Week 7 Day 7: One Fund Can Be Enough

You do not need 15 funds. You do not need a financial advisor. You do not need to understand options or commodities or alternative investments. A single S&P 500 index fund, purchased automatically every month, held for 20-40 years, is a complete wealth-building strategy. The financial industry profits from complexity. Your portfolio profits from simplicity.

Warren Buffett's instructions for his wife's trust after his death: put 90% in a low-cost S&P 500 index fund and 10% in short-term government bonds. That is it. The greatest investor of all time is telling his own family to use one index fund. Not hedge funds. Not individual stocks. Not alternative investments. One index fund. His reasoning: 'I believe the trust's long-term results from this policy will be superior to those attained by most investors -- whether pension funds, institutions, or individuals -- who employ high-fee managers.' If simplicity is good enough for Buffett's family, it is good enough for yours. The caveat: as you near retirement (within 10-15 years), adding bonds to reduce volatility becomes important. But for accumulation years, the S&P 500 alone is a battle-tested strategy.

The academic support for single-fund simplicity is extensive. The Bogle Research Institute has documented that portfolio complexity -- holding more funds, more asset classes, more strategies -- often reduces returns through higher fees, tax drag, and behavioral errors (complex portfolios trigger more tinkering). DeMiguel, Garlappi, and Uppal's influential 2009 study 'Optimal Versus Naive Diversification' found that the simple 1/N equal-weight portfolio outperformed theoretically optimal mean-variance portfolios out of sample because the estimation errors in the 'optimal' approach exceeded the gains from optimization. The simpler approach was more robust. For most individual investors, the primary risk is not suboptimal asset allocation -- it is behavioral error. And behavioral error increases with portfolio complexity. A single-fund strategy minimizes the surface area for mistakes.