Asset Classes

People often mix goals and choices. That creates bad outcomes with clear dollar consequences. Example one. Someone keeps $100,000 in **cash** paying 0.5$100,000 \times (1-0.02)^{20} \approx $67,000. Example two. Another investor puts$100,000 into 100% equities before retirement with a 7% expected return and 16% annual volatility. If sequence of returns gives early 30% drops, a 4% withdrawal plan can deplete the portfolio in 15-20 years rather than lasting indefinitely. Both errors come from treating all money as fungible across time horizons and liquidity needs. The core problem is misaligning the right asset class with the right objective. When that alignment fails - growth money sits in low-return cash, stable needs sit in volatile equities - the results are predictable and measurable. IF a sum of money is earmarked for spending within 1-3 years AND it is placed in equities with 10-20% annual volatility, THEN the probability of a >20% nominal drop before spending may be 20-40% over that window BECAUSE short horizons amplify the chance a large downturn happens before you need the cash. IF long-term growth money is held in cash paying 0-1% while expected stock real returns are 5-7%, THEN forgone compounded gains may be large BECAUSE compound growth rates multiply over decades. This section identifies the mistake. The rest of the lesson shows how the different asset classes behave and how to assign them to goals with numbers and formulas.

Start with definitions. Stocks or equities represent ownership claims on companies. Long run real returns often range 5-8% per year, with annual volatility commonly 12-18%. Bonds are debt instruments; investment-grade bonds often show real returns near 0-3% with volatility 3-8%. Cash includes savings accounts and short-term Treasury bills; real returns commonly -1% to +1% after inflation and volatility near 0-1%, with immediate liquidity. Real assets such as real estate or infrastructure typically generate 2-6% real returns plus rental yields of 2-5%, volatility 8-15%, and partial inflation linkage. Commodities like oil or metals have low long-run returns 0-3%, high short-term volatility 20-40%, and can act as an inflation or supply-shock hedge. Quantitatively combine assets using weighted averages and variance formulas. Expected portfolio return is $E[R_p] = \sum_i w_i E[R_i]$. For two assets, portfolio variance is $\sigma_p^2 = w_A^2 \sigma_A^2 + w_B^2 \sigma_B^2 + 2 w_A w_B \text{Cov}_{A,B}$. Example numeric calculation. Suppose equities expected return $E[R_E]=7\%$ with $\sigma_E=16\%$, bonds $E[R_B]=2\%$ with $\sigma_B=6\%$, correlation $\rho_{E,B}=0.2$. For a 60/40 portfolio $w_E=0.6$, $w_B=0.4$. Then $E[R_p]=0.6\times7\% + 0.4\times2\% = 5.0\%$. Variance computes to $\sigma_p^2 = 0.6^2(0.16)^2 + 0.4^2(0.06)^2 + 2(0.6)(0.4)(0.2)(0.16)(0.06)$. That equals roughly $0.00922$ variance, so $\sigma_p\approx 9.6\%$. Sharpe ratio is $(E[R_p]-R_f)/\sigma_p$. If risk-free cash $R_f=1\%$, Sharpe approx $(5.0\%-1\%)/9.6\%\approx0.42$. These formulas explain trade-offs. Higher expected returns often come with higher volatility and larger potential drawdowns. Equities historically provide growth and compound returns over long horizons. Bonds provide income and reduce volatility. Cash provides liquidity and safety of nominal principal. IF planning horizon is long and one can tolerate 15-20% year-to-year swings, THEN allocating a larger equity weight may increase expected terminal wealth BECAUSE higher expected returns compound over decades according to Compound Interest and the Rule of 72. IF capital must be spent within months, THEN cash or short-term bonds may be preferable BECAUSE they offer near 0% price volatility and immediate access.

What to do instead of rules of thumb. Step 1 - match time horizon to asset class. Short horizon 0-3 months, medium 3 months-5 years, long 5+ years. Step 2 - quantify liquidity need. Emergency fund target commonly 3-6 months of expenses, with ranges 3-9 months for higher income volatility. Step 3 - measure risk capacity versus risk tolerance. Risk capacity is numerical - how many years of expenses your portfolio covers and your replacement income; risk tolerance is behavioral. Step 4 - select mix using IF/THEN/BECAUSE logic. IF money is needed in 0-3 months AND you value access over return, THEN hold it in cash or Treasury bills BECAUSE nominal principal risk is near zero and withdrawals are immediate. IF money is needed in 1-5 years AND you want some return without large drawdowns, THEN favor short-term bonds or a 20-40% equity allocation BECAUSE bonds reduce volatility and equities offer modest upside for medium-term horizons. IF money is for 10+ years AND you can accept 20% drawdowns, THEN shift toward 60-90% equities BECAUSE expected extra 3-5 percentage points in annual return compounds powerfully as shown in Compound Interest. Use rebalancing math and thresholds. Rebalancing maintains target risk exposures. One method is calendar rebalancing annually. Another is threshold rebalancing where you rebalance when an asset's weight crosses plus or minus 5 percentage points from target. Example formula for rebalancing threshold: trigger if $|w_i - w_{i,target}| > \delta$ where $\delta$ is 0.03-0.07. Consider taxes. IF holding municipal bonds in a 24% tax bracket, THEN tax-equivalent yield matters BECAUSE a 3% muni yield roughly equals a 3\% / (1-0.24) \approx 3.95\% taxable yield. Also weigh fees and liquidity. Real assets and private equity may offer higher expected returns but require lockups of 5-10 years and fees of 1-2% or more, which reduce net returns. This framework gives decisions, not mandates. It frames trade-offs numerically so choices map to goals and constraints.

This framework works often but breaks in specific scenarios. Limitation one - changing return regimes. Expected returns are estimates with uncertainty ±2-4 percentage points. IF equity returns happen to be 0-2% real for a decade, THEN heavy equity allocations may underperform historical expectations BECAUSE forward returns can deviate materially from past averages. Limitation two - correlation breakdowns during crises. Many diversification benefits assume correlations around 0.2-0.4. In severe stress, correlations often rise toward 0.7-1.0, increasing portfolio drawdowns. Limitation three - illiquidity and forced selling. Real assets and private investments can be hard to sell quickly. IF an investor needs cash during a market crash AND holdings are illiquid, THEN they may be forced to sell at depressed prices BECAUSE secondary markets can freeze and buyers withdraw. Limitation four - behavioral risk and sequence of returns for retirees. The model does not predict psychology; withdrawal strategies like systematic withdrawals can fail when early returns are poor. Limitations specific to taxes and fees also matter. Municipal tax advantages, capital gains timing, and fund expense ratios can change net returns by 0.5-2.0 percentage points annually. Finally, model inputs are estimates. The math uses point inputs like 7% equity return and 2% bond return, but real outcomes follow distributions. Treat outputs as ranges not certainties. The framework also does not include extreme tail events frequency modeling. IF a once-in-50-year shock occurs, THEN simple mean-variance allocations may misstate true risk BECAUSE tail dependencies and liquidity freezes dominate mean statistics. Documenting these limitations clarifies where to be conservative and where to accept risk.

This lesson builds on Compound Interest (/money/compound-interest) and Opportunity Cost (/money/opportunity-cost) for the compounding and trade-off logic. Mastery here unlocks Asset Allocation (/money/asset-allocation), Portfolio Construction (/money/portfolio-construction), and Retirement Withdrawal Strategies (/money/retirement-withdrawals), because those topics require matching asset-class behavior to cash flow timing, tax treatments, and risk tolerance.