Why is one stock a worse bet than its average suggests?

The stock market's long-run average return is a familiar number. What that number hides is how it was produced — and the how is the strongest argument for portfolios that exists.

An average made by a handful of winners

Hendrik Bessembinder studied roughly 26,000 US-listed common stocks from 1926 to 2016 and found a distribution most investors would not guess:

• About 58% of those stocks delivered a lifetime return below one-month Treasury bills — the closest thing to a risk-free parking spot.
• The best-performing 4% of firms accounted for the entire net wealth the stock market created above Treasury bills across that ninety-year span.
• The most frequently occurring lifetime outcome for an individual stock, once returns are rounded, was a loss of essentially everything.

Read those three lines together and a strange picture appears: the market's healthy average return is real, and it was generated by a small minority of names sitting inside a majority that did not beat cash.

Why that matters for a single pick

The average is a property of the whole, not of a randomly chosen member. Buy one stock and you don't get the average — you get a draw from a wildly skewed distribution whose typical outcome sits far below its mean. Buy the whole basket and you get the average by construction, because the basket is the thing being averaged.

That's the asymmetry. To earn the market's long-run return with one pick, you must land in the thin right tail. To earn it with a broad basket, you must merely be present.

A rounded illustration

Imagine a simplified market of 100 companies over a decade. Ninety-six of them collectively go nowhere — some double, more halve, they roughly cancel. Four of them go up twenty-fold. The market's average is excellent. A single random pick has a 96% chance of a mediocre-to-bad outcome and a 4% chance of the result everyone quotes. Own all 100 and the four winners are inside the basket automatically — they cannot be missed, because nothing was excluded.

The numbers here are invented for clarity; the shape is not. Extreme skew is one of the most consistently documented features of equity markets.

The honest counterweight

None of this says a concentrated holding is wrong, or that anyone should hold one thing rather than another — that is not a call this course makes. It says something narrower and firmer: a single position's expected outcome and its most likely outcome are different numbers, and the gap between them is far larger for one stock than for a broad basket. That is arithmetic, not opinion, and it is the reason the rest of this course exists.

In the data

The left tail is easy to lose because it stops trading. Lehman Brothers, once the fourth-largest US investment bank, near its peak and on its last day as a listed share:

A list of the companies listed today does not contain it, or any of the thousands like it. Build a distribution of single-stock outcomes from today's listings alone and every company that went to zero is absent from it — the skew you measure is the skew of the survivors. The delisted names sit on a separate list, and it is longer than the list of the living.

Try it now

1. The chart below is one company's whole listed history, Apple's — a member of the thin right tail described above. Note where it sits today against its own worst stretch.

2. Now notice what cannot be charted. No equivalent line exists for most of the 58% that trailed Treasury bills, because they stopped trading. The two US lists, counted on 28 September 2026, are below. Compare the common-stock column — the gap is the part missing from every "average stock" figure built from the companies still listed.

3. Write one neutral sentence distinguishing "the average stock" from "the average of stocks." They are not the same object.