The S&P 500 Just Had One of Its Strangest Months in Years. History Says This Usually Doesn't Last.

The S&P 500 fell just 0.5% in September, but market breadth was weak: 78% of constituents declined, 60% fell at least 5%, and the equal-weight index dropped more than 4%. The article says megacap technology leadership has returned amid persistent inflation, higher rates and the Iran war, while AI-related capex may continue supporting earnings growth. It advises against major portfolio changes based on one month and presents an equal-weight ETF as an option to reduce exposure to the largest companies.

Analysis

The key question is whether narrow leadership reflects temporary risk aversion or a durable concentration of earnings growth. In the first case, an equal-weight recovery can follow once macro uncertainty eases; in the second, shifting toward equal weight too early sacrifices exposure to the firms capturing AI-related revenue and returns on capital. Index breadth alone cannot distinguish these regimes. Track earnings-revision breadth and realized returns on AI investment, not just capex announcements: spending is a cost before it becomes customer revenue, and supplier benefits may not accrue evenly.

A single month of divergence is not a timing signal. The nearer-term risk is that macro stress and positioning keep directing flows toward perceived quality, extending cap-weighted leadership. Over 6–18 months, broadening revisions outside the largest constituents would strengthen the case for equal weight; persistent concentration in earnings delivery would weaken it. The historical post-bubble analogy is not a base case: a similar return pattern does not establish similar valuations or fundamentals. The article provides no valuation, positioning, or forward-revision data to resolve that distinction.

Key Decisions for Investors

• Keep a broad cap-weighted core; do not make a wholesale allocation change on one month of relative performance. For portfolios seeking less mega-cap concentration, phase into Invesco S&P 500 Equal Weight ETF (RSP) against Vanguard S&P 500 ETF (VOO), rather than adding an outright market-timing short. Reassess over the next 1–3 months as earnings revisions and breadth data arrive.
• Consider a beta-adjusted long RSP / short VOO relative-value position only as a measured 6–18-month expression of eventual breadth normalization. The payoff is relative, not an assured absolute return; reduce or exit if earnings revisions remain concentrated in the largest constituents or if RSP continues to lag while that concentration persists.
• Monitor quarterly results for evidence that AI spending is translating into monetizable demand and returns, versus rising costs without commensurate revenue. Broadening positive revisions would support adding RSP; a further narrowing of revisions would favor retaining cap-weight exposure.
• Treat easing macro pressure as a potential catalyst, not a forecast. If inflation, rates, or geopolitical risk intensify, quality-seeking flows could prolong narrow leadership; if those pressures recede but breadth and equal-weight relative performance fail to improve, the rotation thesis is not being confirmed.