How Much Do You Really Know About Stock Market Indexes?

The Dow, S&P 500, and Nasdaq Composite have dominated headlines this year, with point swings often used to underscore the volatility that’s made many investors uneasy.

While these indexes are widely recognized as shorthand for the markets and the broader economy, their backstories are far less understood. How they were built, how they’ve evolved, and what they actually measure today may surprise you.

A closer look at how these indexes have evolved over time can offer a more nuanced perspective that helps contextualize market moves and supports long-term strategy.

Where It All Started: The Dow Jones Industrial Average

The concept of a stock market index began in 1896 with Charles Dow and Edward Jones. Their first Dow Jones Industrial Average included just 12 companies, reflecting major industries of the time like agriculture, coal, and steel.¹

Today’s Dow tracks 30 large companies, but the lineup looks completely different. The index has evolved alongside the U.S. economy, shifting from heavy industry to technology, healthcare, and services.¹

Some facts about the Dow you might find interesting:

• The original calculation simply added up the stock prices of 12 companies and divided them by 12.
  
  
• General Electric remained part of the Dow for over 120 years before being replaced in 2018.
  
  
• The Dow is still "price-weighted," meaning a company’s stock price, not its total size, determines its influence on the index.

More Than Just Points: Why Percentage Moves Matter

Headlines often highlight moves like “The Dow drops 500 points,” but point changes alone can be misleading. When the Dow was at 10,000, a 500-point drop meant a 5% decline. With the Dow at around 40,000 today, the same 500-point drop represents barely over a 1% move.

The percentage change, not the number of points, provides a better perspective on market movements. Keeping this in mind can help cut through the noise of daily headlines.

The S&P 500: A Broader View

While the Dow remains iconic, many investors and economists view the S&P 500 as a better snapshot of the broader U.S. market. Updated to 500 stocks in 1957, the S&P 500 includes the largest U.S. companies across 11 sectors, from technology and healthcare to financials. Despite its name, it currently tracks 503 stocks due to multiple share classes.²

A few notable points:

• It is market-cap weighted, meaning that larger companies have a greater impact on performance.
  
  
• Five companies from the original 1957 lineup remain today: General Electric, AT&T, ExxonMobil, Coca-Cola, and Procter & Gamble.
  
  
• The S&P 500 is sometimes viewed not just as a stock market measure but as a broader economic indicator.

The Nasdaq Composite: Tech-Heavy and Fast-Moving

The Nasdaq Composite launched in 1971 as the first electronic stock exchange, quickly becoming the preferred home for emerging and technology-focused companies. Today, it tracks more than 3,300 stocks, with technology accounting for nearly 60% of its overall weight.³

Interesting facts about the Nasdaq:

• According to the number of listed companies, it is the largest stock exchange in the U.S.
  
  
• Major companies like Apple, Microsoft, and Nvidia carry significant weight within the index.
  
  
• The Nasdaq is typically more volatile than the Dow or the S&P 500, largely because of its concentration in growth sectors.

Indexes Are Tools, Not Strategies

It's easy to think of indexes as investment strategies, especially when people say, “Just buy the S&P 500.” But an index is just a snapshot. It doesn't adjust for your risk tolerance, goals, or timeline.

That’s why it’s important to build a strategy that’s aligned with your needs rather than just following the market.

The bottom line is that stock market indexes like the Dow, S&P 500, and Nasdaq help us track performance and trends, but they do not provide the whole picture. They give us reference points, but your investment strategy should always be personal to you.

^Source: 

^{1.  Britannica Money, March 22, 2025}

^{2.  Business Insider, March 27, 2025}

^{3. Business Insider, July 18, 2024}