Diversification is the only "free lunch" in investing — it reduces risk without necessarily reducing returns. By spreading your money across different stocks, bonds, sectors, and countries, you ensure that a loss in one investment is offset by gains in others. When tech stocks crash, your bond holdings or gold may rise.
Harry Markowitz won the Nobel Prize in 1990 for Modern Portfolio Theory, which proved mathematically that diversification reduces portfolio risk. The key insight: what matters is not individual asset risk, but how assets move relative to each other (correlation). Combining assets with low or negative correlation creates a portfolio that is less risky than any single investment.
The ideal diversified portfolio includes 20-30 stocks across different sectors to eliminate company-specific risk. Adding international stocks, bonds, real estate, and commodities further reduces risk. However, over-diversification ("diworsification") dilutes returns. Warren Buffett argues that diversification is for people who do not know what they are doing — concentrated bets work better if you truly understand the business.