S&P 500

If you only follow one number in investing, the S&P 500 is probably it. Maintained by S&P Dow Jones Indices, it covers 500 leading US companies weighted by market capitalization and represents roughly 80% of the total US stock market's value.

Getting into the S&P 500 isn't automatic—a committee selects companies based on market cap ($18B+ minimum), profitability, liquidity, and sector representation. Because the index is market-cap weighted, the biggest companies carry the most influence. In recent years, the top 10 stocks (mostly tech) have accounted for over 30% of the entire index.

The S&P 500's track record is the most cited benchmark in all of investing. Since 1957, it has returned ~10.5% annually including dividends—or ~7-8% after inflation. This long-term performance is exactly why index fund pioneer John Bogle spent his career telling people to just buy and hold the S&P 500.

Here's a stat that drives the point home: most actively managed funds compare themselves to the S&P 500, and most of them fail to beat it over long periods. This reality sparked the index investing revolution—why pay a fund manager 1%+ per year when a 0.03% S&P 500 index fund will likely outperform them?

One thing to keep in mind: the S&P 500's heavy tech weighting and US-only focus means it doesn't represent the global economy. A truly diversified portfolio adds international stocks (like MSCI EAFE and emerging market funds) alongside your S&P 500 holdings.