S&P 500's 9-week rally shows narrowest breadth since dotcom peak

The S&P 500 rose 1.43% to 7,580.06 for a ninth straight weekly gain, but only 20 members hit new highs — matching the dotcom peak's breadth count.

"The speculative price action is likely not over yet, but this narrow breadth is a classic late-cycle signal," said Michael Hartnett, chief investment strategist at Bank of America. "Post-bubble roadmaps since 1929 show it's time to go long bonds and defensives."

Of the 20 stocks that reached records Friday, just seven were not directly tied to artificial intelligence, Hartnett noted. The technology-heavy Nasdaq Composite surged 25% in April and May — its best two-month stretch in more than two decades — driven by semiconductor names. Advanced Micro Devices jumped 50% in May, Micron Technology surged 85%, Samsung Electronics gained 43% and SK Hynix rose 81%. The May rally was concentrated in memory chip makers, with the four companies now valued at or near $1 trillion each. Yet only about 55% of S&P 500 constituents traded above their 200-day moving average as of May 20, according to BCA Research, while advance-decline lines have fallen since mid-April after surging in late March.

The divergence between headline index levels and underlying participation raises the risk of a sharp correction if AI-related momentum stalls. Hartnett is advising clients to rotate into defensive sectors and long-duration bonds, a playbook that has historically outperformed in the 12 months following similar breadth extremes. The Federal Reserve's next rate decision on June 17 will be the next major test for a market priced for perfection.

Breadth Divergence Deepens

A growing number of strategists are flagging the concentration risk. Oppenheimer's Ari Wald wrote in a May 23 analysis that "internals have lagged since the initial April surge." BCA Research strategists led by Arthur Budaghyan warned that "poor breadth is often a sign of underlying stock market vulnerability." BofA reiterated its cautious year-end S&P 500 target even as the index reached new highs, according to a separate note from the bank.

The nine-week winning streak is the S&P 500's longest since a 10-week run that ended in November 2021. But the current advance differs in breadth: the 2021 rally saw broad participation across sectors, while the 2026 version is concentrated in AI-related mega-cap stocks.

Hartnett's post-bubble framework draws on historical precedents dating back to 1929, where the optimal response to narrow, momentum-driven rallies has been to shift into long-duration government bonds and sectors that underperformed during the final months of the bubble. The strategy implies that the current AI-led advance may have limited runway before a regime change in market leadership, with utilities, consumer staples and healthcare among the sectors most likely to benefit from a rotation.

This article is for informational purposes only and does not constitute investment advice.