Asset Allocation by Age: The Math Behind Stock/Bond Ratios

# Asset Allocation by Age: The Math Behind Stock/Bond Ratios

"110 minus your age in stocks" is where most people start — and most conversations end. A 35-year-old: 75% stocks, 25% bonds. A 60-year-old: 50% stocks, 50% bonds. It's a reasonable heuristic with zero personalization and limited empirical justification.

The actual question is: given your time horizon, risk tolerance, and income sources, what allocation maximizes the probability of meeting your financial goals?

The case for equity-heavy allocations in early years is statistical. Over any 30-year period in U.S. market history, equities have outperformed bonds. The risk of a negative real return over 30 years with a diversified equity portfolio is historically near zero — while the risk of bonds failing to match inflation over long periods is real.

Bonds reduce short-term volatility. They do not improve long-term expected returns. For an investor with a 30-year horizon who can tolerate year-to-year drawdowns, bonds add stability at the cost of expected wealth.

The counterargument: behavioral risk. A portfolio that drops 40% in a bear market is only valuable if you hold it. An investor who panic-sells a 100% equity portfolio during a crash locks in permanent losses. A more conservative allocation that you can actually hold through volatility may outperform an aggressive allocation that you abandon.

**Time horizon** — most important. 20+ years: aggressive equity tilt is well-supported. Under 10 years: meaningful bond allocation reduces sequence risk.

**Income stability** — someone with a stable pension or government salary has bond-like human capital and can accept more portfolio equity risk. Someone with variable income should hold more bonds as an income buffer.

**Spending flexibility** — a retiree who can reduce spending in a bad market year has more tolerance for equity volatility than one with fixed expenses.

**Other assets** — Social Security, pensions, real estate, and rental income all act as bond-like assets in a total wealth framework.

**Behavioral risk tolerance** — what you can actually hold during a 30–40% drawdown without selling. Self-knowledge matters.

Target-date funds use a glide path — gradually reducing equity exposure as retirement approaches. The logic: reduce sequence-of-returns risk near the most vulnerable window (first decade of retirement). The debate is about how aggressive the glide path should be and whether bonds or cash are the better risk reducer.

Vanguard's research suggests that many target-date funds are excessively conservative in the early retirement years — the right allocation at 65 may still be 60–70% equities given life expectancies of 20–30 more years.

U.S. investors are home-country biased. Global market cap weight is approximately 60% U.S., 40% international — yet most U.S. investors hold 80–100% U.S. equities. The evidence on whether international diversification adds return is mixed, but it reduces concentration risk in a single country's equity market and historically reduces volatility.

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*Related: [Index funds vs. active management](./index-funds-vs-active-management) — once you know your allocation, how to implement it. [Rebalancing](./rebalancing) — maintaining your target allocation over time.*