The idea that diversification is a good strategy in portfolio allocation is the cornerstone of Modern Portfolio Theory (MPT). MPT is the brainchild of Nobel Prize–winning economist Harry Markowitz. In a paper he wrote in 1952 for his doctoral thesis, Markowitz argued that investors must look at a portfolio's overall risk/reward ratio. Although this sounds like common sense today, it was a groundbreaking idea at the time.
Before Markowitz's paper, most investors constructed their portfolios based on a risk/reward ratio analysis of individual securities. Investors chose a security based on its individual risk profile and ignored how that risk profile fit within a broader portfolio. Markowitz argued (successfully) that investors could construct more profitable portfolios if they looked at the overall risk/reward ratio of their portfolios.
Therefore, when considering an individual security, you need to not only assess its individual risk profile, but also take into account how that risk profile fits within your general investment strategy. Markowitz's idea that holding a group of different securities reduces a portfolio's overall volatility is one of the most important ideas in portfolio allocation.
