Correlation measures whether two assets move together. If your 10 holdings rise and fall at the same rhythm, you are not diversified: you own the same risk under ten different names. The line between real protection and false safety comes down to a single number, sitting between -1 and +1.
True diversification is not about multiplying holdings, but about combining assets with low correlation. A three-position portfolio of well-chosen assets (for example: world stocks + bonds + gold) can be more diversified than twenty tech stocks. Worth remembering: think in asset classes, not in number of holdings.
+1 = perfectly correlated: both assets always move in the same direction. 0 = no correlation: movements are independent. -1 = perfectly inverse: when one goes up, the other goes down. In practice, perfect correlation never exists, but the trends are clear.
US stocks / European stocks: ~0.85 (highly correlated, no real geographic diversification). Stocks / Government bonds: ~-0.3 (slightly inverse, a good diversifier). Stocks / Gold: ~0.05 (nearly independent, an excellent diversifier). Bitcoin / Nasdaq: ~0.55 since 2021 (so much for the 'crypto decoupling' story).
In normal times, asset classes move to different rhythms. In a crisis, they all converge toward +1. In 2008, stocks, listed real estate, and commodities fell together. Only government bonds and cash held up. Practitioners call it 'correlation breakdown': it is precisely the moment when naive diversification stops protecting you.
For a robust FIRE portfolio: combine global stocks (growth) + government bonds (crash protection) + a small allocation to gold or TIPS (inflation protection). Check historical correlations before adding a position. If correlation exceeds 0.7 with an existing asset, the diversification benefit is marginal.