Five Companies, 30% of S&P 500: How Market Giants Pulled Further Ahead in a Decade

The researchers examined 554 distinct companies across the decade, producing 4,443 company-year observations, with usable data for between 391 and 488 companies annually.

Owning a slice of the S&P 500 can feel like spreading your money across hundreds of businesses, though the growing weight of its biggest companies tells a more complicated story about how much influence a handful of names can carry.

The study 'Firms Diverged Faster than Households: Market-Capitalization Concentration in the S&P 500,' published in the Journal of Risk and Financial Management, examines how that balance changed between 2016 and 2025. Researchers Sarthak Pattnaik, Chhayank Jain and Eugene Pinsky found that the five largest companies' share of the index's measured market value more than doubled, with digital businesses playing a substantial role in the widening gap.

1. A Bigger Share of the Market Ended Up in Fewer Hands

The five largest companies accounted for 13.4% of measured market capitalisation in 2016 and 30.0% in 2025, meaning roughly three dollars in every ten of the sample's stock-market value belonged to just five businesses by the end of the period. The ten largest companies expanded their share from 21.9% to 41.2%. Market capitalisation measures a company's stock-market value, calculated from its share price and outstanding shares; it does not measure revenue, profits or the number of customers a business serves.

The researchers examined 554 distinct companies across the decade, producing 4,443 company-year observations, with usable data for between 391 and 488 companies annually. Their combined measured market value grew from $18 trillion to $61.3 trillion.

The Gini coefficient, which rises as a distribution becomes more unequal, climbed from 0.586 to 0.719. A separate concentration measure translated the index's weights into the equivalent of approximately 112 equally weighted holdings in 2016 and 43 in 2025. The upward pattern included a clear interruption in 2022, when the largest technology companies lost more value than the broader index and concentration dropped before climbing again.

The household comparison gives the change some perspective: the wealthiest 1% of US households increased their share of national net worth from about 30.7% to 31.4%, a gain of 0.65 percentage points using unrounded figures. The five largest companies gained approximately 16.7 percentage points of measured index value.

Across comparisons using the largest five, ten or twenty-five companies, corporate concentration moved 24 to 30 times as far as household wealth concentration. These figures describe changes in different populations and different kinds of wealth; they cannot establish that companies and households became equally unequal or that one change caused the other.

2. Digital Businesses Took More of the Value

Digital companies captured a growing share of the value among the largest businesses, increasing their portion of the study's estimated upper group from 43% to nearly 69%. Their share of the number of companies in that group barely changed, suggesting the shift came largely from those businesses becoming more valuable.

The researchers grouped information technology and communication services companies with nine additional platform businesses, including Amazon and Tesla. Two measures that separate inequality into components attributed 32% and 46% of the decade's increase to the widening average-size gap between digital and non-digital companies.

Differences among companies inside those two groups accounted for the remainder, making the story broader than technology businesses simply pulling away from everyone else. The definition of "digital" affected the size of the result. Restricting the category to the two industry sectors reduced the gap's contribution to roughly 23% and 32%, preserving a substantial digital connection.

Low reproduction costs, network effects and scalable intangible assets offer possible explanations for digital businesses' growing weight. Acquisitions of emerging competitors could produce similar patterns. The study measures the outcome without establishing which mechanism caused it.

3. Hundreds of Holdings Can Still Depend Heavily on a Few Names

The largest companies became more influential in the index's fluctuations, with the top five's contribution to measured return variance rising from 15.7% to 38.0%. The top ten contributed 53.4% by 2025, accounting for more than half of this measure of overall movement.

A portfolio's number of holdings tells only part of its diversification story because individual stocks have different levels of volatility and do not move together in the same way. The drop from 112 to 43 equivalent equally weighted holdings describes concentration in the weights. It does not mean investors lost more than half their diversification benefits.

The study's diversification ratio, which incorporates stock volatility and how returns move together, changed much less: from 1.75 in 2016 to 1.69 in 2025, with considerable variation between those years. For fund managers using equal weights or limits on individual holdings, the growing influence of the largest companies makes their exposure increasingly different from the capitalisation-weighted benchmark. The paper does not calculate the resulting tracking risk for any particular investment strategy.

4. The Findings Survived Checks, but the Explanation Remains Open

The researchers reconstructed each year's actual index membership, reconciled share prices and share counts for stock splits, and combined multiple share classes into single company observations. Using today's members to represent earlier years would favour businesses that survived and could create a misleading concentration trend.

Missing historical records remained a limitation, with coverage increasing from roughly 81% of the 2016 roster to 97% in 2025. Tests that assigned plausible values to missing companies, examined a consistent set of firms and changed statistical assumptions preserved the main concentration finding. Estimates of the Gini increase were more sensitive, and attempts to correct its statistical bias depended on the assumed distribution.

The researchers could not reliably identify a precise mathematical rule describing the largest companies' sizes. Two established approaches disagreed about the trend, and a proposed power-law model failed a fit test in seven of ten years. The evidence supports a market containing exceptionally large companies without establishing a dependable formula for that pattern.

The sector analysis excluded 414 company-year observations without industry labels. Ten annual observations offer a limited historical window, and the S&P 500 represents a selected group of large listed companies rather than the entire American economy.

Artificial intelligence raises a fresh question for future research because serving AI systems requires computing resources for every query. Control of scarce chips and infrastructure could concentrate value further; usage costs and wider access to productive tools could change the pattern in other directions.