Past performance is not a guarantee of future returns. This familiar disclaimer highlights the fact that a particular investment strategy may work well in some periods and poorly in others, limiting the inference that can be drawn from past returns.The concern is heightened when a proposed investment strategy is backtested by using historical data. Consider an investment strategy that can be pursued today with readily available securities. If those securities were unavailable in the past, then the strategy has no true antecedent. Backtesting must be conducted with proxies for the securities, and the choice of proxies can have a direct effect on measured returns. In addition, the introduction of new securities can have an indirect effect; a strategy that was seemingly profitable in the past might have been less profitable if the new securities had been available and thus made the strategy accessible to a broader class of investors. The matter is confounded by the specific attributes of the backtesting period, concerns about statistical significance, and a plethora of metrics used by investors to evaluate strategy performance.In our study, we considered these issues by carefully examining the historical performance of four simple strategies based on two asset classes: U.S. equity and U.S. Treasury bonds. 1 We included a market or value-weighted portfolio, which is the optimal risky portfolio in the capital asset pricing model (CAPM), and a 60/40 mix, which is popular with pension funds and other long-horizon investors. We also included two risk parity strategies. Risk parity attempts to equalize risk contributions across asset classes; early formulations of risk parity can be found in and Kessler and Schwarz (1996). 2 Risk parity has been popular since the 2008 financial crisis, as frustrated investors have struggled to meet return targets by levering low-risk or low-beta assets, and it is sufficiently mainstream to be featured in the Wall Street Journal. 3 A diverse collection of risk parity strategies can be constructed by varying asset classes, grouping schemes, and risk estimates. 4 An essential element of risk parity is leverage, which distinguishes the two risk parity strategies in our study. Because an unlevered risk parity strategy tends to have relatively low risk and thus relatively low expected return, a risk parity strategy must be levered in order to have even a remote chance of achieving a typical return target. 5 The notion that levering a low-risk portfolio might be worthwhile dates back to Black, Jensen, and Scholes (1972), who provided empirical evidence that the risk-adjusted returns of low-beta equities are higher than the CAPM would predict. Black (1972) introduced a zero-beta portfolio, considered by some to be the antecedent of risk parity. Nearly four decades later, Frazzini and Pedersen (2010) developed a compelling theory of leverage aversion in which risk parity emerges as a dominant strategy, and this dominance is supported by the empirical study in Asness, Frazzini, and Pedersen (201...