He Thinks His $500,000 S&P 500 401(k) Protects Him From Big Losses. Just 10 Holdings Make Up Nearly 40% of the Fund. A 25% Drop Could Cost Him Almost $49,000

The Full Benefits Desk desk. Editor: Gerelyn Terzo.

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Consider a hypothetical 58-year-old with $500,000 in an S&P 500 index fund inside his 401(k). He figures that owning hundreds of companies protects him from any single collapse. Financial reporting on AI investment concerns puts that belief to the test, and so have the past few weeks of trading.

The Nasdaq has rebounded from its recent lows but still posted another weekly decline in early November. Bloomberg described Goldman Sachs (NYSE:GS | GS Price Prediction) traders taking calls from investors seeking answers to an AI rout. The VIX, Wall Street’s fear gauge, jumped to 28 on November 5 and has since fallen to about 19, suggesting that investors are less worried about near-term market volatility, though the risk hasn’t disappeared.

7 Tips for Surviving a Market Sell-Off

An S&P 500 fund like the Vanguard S&P 500 ETF (NYSEARCA:VOO) weights each company by its market value. Bigger companies get bigger slices. Each new dollar you contribute follows those same weights.

NVIDIA (NASDAQ:NVDA) has a market value of about $4.1 trillion. It takes up a far larger share of the index than hundreds of smaller members. Its beta is about 2.2, a measure of how strongly the stock has historically responded to market movements. When companies like that dominate the top of the fund, the fund’s results depend heavily on how a single theme performs.

Strong gains made the index even more top-heavy. VOO is up about 11% this year and roughly 74% over five years, including reinvested dividends. As the biggest winners grow, they account for more of your balance. As they grow, you never have to place a trade for that to happen.

VOO’s 10 largest holdings accounted for 39.1% of its assets as of September 30, 2026. On a $500,000 balance, that means about $195,500 rides on just 10 holdings.

If those 10 holdings fall 25% and the others stay flat, the account loses about $48,900. That’s nearly 10% of his entire 401(k), from just 10 holdings in a fund that owns hundreds.

The real-world consequences could be worse. In a broad selloff, other holdings may fall alongside the AI giants, including suppliers, utilities serving data centers and other technology leaders.

For someone close to retirement, the timing makes this worse. A 4% withdrawal on his balance is $20,000 a year. The nearly $49,000 loss would equal more than two years of his planned $20,000 annual withdrawals, even before he takes his first distribution. Early market losses do more lasting damage. A bad market in the first years of withdrawals is worse than one a decade in, which is the whole subject of our free guide to defending the first five years.

Real diversification means owning assets that don’t all fall for the same reason. Many 401(k) menus make overlap easy. The U.S. stock portion of a target-date fund usually tracks a broad index led by the same megacaps. A large-cap growth fund often holds them at even heavier weights, and if you pair either one with an S&P 500 fund, your exposure to those top names goes up.

Target-date funds took in 64% of plan contribution dollars at Vanguard in 2024. Many savers combine them with other funds on the menu, which stacks the same exposure on top of itself.

Allocation matters too. Fidelity found that 7% of savers in their 50s have every dollar in stocks. Vanguard’s participants age 65 or older held a median of 50% in equities. Someone with every dollar in one S&P 500 fund is taking more stock risk than most people his age.

Use the 25% scenario as a stress test, then compare it against your own balance and fund weights, and look up your actual numbers before responding to the next AI trade.

Contact [email protected] for any questions or corrections.

Gerelyn Terzo is the author of dividend investing handbook "Dividend Investing Strategies: How to Have Your Cake & Eat It Too." A veteran financial journalist, she covers agri-finance for outlets like Global AgInvesting and the broader stock market and personal finance for 24/7 Wall Street. She began at CNBC and later helped launch Fox Business in New York. Gerelyn currently resides in Woodland Park, Colorado and dabbles in nature photography as a hobby.

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