Asset allocation — the decision of how to divide your investment portfolio across different asset classes — is the most consequential investment decision most people make. The landmark 1986 research by Brinson, Hood, and Beebower found that asset allocation policy explains over 90% of the variability in portfolio returns over time. Put simply: whether you hold Apple stock or Microsoft stock matters far less than whether your portfolio is 80% equities or 40% equities. This insight underpins the modern approach to investment management.
The major asset classes behave differently under various economic conditions, which is what makes diversification across them valuable. Equities (stocks) offer the highest long-term expected return (historically ~7% real per year for the US market) but the most volatility — during the 2008–2009 crisis, a 100% stock portfolio fell ~57%. Bonds provide income and relative stability; they often (though not always) rise when stocks fall, dampening portfolio volatility. Cash and cash equivalents preserve capital but lose purchasing power to inflation. Real assets (REITs, commodities, TIPS) provide inflation protection and partial diversification from traditional stocks and bonds.
The "right" allocation is deeply personal and depends on three variables: time horizon (when you'll need the money — longer horizons justify more equity exposure because you have time to recover from downturns), risk tolerance (your psychological ability to hold through market declines without panic-selling — an underappreciated factor), and risk capacity (your financial ability to absorb a loss — if losing 30% would force you to change your retirement plans, you can't afford 30% downside even if you could psychologically handle it).
Common allocation guidelines provide useful starting points. The "110 minus age" rule suggests a 30-year-old hold 80% stocks and 20% bonds; a 60-year-old holds 50/50. More aggressive versions use 120 or 125 minus age, reflecting longer life expectancies and the need for portfolios to last 30+ years in retirement. Target-date funds automate this process — a Vanguard Target Retirement 2055 Fund holds approximately 90% equities today and will gradually shift to ~50% equities by 2055, requiring zero active management from the investor.
Three formal approaches to managing allocation: Strategic allocation sets fixed targets (e.g., 70/30 stocks/bonds) and rebalances to those targets periodically — the simplest, most evidence-backed approach for individual investors. Tactical allocation temporarily shifts allocation based on market conditions or valuations (e.g., reducing equity exposure when Shiller CAPE is elevated); evidence for this approach outperforming is mixed and it introduces timing risk. Dynamic/glidepath allocation systematically reduces equity exposure as a specific date approaches — the approach used by target-date funds and by FIRE adherents managing through retirement.
Rebalancing — returning your portfolio to its target allocation when drift occurs — is both necessary and psychologically difficult. Markets inevitably cause drift: after a bull market, stocks may grow from 70% to 80% of the portfolio, increasing risk beyond the intended level. Rebalancing back to 70% means selling some stocks (which have risen) and buying bonds (which have lagged) — effectively a systematic, emotion-free "sell high, buy low" process. Most financial advisors recommend rebalancing when any asset class drifts more than 5% from its target, or on an annual schedule, whichever comes first.