A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Diversification across asset classes (stocks, bonds, real estate) reduces portfolio risk without proportionally reducing expected returns. Asset allocation—the percentage split between asset types—should match your risk tolerance and time horizon; younger investors tolerate higher stock allocations while retirees need more bonds for stability.
You already know from your prerequisites that diversification reduces risk, and that your risk tolerance — your ability and willingness to absorb losses — should guide how aggressively you invest. Now let's put those ideas together into a practical framework: asset allocation, which is the decision about what percentage of your total portfolio to put into each category of investment.
The major asset classes are stocks (ownership stakes in companies), bonds (loans to governments or corporations), and real estate (either physical property or REITs that trade like stocks). These asset classes behave differently under the same economic conditions — when stocks fall sharply in a recession, high-quality government bonds often hold their value or rise, because investors flee to safety. This is called negative correlation, and it's what makes combining asset classes more powerful than just owning more of the same thing. Adding bonds to a stock portfolio doesn't just reduce the maximum gain — it significantly reduces volatility, meaning the ride is smoother even if the destination is similar.
Your time horizon — how many years before you need the money — is the dominant factor in asset allocation. The reason is simple: stocks are volatile year-to-year but reliably grow over decades. If you're 30 years from retirement, a 40% stock market drop is a temporary setback with decades to recover. If you're 3 years from retirement, that same drop at the wrong moment could devastate your actual retirement income. This is why a common rule of thumb (like "110 minus your age = stock percentage") shifts investors from growth-oriented to preservation-oriented allocations as they age. This gradual shift is called a glide path.
A concrete example: a 25-year-old investor with high risk tolerance might hold 90% stocks (split between domestic and international) and 10% bonds. A 60-year-old approaching retirement might hold 50% stocks, 40% bonds, and 10% real estate. Neither portfolio is "right" in absolute terms — the right allocation is the one you can actually hold through a downturn without panic-selling. The biggest portfolio mistake is not choosing the wrong allocation; it's choosing an allocation too aggressive for your emotional tolerance and abandoning it during a crash.
Crucially, your allocation drifts over time as different assets grow at different rates — a 60/40 portfolio might become 70/30 after a strong stock year, taking on more risk than you intended. Rebalancing — periodically selling what grew and buying what lagged — restores your target allocation. This mechanical process has the counterintuitive effect of selling high and buying low, which is precisely what disciplined investing requires but emotion makes difficult without a system.