Deep risk: How History informs Portfolio Design is the third installment in the investing for adults series. this series is not for novices. This booklet takes portfolio design beyond the familiar "black box" mean-variance framework. Most importantly, the short-term volatility of financial assets, commonly measured as standard deviation, is a highly imperfect measure of the actual long-horizon perils faced by real-world investors subject to the vagaries of financial and military history. These risks have names-inflation, deflation, confiscation, and devastation-and any useful discussion of portfolio design of necessity incorporates their probabilities, consequences, and costs of mitigation. You're an investment adult, so you know that the future efficient frontier lies well beyond our ken; presumably you already know all about the mechanics, long-term benefits, as well as the uncertainties, of wide diversification and factor tilt using low-cost, efficient vehicles and the risk/reward spectrum between all-fixed- income and all-equity portfolios. This booklet contains no magic formula for the "perfect portfolio," but rather, with luck, a framework within which to think more clearly about risk.