Modern portfolio theory is based on the thought that one may be able to maximize expected return, given a level of market risk, by constructing a portfolio of assets based upon an “efficient frontier”. Modern Portfolio Theory is an extension of diversification, and is the idea that owning assets across various asset classes exposes you to less risk than owning just one. MPT argues that an investment’s risk and return profile should not be viewed in isolation, but looked at in aggregate with the portfolio as a whole.
Modern Portfolio Theory was first introduced by Harry Markowtiz in 1952, for which he was awarded a Nobel Prize in Economics.