Why the S&P 500 index can mask deteriorating health
The S&P 500 is market-cap weighted, meaning the 10 largest stocks control roughly 30% of the index. When those mega-caps rally on AI excitement while the median stock declines, the index looks strong on the surface but the underlying market is weak. Breadth indicators measure this hidden divergence by counting how many individual stocks are making new highs or trading above their 200-day average. If only 20% of stocks beat the 200-day average while the index hits a new high, that's a red flag.
The advance-decline line is the simplest breadth tool: it rises when more stocks go up than down in a day, and falls vice versa. Over months, a declining advance-decline line paired with a rising index signals the rally is fragile and built on a shrinking number of mega-caps.
Using breadth to forecast reversals
Market tops often form with breadth divergence: the index is near all-time highs but breadth indicators are rolling over weeks before the actual correction. The converse happens at bottoms, where the index is near lows but breadth meters are improving, signalling a reversal is imminent. Traders use breadth as an early warning system rather than a timing tool by itself.
Extreme breadth readings also matter. If 90% of stocks are trading above their 200-day average, the market is extended and vulnerable to mean reversion. If only 10% are, the market is oversold and due for a bounce. These are contrarian indicators, not trend followers.