S&P 500: You're Less Diversified Than You Think, These ETFs Prove It

Published in Stocks and Investing on Sunday, August 24th 2025 at 18:51 GMT by Seeking Alpha
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SP 500: How “Diversified” Your ETF Is Really?

When most investors hear “S&P 500,” they immediately picture an automatically diversified basket of 500 blue‑chip companies. The reality, however, is far less balanced. A recent in‑depth article on Seeking Alpha—titled “SP 500: You Are Less Diversified Than You Think. These ETFs Prove It” (https://seekingalpha.com/article/4816324-sp500-you-are-less-diversified-than-you-think-these-etfs-prove-it)—demonstrates that the most popular index funds, such as SPY, VOO, and IVV, are surprisingly concentrated in a handful of tech giants. Below is a comprehensive, paraphrased recap of the key findings, along with supplementary information from the linked resources the author cites.

The Core Problem: Concentration in “Diversified” Index Funds

Concentrated Ownership of the Largest Companies
The article opens with a striking statistic: the top 20 holdings of the S&P 500 account for roughly 30 % of the index’s total market capitalization. Even more eye‑opening is that the top 10 hold around 20 %. When you hold an ETF that tracks the index, you inherit this concentration. In practice, that means most of your portfolio is exposed to the performance of a handful of large, often technology‑focused firms.
Dominance of the Tech Sector
The author points out that the tech sector alone represents more than 25 % of the index’s weighting, and its nine biggest companies—Apple, Microsoft, Amazon, Alphabet, Meta, Nvidia, Tesla, Adobe, and Salesforce—together hold about 15 % of the total market cap. This sector concentration is reflected in the ETFs themselves: SPY, VOO, and IVV all show similar tech‑heavy weightings.
ETF‑Level Concentration Ratios
By examining the holdings disclosed by the ETFs (available via the “Holdings” tab on each fund’s website), the article shows that the top 10 holdings in SPY account for over 17 % of the fund’s net asset value (NAV). VOO’s top 10 are slightly higher, around 18 %, while IVV is similar. Even the “top 25” group in each ETF captures roughly 30 % of the NAV.
Implication for Individual Investors
A concentrated portfolio means that an adverse event—such as a product failure, regulatory setback, or earnings miss—in one of these big names can disproportionately affect your returns. In contrast, a truly diversified portfolio would spread risk more evenly across a wider swath of issuers and sectors.

The Data Sources and Tools the Article Uses

ETF Holdings Pages
The author references the publicly available holdings lists on the fund provider’s website. For example, the Vanguard S&P 500 ETF (VOO) has a “Holdings” link that lists all 500 constituents with weightings.
Morningstar and Yahoo Finance
These platforms provide both the index and ETF fact sheets, offering sector breakdowns, market‑cap weightings, and performance data. Morningstar’s “Fund Screener” is highlighted as a useful tool for assessing concentration metrics such as the Herfindahl–Hirschman Index (HHI).
Bloomberg’s Sector Data
A quick glance at Bloomberg’s sector‑level data (linked in the article) confirms the dominance of technology, followed by healthcare, consumer discretionary, and communication services.

How Much Concentration Is Acceptable?

The article delves into the debate over how “diversified” an investment truly needs to be. A key point is that a high concentration is not inherently bad if you understand the risks. However, it can be detrimental if you’re unaware of the exposure. The article encourages readers to look at metrics like:

Concentration Ratio (CR) – the sum of the top ten holdings’ weightings.
Herfindahl–Hirschman Index (HHI) – a more sophisticated measure that squares each weight, summing the squares for a number that can range from 0 (perfect diversification) to 1 (single‑holder).

For SPY, VOO, and IVV, the HHI hovers around 0.10–0.12, indicating moderate concentration relative to an ideal, perfectly diversified portfolio (which would have an HHI of approximately 0.002).

Practical Ways to Mitigate Concentration Risk

Choose a Total‑Market ETF
Funds like the Vanguard Total Stock Market ETF (VTI) or the iShares Russell 3000 ETF (IWV) spread holdings across the entire U.S. equity universe, dramatically reducing sector and top‑holder concentration.
Add Sector or Thematic Funds
Instead of relying solely on a large‑cap blend, investors can add mid‑cap or small‑cap funds (e.g., IWM or VTV) or thematic ETFs that focus on specific sectors (e.g., healthcare, clean energy). This layering can help balance the heavy tech tilt.
Use Factor‑Based Funds
Some investors prefer factor ETFs (e.g., value, momentum, quality) which select stocks based on systematic characteristics, potentially smoothing out idiosyncratic risk associated with top names.
Track the Index Yourself
If you’re comfortable with DIY investing, you could create a self‑managed portfolio that mirrors the S&P 500 but includes a cap on any single holding—say, limiting each stock to no more than 2–3 % of the portfolio.

Key Takeaway: Know What You’re Buying

The article’s core message is simple: you’re not as diversified as you think when you invest in the most common S&P 500 ETFs. The concentration in a handful of top names—especially tech—creates an unintentional exposure to a specific group of companies and industries. By proactively assessing the underlying holdings, understanding concentration metrics, and considering diversification strategies, investors can better align their portfolios with their risk tolerance and long‑term goals.

For further detail, the author provides direct links to the ETF fact sheets and holdings pages, allowing readers to audit the portfolios themselves. This transparency empowers investors to make informed decisions, ensuring that the “diversification” they assume is truly reflective of the risk profile they’re comfortable with.

References (via article links)