A.D. Roy: The Forgotten Father of Portfolio Theory

The notion that asset diversification reduces risk is ancient and can be traced as far back as the Talmud which states, “A man should always keep his wealth in three forms: one-third in real estate, another in merchandise, and the remainder in liquid assets” (Baba Metzia, verse 42a). Somewhat more recently, in 1738, Daniel Bernoulli observed, “it is advisable to divide goods which are exposed to some small danger into several small portions rather than to risk them all together” (1738/1954, p. 30). Arguably, however, it was not until 1935 that the future Nobel laureate J. R. Hicks offered some early direction for modern portfolio theory. Although his research was more concerned with explaining the demand for money, he points out two important considerations for modeling risk. Hicks writes, “The risk factor comes into our problem in two ways: First, as affecting the expected period of investment, and second, as affecting the expected net yield of investment” (Hicks, 1935, p. 7). Regarding Hicks' first point, bothMarkowitz (1952)andRoy (1952)emplace their analyses in a one-period investment horizon. Second, and even more relevant to modern portfolio theory, is Hicks' suggestion of using an expected value calculated with subjective probabilities. Hicks continues, “It is convenient to represent these probabilities to oneself, in statistical fashion, by a mean value, and some measure of dispersion” (1935, p. 8). Clearly, Hicks comes very close to articulating a mean–variance solution. Crucially, and unlike Roy or Markowitz, Hicks does not develop this line of reasoning nor does he suggest the particular use of variance or standard deviation as that measure of risk. Nonetheless, Hicks' suggestion anticipates the work of Markowitz and Roy.1