A surge in passive investing has reshaped how individuals gain exposure to markets. Index funds promise diversification at low cost, yet this promise can become misleading when market concentration rises. The distinction between owning “the market” and owning a broad set of businesses has become increasingly material.
Market-cap-weighted indices allocate more capital to companies as their valuations rise. This creates a feedback loop: the largest firms dominate index returns, and investors tracking those indices become more exposed to them over time. Research from S&P Dow Jones Indices shows that a small subset of companies has recently driven a disproportionate share of returns in major indices like the S&P 500.
This concentration has historical precedent. A study by National Bureau of Economic Research found that a narrow group of “superstar firms” has accounted for an increasing share of profits and market value in recent decades.
The implication is not that index investing is flawed, but that its risks are often misunderstood. Investors may assume they are broadly diversified across sectors and business models, while in reality their returns hinge on the continued success of a handful of firms. This creates hidden exposure to specific economic forces, such as technological disruption or regulatory change, that disproportionately affect those dominant companies.
By contrast, a portfolio deliberately constructed at the business level allows for explicit consideration of valuation, leverage, and capital allocation. It avoids mechanically increasing exposure to companies simply because their prices have risen.
The gap between perceived and actual diversification tends to widen in buoyant markets. It is precisely when returns are strong and narratives compelling that concentration risk is most easily overlooked.
Source: S&P Dow Jones Indices; NBER