How a few megacaps mask breadth

The previous lesson said the index and the average stock can disagree. This lesson shows the arithmetic of why, and why the disagreement has grown as the largest companies have grown.

Concentration by the numbers

In a cap-weighted index of 500 stocks, an equal share would be 0.2% each. In practice the distribution is extremely skewed. At various points in recent years the ten largest companies in the S&P 500 have accounted for roughly 30-35% of the index, and the single largest name for 6-7%. The bottom 250 names together weigh less than the top three.

Here is a stylised index to make the mechanics visible. Five "megacaps" hold 30% of the weight; the other 95 stocks share the remaining 70%.

Ninety-five stocks fell. The index rose. Advancers to decliners: 5 to 95. A trader reading only the index sees a green day; a trader reading the advance-decline data sees a red day with a green headline.

Now flip it:

Both days print a small green index close. One had 5% of stocks rising; the other had 95%. They are not the same day and should not be traded the same way.

What the sketch looks like on a chart

Index (cap-weighted)             Equal-weight index
      /                                 ___
     /                               __/   \
    /                            ___/       \__
___/                          __/              \
                                                \
  new high                    lower high, rolling over

When the two lines diverge like this for weeks, the index is being carried by a shrinking group. It can go on longer than anyone expects, so the divergence is not a sell signal. It is a warning that the index has less support than it looks, and that stock-picking outside the leaders is fighting the tape.

Key idea: A cap-weighted index can rise while most of its members fall, because weight is concentrated in a handful of names. Breadth data exists to expose that gap.

Why concentration has grown

Three forces have pushed weight toward the top:

1. Winners compound. Cap-weighting is momentum by construction: whatever grows gets a bigger share, which attracts index flows, which grows it further.
2. Passive flows. Money into index funds buys every stock in proportion to weight, so the largest names receive the largest bids regardless of their individual prospects.
3. Sector shape. When the largest companies share a sector, a sector story becomes an index story. A rate shock that hits long-duration growth stocks moves the index far more than the same shock would have in an era of more balanced weights.

None of this is a complaint about indexing. It is a description of why the index is a less reliable proxy for "the market" than it used to be.

Three ways to see through the mask

Equal-weight vs cap-weight ratio. Divide the equal-weight index by the cap-weighted one and plot the ratio. Rising means the average stock is outperforming (broad rally); falling means leadership is narrowing. Module 3 covers this in detail.

Advance-decline line. A running total of advancers minus decliners. Every stock counts once, so the megacaps cannot dominate. When price makes a new high and the A/D line does not, the rally is narrow.

Percent of stocks above a moving average. If the index is at highs but only 45% of its members are above their 50-day average, more than half the index is in a short-term downtrend. That is a narrow market by definition.

These thresholds are illustrative starting points, not laws. The lesson on thresholds in Module 2 will say more about that.

What narrow breadth does and does not mean

It does not mean the index must fall. Narrow leadership can persist for many months; the late 1990s and parts of the 2020s are examples where the "narrow market" warning was early by a long way.

What it does mean:

• The index is more fragile: a stumble in a few names moves the whole thing.
• Long setups outside the leaders have a worse base rate.
• Short setups in the weak majority have a better base rate than the index suggests.
• Realised index volatility is often lower than the stress underneath, because the leaders' strength cancels the laggards' weakness in the average. That can unwind quickly.

The practical use is not prediction but selection. On a narrow day you either trade the leaders, trade the weakness, or reduce size. You do not buy a random breakout and expect the "strong market" to carry it.

Try it: Look up the current weight of the top ten stocks in the S&P 500 (index providers and most ETF issuers publish holdings). Then work out what the other 490 stocks must do, on average, to offset a 2% drop in the top ten. Write the number down. That is the mask in one figure.

Recap

• Cap-weighting concentrates influence: a handful of stocks can outweigh hundreds.
• The same small green index close can hide 95% of stocks rising or 95% falling.
• Concentration has grown through compounding, passive flows and sector overlap.
• Equal-weight ratios, A/D lines and percent-above-MA measures each expose narrow leadership.
• Narrow breadth is a fragility warning and a stock-selection filter, not a timing signal.