I've been investing for 12 years and asset allocation confused me for years until I drew it out visually.

Here's the simple mental model: your total investment portfolio is a pie. Each slice is an asset class. The percentages determine how much risk you're taking and how correlated your returns are.

A simple aggressive portfolio: 80% US stocks (VTI), 20% international stocks (VXUS). A moderate portfolio: 60% VTI, 20% VXUS, 20% bonds (BND). A conservative portfolio shifts more toward bonds.

The logic: stocks are volatile but grow more over time. Bonds are stable but grow less. International stocks move somewhat differently from US stocks, providing diversification. Adding bonds dampens volatility at the cost of expected return.

Your allocation should reflect two things: your time horizon and your actual (not theoretical) ability to tolerate watching the portfolio drop 30-40% without selling.