Diversification — the Only Free Lunch

The math

A portfolio's risk is NOT the average of its parts: combining assets that do NOT move together cancels some volatility — the portfolio can be lower-risk than any single holding while keeping the return. Markowitz's insight (modern portfolio theory): what matters is each asset's contribution to PORTFOLIO risk, not its standalone risk. This is the only place in finance where you get something (risk reduction) for nothing (no expected-return sacrifice).

The correlation engine

Correlation (−1 to +1) measures co-movement. At +1, adding assets reduces nothing (ten tech stocks ≈ one tech stock, day 32). At 0, risk falls with the square root of holdings. At negative correlation, risk collapses fastest. The catch: correlations RISE in crises — everything correlates to 1 in a panic, so diversification works best on ordinary days. It reduces ordinary risk, not catastrophe risk (that is what position sizing and cash are for).

What real diversification needs

Different DRIVERS, not different names: mix asset classes (stocks/bonds/real assets), geographies, and return mechanisms. Five semiconductor makers share one driver (chip demand); a stock-bond-REIT-gold mix has four. The question for every holding: "what drives this, and what else in the portfolio shares that driver?"

Sources

• Modern Portfolio TheoryInvestopedia
• The Little Book of Common Sense InvestingJohn Bogle