What is Modern Portfolio Theory?
Modern Portfolio Theory (MPT), introduced by Harry Markowitz in his 1952 paper 'Portfolio Selection,' demonstrates that an investor can reduce portfolio risk without sacrificing expected return by combining assets whose prices do not move in perfect lockstep. The key insight is that portfolio risk depends not only on individual asset volatilities but critically on the correlations between them. Markowitz received the Nobel Prize in Economics in 1990 for this work.
Diversification in Practice
A commonly cited rule of thumb is that holding roughly 25 to 30 uncorrelated stocks eliminates most unsystematic (company-specific) risk, although the exact figures vary by study and period. A portfolio split between U.S. equities and U.S. bonds (the classic 60/40 mix) has historically captured a large share of the stock market's return with noticeably lower volatility, though the exact figures depend on the period measured. Adding international equities, real estate, and commodities further improves the risk-return profile because these asset classes have historically exhibited low correlations with domestic stocks, particularly during market stress.
Key Considerations
MPT relies on historical correlations, which can spike toward 1.0 during crises, precisely when diversification is needed most. During the 2008 financial crisis, nearly all risky asset classes fell simultaneously. MPT also assumes investors are rational and markets are efficient, which behavioral finance research has challenged. Despite these limitations, the core principle that diversification reduces risk remains one of the most robust findings in finance and should guide every investor's allocation decisions.