Benartzi and Thaler (2001) called the bias '1/N' after watching 401(k) participants split contributions equally across whatever fund options the plan offered — typically allocating ~1/N to each fund regardless of whether the funds were genuinely diverse. A plan with 6 stock funds and 1 bond fund yielded 86% stock allocations on average; the same population given 1 stock fund and 6 bond funds went 14% stocks. The portfolio was determined by the menu, not by the investor's actual risk preference.
| Naive-Diversification View | Correlation-Aware View | Why It Matters |
|---|---|---|
| 20 tech stocks across 5 sub-sectors = diversified | 20 tech stocks have ~0.7-0.9 cross-correlation in a sell-off — effectively one position | The 2022 NASDAQ drawdown took down all 20 simultaneously — diversification was illusory |
| 5 S&P 500 ETFs from different issuers = diversified | All 5 hold roughly the same 500 names — you own the same beta 5× | Net exposure is identical to one ETF; the count creates a false sense of breadth |
| 10 large-cap dividend stocks across sectors = diversified | Most large-cap dividend stocks correlate ~0.6-0.8 in equity drawdowns | Cross-asset diversification (bonds, cash, international, alternatives) does more work than stock-count |
Markowitz (1952) defined true diversification as adding low-correlation positions: a position correlated 0.3 with the rest of the portfolio reduces variance more than 5 positions correlated 0.8. The math says you need ~15-20 securities to capture most diversification benefit IF correlations are low — but if all 20 securities are large-cap US equities, you have captured the diversification benefit of 1-2 securities and stopped early.
Forget counting positions. Instead, group your portfolio into three buckets: (1) **same beta exposure** (e.g., US large-cap equity — likely VTI, SPY, individual mega-cap stocks, most US large-cap mutual funds); (2) **distinct risk factors** (international developed, emerging markets, US small-cap, real assets, commodities); (3) **low-correlation diversifiers** (high-quality bonds, gold, cash). A portfolio with 30 positions that all live in bucket 1 is undiversified; a portfolio with 5 positions spread across all three buckets is meaningfully diversified. The bucket count matters more than the position count.
Sit with the ideas.
Benartzi and Thaler's 1/N study (2001) found that 401(k) participants split contributions roughly equally across the funds offered, regardless of those funds' content. Which conclusion is most directly supported?