Index Fund Machine Ch. 1: Somebody Chose Those Stocks

Investment Background

This library already contains six books arguing you should index.

A Random Walk gave the cost arithmetic. Winning the Loser's Game gave market structure. The Psychology of Money gave psychology. Stocks for the Long Run gave the historical record. Where Are the Customers' Yachts? gave the agency conflict. Expected Returns gave the premium structure.

This book is not a seventh argument.

It assumes you are already convinced, then asks a question none of them ask:

Do you know what you actually own?

This is a book about the container, not about whether to use one. Your retirement core may sit inside that container for thirty years, and most people have never opened it.

The Wall Street Translation

An Index Is Not a Photograph

The most common misconception treats an index as a neutral measurement of the market — as objective as a thermometer reading.

It is not. An index is a rulebook, defined by a committee or a methodology document. And rules embody choices.

Consider some real ones:

Choice one: which companies are included?

A "US total market" index must answer: how small must a company be before it is excluded? The United States has thousands of listed companies, many tiny and barely traded.

Different providers answer differently, so two funds both called "total market" can differ by thousands of holdings.

Choice two: how are they weighted?

The single most consequential choice.

Weighting	Meaning	Consequence
Market-cap weighted	Bigger company, bigger weight	Most common. You automatically hold more of what has risen
Equal weighted	Every company weighted the same	Tilts toward small companies; requires frequent rebalancing
Fundamentally weighted	Weighted by earnings, book value, etc.	An active decision dressed as an index

Market-cap weighting has a property worth understanding: it is the only weighting that requires no trading to maintain.

If a company's share price doubles, its weight in a cap-weighted index rises automatically and the fund buys nothing.

An equal-weighted index must sell what rose and buy what fell to stay equal-weighted — generating trading costs and taxable events.

That is the structural reason cap-weighted index funds cost so little, not merely that they are "simple."

Choice three: when do constituents change?

Indices periodically add and remove companies. And the timing of those changes is public.

Which creates a real cost: when every index fund must buy the same newly added stock on the same day, that stock's price rises beforehand. The funds buy at the higher price.

This is the "index rebalancing effect," a widely studied phenomenon. Our article "Index Rebalancing" discusses its trading side.

A Consequence of Cap-Weighting That Must Be Understood

Worth separate treatment, because it is frequently misread as a defect.

In a cap-weighted index, your holdings automatically concentrate in the best performers.

When a handful of companies become enormous, they occupy a large share of the index.

This is often criticized as "the index is no longer diversified."

A balanced answer is required:

Where the criticism is right: concentration genuinely has risen, and that means your portfolio is more sensitive to the fate of a few companies. That is a real change in risk.

Where it is wrong: cap weighting reflects the market's actual composition. If you do not hold cap weights, you are making an active decision — claiming you know the correct weights better than the market does.

A Random Walk Chapter 3 addresses this objection ("indexes have become highly concentrated"). What this chapter adds is the mechanical explanation: concentration is not a design flaw but the inevitable result of making no active decision.

And the real response is not abandoning cap weighting but diversifying globally — because different countries' concentrations do not fall on the same companies.

"Smart Beta": A Boundary Worth Stating

Recent years have produced many products described as indices that in fact embed active choices.

They are usually called "smart beta" or "factor indices."

This chapter's framework gives a clean test:

If an index's rules embed a judgment about which stocks are better, it is an active strategy expressed in rule form.

That does not make it bad. But it means:

• It should be judged by active-strategy standards (performance versus the index, tracking error, fees).
• It does not enjoy the broad index's core advantage — that nobody has to be right.

What Works on Wall Street covers those factors (value, momentum, size) at book length. This chapter does not repeat it; it only marks the boundary: those are strategies, not the market.

Executable Trading Rules

1. Read the index methodology document for your core holding, once. The chapter's most practical line. Every major index provider publishes it. Look for three things: inclusion criteria, weighting method, rebalancing frequency. Fifteen minutes, once in your life, and it permanently changes your understanding of what you own.

2. Check how many companies your "total market" fund actually holds. The number is published on the fund page. If two funds both called total market hold very different counts, they are not the same thing.

3. Prefer a cap-weighted broad index as your core. Not because it performs better, but because it is the only approach requiring nobody to be right while structurally minimizing maintenance cost.

4. Reclassify "smart beta" as an active strategy. Then require it to justify itself by active standards (Chapter 4 gives the fee and tracking-error checks).

5. Address concentration with global diversification, not by abandoning cap weighting. The correct response to the concentration critique, and it requires no judgment about which company is overvalued.

Relevance to a Retirement Portfolio

This chapter's practical meaning for retirees is concrete.

Your retirement core is most likely a broad index fund. And this chapter says: that fund's character is determined by a rulebook you have never read.

The application:

When choosing between two apparently identical funds — say two "US total market" funds — the difference is usually not in the name but in the methodology:

Check	Why it matters
Which index it tracks	Determines inclusion scope and weighting
Number of holdings	Thousands versus hundreds is a different diversification
Rebalancing frequency	Affects trading costs and tax efficiency
Expense ratio	The only certain variable (Customers' Yachts ch03)

Those four together usually matter far more than which brand it is.

Our standard position bears restating: this chapter is not advice to switch funds frequently. Switching creates taxable events (Poor Charlie's Almanack ch05: do not interrupt compounding).

It is advice that when you make the choice you may make only once or twice in a lifetime, you know what you are choosing.

Chapter 2 covers the container's next layer: how a fund actually goes about holding those stocks, and why it cannot do so perfectly.