Modern Portfolio Theory (MPT)
Modern Portfolio Theory (MPT), introduced by Harry Markowitz in 1952, changed how investors think about portfolios. Instead of analyzing individual securities in isolation, MPT focuses on how securities interact within a portfolio.
Central Principle
- Investors can construct portfolios that maximize expected return for a given level of risk
- Risk is measured by standard deviation of portfolio returns (total risk)
- Key insight: combining assets that are not perfectly correlated reduces overall portfolio risk
- MPT assumes investors are risk-averse - given equal returns, they prefer the less risky portfolio
- Portfolio risk depends on three factors: (1) weight of each asset, (2) standard deviation of each asset, (3) correlation between assets
Think of it this way: Before MPT, investors analyzed stocks one at a time, asking "Is this a good stock?" MPT changed the question to "How does this stock fit with my other investments?" A stock might be risky on its own, but if it tends to go up when your other stocks go down, adding it actually reduces your overall portfolio risk.
Correlation and Its Effect on Diversification
Correlation coefficient (r): ranges from -1.0 to +1.0
| Correlation | Meaning | Diversification Benefit |
|---|---|---|
| r = +1.0 | Perfect positive correlation (assets move in lockstep) | None |
| r = 0 | No correlation (movements are unrelated) | Good |
| r = -1.0 | Perfect negative correlation (assets move opposite) | Maximum |
- In practice, most asset pairs have correlations between 0 and +1
- Diversification benefit begins at any correlation below +1.0
- The lower the correlation, the greater the risk reduction
Exam Tip: Gotchas
- Correlation of +1.0 provides ZERO diversification benefit - the portfolio's risk is simply the weighted average of individual risks. Any correlation below +1.0 reduces portfolio risk below the weighted average. The exam loves testing whether candidates know that diversification works at any correlation less than +1.0, not just negative correlations.
Efficient Frontier
The efficient frontier is the set of portfolios offering the highest expected return for each level of risk (or lowest risk for each return).
- Appears as a curved line on a risk-return graph representing all optimal portfolios
- Portfolios on the frontier: offer the highest return for each level of risk
- Portfolios below the frontier: suboptimal (same risk, lower return, or same return, higher risk)
- Portfolios above the frontier: impossible - the frontier is the upper boundary of attainable risk-return combinations
- No rational investor would choose a portfolio below the efficient frontier
- Adding the risk-free asset creates the Capital Market Line
Think of it this way: Imagine plotting hundreds of possible portfolios on a graph with risk (standard deviation) on the x-axis and expected return on the y-axis. The efficient frontier is the "best case" boundary curve connecting all the portfolios that offer the maximum return at each risk level. There is no "above" - if you could earn a higher return at the same risk, that portfolio would already be on the frontier.
Where Individual Securities Plot
A common misconception is that a high-flying individual stock should plot above the efficient frontier because it offers high potential return. It does not. Individual securities always plot inside the frontier, below and to the right of the curve.
- An individual stock carries two kinds of risk: systematic (market) risk and unsystematic (company-specific) risk
- A portfolio on the frontier is diversified - the unsystematic risk has been eliminated through correlation effects
- An individual stock still carries that unsystematic risk, so it has more total risk for the same expected return than an efficient portfolio
- Plotted on a risk-return graph, the single stock sits inside the curve: same return as a frontier portfolio could be achieved with less risk, or more return could be achieved at the same risk
| Position on Graph | What Plots There |
|---|---|
| On the frontier | Diversified, optimal portfolios |
| Below and to the right (inside) | Individual stocks, suboptimal portfolios |
| Above the frontier | Nothing - unattainable by definition |
| Leftmost point of the frontier | Minimum variance portfolio (still diversified) |
Exam Tip: Gotchas
- The exam loves the trap "where does a single stock plot vs. the efficient frontier?" Wrong answer to avoid: above the frontier. Nothing plots above the frontier. The frontier is the upper boundary of attainable risk-return space.
- Individual securities plot inside the curve (below and to the right) because they carry unsystematic risk that diversified portfolios on the frontier have eliminated.
- For the same level of risk as an individual stock, a diversified portfolio on the frontier always offers a higher expected return. That is the entire payoff of diversification.
Think of it this way: Imagine you and a fully diversified portfolio both take on the same amount of total risk (same standard deviation). The diversified portfolio earns more, because none of its risk is wasted on company-specific noise. Your single stock is paying for company-specific risk it gets nothing back for.