The S&P 500: clear and present danger for investors in US stocks

The S&P 500 is often hailed as a broad barometer of American corporate health, a window into the fortunes of the world’s largest economy. Yet beneath its reassuring name lies a structural flaw that is becoming increasingly difficult to ignore: an alarming concentration of market power in a handful of companies.

The upper echelons of the index are now dominated by a select few tech giants whose combined weight exerts disproportionate influence. As of mid-2025, just ten companies account for over 35% of the index’s value. This imbalance introduces a clear and present danger. When performance is so heavily skewed towards a narrow cluster of firms, even minor missteps from one of these market leaders can send ripples across the entire index.

This is not the first time market concentration has triggered concern. Periods of heightened reliance on a few dominant firms have historically preceded bouts of underperformance for the wider index. Notably, such episodes have often marked inflection points, followed by rebalancing, during which smaller and mid-sized companies tended to outperform their behemoth counterparts.

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The present cycle is no exception. As investor enthusiasm fuels ever-higher valuations in megacap stocks, a growing cohort of market participants is exploring ways to counteract the risks of overconcentration. There are several strategies to consider.

Global funds can help

Diversifying beyond the US is a natural first step. Global equity funds still lean heavily towards American firms (roughly two-thirds of global indices are comprised of US stocks) but they dilute the dominance of the S&P 500’s top-tier names. Exposure to developed and emerging markets introduces different economic drivers, currencies and sector dynamics, all of which help to spread risk.

Domestically, US equal-weighted indices offer an intriguing alternative. Unlike traditional market-cap indices, in which the largest firms exert the greatest influence, equal-weight indices assign identical weights to each constituent.

The effect is a reallocation of investor attention: from a handful of titans to the collective strength of the market’s long tail. Historically, such indices have produced stronger long-term returns,albeit with greater volatility.

Another route lies in so-called fundamentally weighted indices. These “smart beta” strategies select and weight stocks based on underlying metrics like earnings, revenue, or book value, rather than investor sentiment or momentum. The result is a portfolio more rooted in economic reality, with a bias toward undervalued or overlooked firms. These indices can act as a ballast in frothy markets where hype often overshadows substance.

The advantage of capped indices

A more nuanced solution comes via capped indices, which impose limits on how much any single stock can represent. When a stock grows too large, it is reweighted to prevent it from dominating. This does not eliminate exposure to market leaders, but it reduces the potential shock if one falters. For those wary of abandoning the S&P 500’s traditional constituents but unwilling to accept its risks wholesale, capped indices offer a compromise.

Each of these strategies carries trade-offs. Fees are often higher, and liquidity may be thinner for niche products. Moreover, diversification is no panacea: it may reduce risk, but not eliminate it. Still, the real risk lies in complacency. In an era where passive investing has become the norm, many portfolios are more exposed to concentrated risk than investors realise.

What is clear is that the S&P 500, while historically reliable, is no longer as diversified as its name implies. As a small cadre of companies exerts ever-greater influence over index performance, the case for exploring alternatives becomes more urgent. Investors may do well to look beyond the usual suspects, before market gravity does it for them.

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