Before Markowitz, investors thought about individual stocks in isolation: is this a good stock? MPT changed the question to: how does this stock affect the portfolio as a whole? A risky stock might actually reduce portfolio risk if it moves opposite to the other holdings. The portfolio, not the individual asset, is what matters.
The core insight: correlation and diversification
MPT shows that portfolio risk depends not just on the riskiness of individual assets but on how they move relative to each other — their correlation. When assets are perfectly correlated (correlation = 1), combining them doesn't reduce risk. When they're uncorrelated (correlation = 0) or negatively correlated (correlation = -1), combining them can dramatically reduce portfolio volatility.
The Efficient Frontier
Markowitz showed that for any set of assets, there's a curve of "efficient" portfolios — each one offering the maximum return for a given level of risk, or minimum risk for a given return. This is the Efficient Frontier. Every portfolio on the frontier is rational to hold; every portfolio inside (below) the frontier is suboptimal — you could get either more return or less risk for the same trade-off.
Combined with CAPM, this led to the concept of the "market portfolio" — the one specific combination of risky assets that lies on both the efficient frontier and the Capital Market Line — which is essentially the argument for index investing.
Limitations
MPT assumes: normally distributed returns (ignores fat tails), stable correlations (they spike in crises), rational investors (behavioural finance says otherwise), and ability to short and borrow freely. Correlations are estimated from historical data and break down precisely when diversification is needed most — in a crisis. The 2008 crash showed that assets assumed to be uncorrelated (including hedge funds, real estate, and equities) all fell together.
What this means for you
The practical legacy of MPT is the principle that portfolio construction matters as much as asset selection. Adding an asset with lower expected return but low correlation to your portfolio can actually improve your overall risk-adjusted outcome. This is why a small allocation to gold, commodities, or global bonds often improves portfolio efficiency — not because of their individual returns but because of how they interact with the rest of your holdings.