought for safety is a concentrated position wearing a diversified costume. The questions worth asking are simple, and hardly anyone asks them. What am I actually exposed to? Why did it get this concentrated? What happens when the flows that pumped it up start running the other way?

The Diversification Illusion

Open up a cap-weighted index and the ownership is lopsided. A small cluster of the biggest companies can account for a third of the entire index's value, and an even larger share of its recent returns. The last time I ran this for a family that thought it owned five hundred companies, ten of those names were a third of the money. You hold five hundred. A handful of them move your portfolio. The other several hundred are, in practice, rounding error.

That's not diversification. It's a concentrated bet on a few mega-cap companies, most of them in the same industry, exposed to the same handful of forces. You didn't choose that bet. You inherited it by owning the market in the shape it happens to be in right now, and it feels safe only because someone labeled it the whole market.