During the 1950s, I decided, as did many others, that many practical problems were beyond analytic solution and that simulation techniques were required. At RAND, I participated in the building of large logistics simulation models; at General Electric, I helped build models of manufacturing plants.

Portfolio theory, as used by most financial planners, recommends that you diversify with a balance of stocks and bonds and cash that's suitable to your risk tolerance.

I was born in Chicago in 1927, the only child of Morris and Mildred Markowitz, who owned a small grocery store. We lived in a nice apartment, always had enough to eat, and I had my own room. I never was aware of the Great Depression.

If the investor doesn't have enough time and skill to investigate individual stocks or enough money to diversify a portfolio, the right thing to do is to invest in exchange-traded funds that give you exposure to asset classes. It does make sense for the individual investor to think in terms of holding individual asset classes.