Week 16 Day 7: Your Stock-to-Bond Ratio Is the Biggest Decision

Brinson, Hood, and Beebower's famous 1986 study found that asset allocation explained about 90% of the variation in portfolio returns. Picking individual stocks, timing the market, or choosing the best fund manager explained the remaining 10%. The single most important investment decision you make is how much to put in stocks vs. bonds.

To set your allocation, answer two questions: (1) How long until you need the money? Longer horizons tolerate more stocks. (2) How would you feel if your portfolio dropped 40%? Be honest. Here are the tradeoffs. 100% stocks: highest expected return (10%/year), but maximum volatility. Worst year: -43%. You need iron nerves. 80/20 stocks/bonds: strong returns (9.5%/year) with moderate cushion. Worst year: -35%. Good for most people under 50. 60/40 stocks/bonds: balanced (8.8%/year). Worst year: -27%. Classic moderate allocation. 40/60 stocks/bonds: conservative (7.5%/year). Worst year: -18%. Good for money needed in 5-10 years. 20/80 stocks/bonds: very conservative (6%/year). Worst year: -10%. Near-term money or very risk-averse investors. Pick the allocation whose worst year you could endure without selling. Then automate it and stop checking daily.

The Brinson-Hood-Beebower (BHB) result is frequently misquoted. The study found that asset allocation explained approximately 93.6% of the variation in returns over time for a given portfolio -- not that it explained 93.6% of the level of returns or the difference between portfolios. Ibbotson and Kaplan (2000) clarified: asset allocation explains about 90% of the variability of a single fund's returns over time, about 40% of the variation in returns across funds, and approximately 100% of the level of returns (since costs and active management roughly cancel out in aggregate). The practical implication remains: setting and maintaining an appropriate stock/bond allocation is the single highest-impact decision an individual investor makes. The second biggest decision is costs (index vs. active). Everything else -- factor tilts, alternative assets, tactical shifts -- is marginal. William Bernstein summarizes this as: 'The most important financial decision you will ever make is your stock/bond mix. Get it right and the rest is details.'