The stock market is one of the most consequential institutions in modern economic life — holding the retirement savings of hundreds of millions of Americans, driving capital allocation across the economy, and generating returns that have compounded wealth for long-term investors over decades. It’s also widely misunderstood by the people whose financial futures depend on it. Most investors have a vague sense of what the market is without a clear understanding of how it actually functions — how prices are determined, what causes them to move, and what the historical return data actually tells us about reasonable expectations. This article provides that understanding.
What a Stock Actually Is
A share of stock is a fractional ownership interest in a company. When a company sells shares to the public through an initial public offering (IPO) or subsequent offerings, it’s selling fractional ownership stakes in the business and raising capital that it can use for operations, investment, or debt repayment. As a shareholder, you own a proportional claim on the company’s assets and future earnings. If a company has 100 million shares outstanding and you own 1,000 shares, you own 0.001% of the company — a tiny fraction, but a genuine ownership interest that entitles you to your proportional share of dividends declared, assets in a liquidation, and votes on shareholder matters.
The value of that ownership interest is ultimately derived from the present value of the company’s future earnings stream. A company expected to generate large and growing profits for decades is worth more per share than a company with flat or declining earnings, because shareholders’ proportional claim on the future profit is more valuable. This earnings-based valuation is the theoretical foundation for stock prices — and the reason stocks in profitable, growing businesses have historically generated positive long-term returns, because the underlying businesses were generating real economic value that accrued to their owners.
How Stock Prices Are Set
Stock prices are set continuously throughout each trading day by the interaction of buyers and sellers on exchanges — primarily the New York Stock Exchange and NASDAQ for US stocks. At any moment, the price of a stock is the most recent price at which a willing buyer and a willing seller agreed to transact. The bid price is what buyers are currently offering to pay; the ask price is what sellers are currently willing to accept. When a buyer agrees to pay the ask price (or a seller agrees to accept the bid), a transaction executes and that price becomes the new market price.
This continuous auction mechanism means stock prices reflect the collective real-time opinion of all market participants about what the stock is worth — incorporating all publicly available information about the company, the industry, the economy, and investor sentiment. The efficient market hypothesis, developed by Eugene Fama (who won the Nobel Prize for the work), holds that prices in liquid public markets like the US stock market rapidly incorporate all available public information, making it very difficult for any individual investor to consistently identify and profit from mispricing. This is why passive index investing — accepting the market’s collective price rather than trying to beat it — has proven so persistently effective: it acknowledges that the market’s aggregate judgment is difficult for individuals to reliably improve upon.
What Actually Moves Stock Prices
Stock prices move when new information causes investors to revise their estimates of a company’s future earnings. Earnings reports that exceed expectations drive prices up; earnings that disappoint drive them down. Changes in interest rates affect stock valuations because they change the discount rate used to calculate the present value of future earnings — higher rates make future earnings worth less today, which reduces stock valuations broadly. Economic data that affects expected corporate earnings — employment, consumer spending, manufacturing activity — moves markets in proportion to how much it changes earnings expectations. Geopolitical events, regulatory changes, technological disruptions, and leadership changes at major companies all move prices to the extent that they affect expected future earnings.
In the short run, sentiment and psychology influence prices significantly beyond what earnings fundamentals would justify. Fear drives prices below fundamental value; euphoria drives them above. Market bubbles — periods of extreme overvaluation driven by collective excitement about a new technology, asset class, or economic narrative — and market crashes — periods of extreme undervaluation driven by fear and forced selling — both represent temporary departures from fundamental value that eventually correct. In the long run, stock prices track corporate earnings with reasonable fidelity, and corporate earnings track economic output. This is why long-run stock market returns have historically been positive — the economy and corporate earnings have grown over time, and stock prices have followed.
Market Indices: What They Actually Measure
When news reports say “the market was up 1.2% today,” they’re typically referring to a stock market index — a composite measure of the performance of a defined group of stocks. The S&P 500 tracks the 500 largest US public companies by market capitalisation, weighted by each company’s market cap (so larger companies influence the index more). The Dow Jones Industrial Average tracks just 30 large companies using a price-weighted methodology, which makes it less representative than the S&P 500 but more widely cited historically. The NASDAQ Composite includes all stocks listed on the NASDAQ exchange and is heavily weighted toward technology companies.
Index funds that track these benchmarks — like the Vanguard S&P 500 ETF or Fidelity’s FXAIX — give individual investors proportional exposure to all the companies in the index at extremely low cost, generating returns that match the index’s performance minus a tiny expense ratio. The long-run performance of these indices provides the empirical basis for stock market investing as a wealth-building strategy: the S&P 500 has generated total returns (including dividends) of approximately 10% per year on average since its creation in 1957, with significant variation from year to year but consistent positive returns over any 20-year period in its history.
Why Markets Decline and Why That’s Normal
Market declines — called corrections when they’re 10% to 20% from a recent peak, and bear markets when they exceed 20% — are a normal and regular feature of stock market history, not anomalies. Since 1950, the S&P 500 has experienced roughly 38 corrections of 10% or more. Bear markets occur roughly every three to five years on average. The market declined more than 50% in both the 2000 to 2002 tech bust and the 2008 to 2009 financial crisis. It fell 34% in five weeks in early 2020 before recovering to new highs within months.
These declines are not failures of the market system — they’re the mechanism by which markets reprice risk and adjust valuations when economic reality diverges from prior expectations. The investor who stays invested through these inevitable declines participates in the recovery; the investor who sells during a downturn locks in losses and typically misses the recovery that follows. This is the core behavioural challenge of stock market investing: the volatility that produces long-run returns also produces short-run losses that test investors’ resolve. Understanding that declines are normal, expected, and temporary on a long enough time horizon is the foundational knowledge that enables investors to stay the course when staying the course feels psychologically most difficult.
What the Historical Returns Actually Tell Us
The S&P 500’s long-run average annual return of approximately 10% nominal (7% after inflation) is widely cited as the basis for long-term financial planning. A few caveats are worth understanding. This is an average over long periods — individual years vary from roughly negative 40% to positive 50%, and decade-long returns can be significantly below or above the long-run average. The US stock market has had an unusually strong historical return compared to international markets, and whether this outperformance continues is uncertain. Returns for any given investor depend critically on the sequence of returns they experience — someone who retires and begins drawing from their portfolio at the start of a major bear market faces significantly worse outcomes than the average return implies, even with identical average annual returns. These caveats don’t undermine the case for long-term stock market investing — they contextualise it, ensuring that investors hold realistic expectations rather than assuming the average will arrive smoothly and predictably.
Investing in International Markets
The US stock market is large — representing roughly 60% of global market capitalisation — but it’s not the whole market, and whether to invest exclusively in US stocks or to diversify internationally is a genuinely debated question in portfolio construction. The case for international diversification is that it reduces concentration in a single country’s economic and political trajectory and provides exposure to growth in economies that may outperform the US over specific future periods. The case against heavy international allocation is the US market’s historical outperformance relative to international markets over recent decades, the currency risk inherent in foreign investments, and the global revenue diversification of many large US multinationals. Most major financial economists recommend some international allocation — typically 20% to 40% of the equity portion — as a diversification measure, while acknowledging that the optimal allocation is uncertain and reasonable people disagree. The practical implementation through a total international stock index fund (like Vanguard’s VXUS or equivalent) is straightforward and low-cost, making the diversification decision primarily about allocation percentage rather than execution complexity.
