If your 20 holdings are all tech companies, they'll likely rise and fall together — that's not real diversification. True diversification comes from combining assets that don't move in lockstep.
It's tempting to think "I own 20 different stocks, so I'm diversified." But diversification isn't about how many holdings you have — it's about how independently they move relative to each other. Correlation measures exactly that, on a scale from -1 to +1. A correlation of +1 means two assets move perfectly in lockstep; -1 means they move perfectly opposite each other; 0 means their movements are unrelated.
Twenty tech stocks are typically highly correlated — when the sector rallies or sells off, most of them move together, because they share the same economic drivers (interest rates, sector sentiment, tech spending cycles). Owning 20 of them concentrates your risk in "whatever moves tech stocks" almost as much as owning just two or three would. Real diversification comes from combining assets with low or negative correlation — different sectors, different asset classes (stocks and bonds, for instance), or different geographies.
The payoff of low correlation is genuine: when you blend two assets that don't move in lockstep, the combined portfolio's volatility can be lower than a simple average of the two individual volatilities — smoothing out the ride without necessarily sacrificing much expected return. That "free" risk reduction from combining imperfectly-correlated assets is one of the few things in investing sometimes called a "free lunch."
Twenty tech stocks typically cluster around +0.7 to +0.9 — close to the "lockstep" end. Stocks and long-term government bonds have often sat closer to 0 or even slightly negative over long periods, though this relationship isn't fixed and can shift, especially during periods of high inflation.
Real correlations vary by period and by the specific holdings, so here's a clean illustrative comparison to see the mechanics clearly.
| Portfolio | # Holdings | Avg. Correlation | Individual Volatility | Portfolio Volatility |
|---|---|---|---|---|
| 20 Tech Stocks | 20 | ~0.80 | ~35% | ~32% |
| Mixed (Stocks + Bonds + Gold) | 20 | ~0.25 | ~35% | ~19% |
Both portfolios hold 20 positions with similar individual volatility — but the highly-correlated tech portfolio barely reduces overall volatility versus a single stock, while the mixed portfolio's lower average correlation cuts volatility by nearly half. Same number of holdings, dramatically different diversification benefit — purely because of correlation.
Enter two assets' individual volatility and their correlation — see the blended volatility of an equal-weight combination, and how much diversification actually helped.
Model: equal-weight (50/50) blend. Portfolio volatility = √(0.25×σA² + 0.25×σB² + 0.5×ρ×σA×σB) — the standard two-asset portfolio variance formula. Lower correlation (ρ) means more of the individual volatility "cancels out" in the blend.
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Now that you can judge real diversification, the next step is keeping your carefully-built mix from quietly drifting away from target.