Harry M. Markowitz, an economist who launched a revolution in finance, upending traditional thinking about buying stocks and earning the Nobel Prize in economic science in 1990 for his breakthrough, died on June 29 in San Diego. He was 95.

Until Dr. Markowitz came along, the investment world assumed that the best stock-market strategy was simply to choose the shares of a group of companies that were thought to have the best prospects. But in 1952, he published his dissertation, “Portfolio Selection,” which overturned this common sense approach with what became known as modern portfolio theory.

The heart of his research was grounded in the relationship between risk and reward. He showed that the risk in a portfolio is less dependent on the riskiness of its component stocks and other assets than how they relate to one another. It was the first time that the benefits of diversification had been codified and quantified, using advanced mathematics to calculate correlations and variations from the mean.

Dr. Markowitz won renown in two other fields: “sparse matrix” techniques for solving large mathematical optimization problems, and the development of Simscript, which is used for programming computer simulations of systems like factories, transportation and communications networks. “I’m not a one-shot Nobel laureate,” he said in an interview with The New York Times.