The Core Principle
Diversification reduces portfolio risk by combining assets that do not move in perfect lockstep. When some positions decline, others may hold steady or rise, dampening overall portfolio volatility.
The mathematical basis is simple: portfolio variance decreases as you add assets with imperfect correlation (correlation less than 1.0). The lower the correlation between holdings, the greater the diversification benefit.
Sector and Industry Diversification
Owning 20 technology stocks is not diversified. If the sector falls, all positions suffer simultaneously. Meaningful diversification requires exposure across sectors: technology, healthcare, financials, consumer goods, industrials, energy, and others.
The Apter Portfolio page shows sector allocation percentages and flags concentration risk when any single sector exceeds a threshold. This helps identify hidden sector bets.
Position Sizing
Even a well-diversified sector allocation is undermined if one position is disproportionately large. A common guideline is limiting individual positions to 5-10% of portfolio value, though this depends on conviction level and total portfolio size.
Position sizing is about managing downside: if your largest position drops 50%, how much does the total portfolio lose? If the answer is uncomfortable, the position is too large.
Correlation and False Diversification
During market stress, correlations between most equities increase — stocks that normally behave independently start moving together. This means diversification benefits are weakest precisely when you need them most.
True diversification across asset classes (stocks, bonds, commodities, real estate) provides more resilient protection than diversification within a single asset class. Within equities, geographic diversification (domestic vs. international) also adds value.
Over-Diversification
Adding beyond 30-40 individual stocks provides diminishing risk reduction. Each additional position adds complexity and monitoring burden while contributing minimal diversification benefit. At some point, you are effectively creating your own index fund with higher costs.
The goal is sufficient diversification to manage risk without diluting your highest-conviction ideas.