Investors who have spent years in the A-share and Hong Kong stock markets have almost certainly heard the name "S&P 500." It is both a bellwether of global capital markets and one of the most intuitive answers to the question "Why buy U.S. stocks?" Buffett left behind that widely circulated instruction—advising his family to allocate the vast majority of his estate to S&P 500 index funds—and what lies behind it is precisely the index's unique and powerful underlying design logic: it is not a static list of stocks, but a continuously running, self-evolving "core asset screening machine." Understanding how this machine works means understanding the true source of the miracle of long-term compounding in U.S. stocks.
Not a "Stock Pool," but a "Screening Machine"
Many investors' first impression of the S&P 500 is "the 500 largest companies in America," but this understanding is incomplete. As a committee-governed, market-cap-weighted index, the S&P 500's constituent selection follows a whole set of rigorous and transparent rules, the most fundamental hard constraint among them being profitability.
Entry Threshold: Profitability Is a Hard Requirement
In addition to conventional metrics such as market capitalization, liquidity, and public float, a company must maintain positive cumulative net profit over the most recent full fiscal year as well as the most recent quarter to be eligible for inclusion in the index. This means that even a star company with a massive market cap and plenty of buzz will find it hard to knock on the S&P 500's door if it keeps posting losses on its books. Unlike certain index-construction philosophies where "bigger is king," the S&P 500 has insisted from the very beginning that its constituents must be "profitable companies." This seemingly simple rule has built the first moat around the index's long-term quality.
Dynamic Turnover: The Elimination Mechanism Never Stops
The S&P Dow Jones Indices committee regularly evaluates constituent performance, removing companies whose market caps have shrunk, whose earnings have deteriorated, which have been acquired, or which have long failed to meet the standards, while admitting high-quality enterprises better aligned with the new economy. This "out with the old, in with the new" mechanism keeps the index consistently representative of core U.S. assets. Looking back over the past few decades, countless former industry giants have faded away due to complacency, while many newcomers have taken the stage through index rebalancing—all of it happening silently. Without any active action on the investor's part, one automatically keeps pace with the structural shifts of the American economy; this is precisely the most underrated value of index investing.
A Condensed History of American Economic Evolution
Open up the S&P 500's constituent list, and what you see is in fact a condensed history of American industrial change. From the early days of railroads, steel, and oil, through the middle era of automobiles, finance, and consumer goods, to today, when the information technology sector has long held the top weighting, the index's sector structure has always beaten in step with the pulse of the American economy.
The Shifting Sector Weights Across Eras
<A defining feature of the US stock market is its "winner takes all" nature. Leading companies, backed by technological moats, economies of scale, and global footprints, tend to keep widening their competitive advantage, with earnings growth significantly outpacing the industry average. The S&P 500 captures this leadership effect in full: buying the index is equivalent to holding the industry-leading companies across US sectors in proportion to their market capitalization, naturally sharing in the leadership premium and the fruits of globalized operations.
Why Index Investing Wins: Three Underlying Principles
1. Putting Survivorship Bias to Positive Use
The biggest risk of active stock picking is holding on to declining companies for the long term. By periodically removing losers and adding winners, the S&P 500 turns the "survivorship bias" at the individual stock level into a structural advantage for index investors. Buying the index is essentially buying a self-updating asset machine—bad companies are moved out, good ones are brought in—so investors need no pinpoint judgment, only long-term commitment.
2. Extremely Low Costs and Friction
Index fund fees are far lower than those of actively managed products, and
In recent years, the weight of leading technology companies in the index has kept climbing, and the S&P 500's performance has become deeply tied to a handful of tech giants. This means that if a core narrative such as artificial intelligence were ever to be disproven, the index's potential for drawdown would be far from negligible. A prudent approach is to balance it with an appropriate allocation to equal-weight indices or other regional assets, reducing exposure to any single narrative and strengthening the portfolio's resilience.
Real-World Frictions: Currency and Taxes
For investors whose assets are denominated in Chinese yuan, fluctuations in the yuan-dollar exchange rate can amplify or erode part of the returns, so this should be factored into expectations when allocating assets. At the same time, the U.S. dividend withholding tax on non-resident investors, along with the tightening or loosening of quotas on cross-border investment channels, are practical factors that must be taken into account when drawing up a plan. Only by clearly accounting for these friction costs can one set reasonable expectations for real returns.
Conclusion: Mechanisms Are More Reliable Than Predictions
"Why buy U.S. stocks?" The S&P 500 offers the simplest and most compelling answer: rather than betting on whether any one company can remain great forever, invest in a mechanism that continuously filters out great companies. For investors pursuing global asset allocation, understanding the survival-of-the-fittest rules that govern the S&P 500's constituents amounts to holding the first key to unlocking the long-term compounding miracle of U.S. equities. Index first, individual stocks later; core holdings first, satellite positions later—this may well be the most prudent path, and the one most aligned with long-termism, for ordinary investors entering the U.S. stock market.
