Modern Portfolio Theory: Definition & Examples

Instructor: Ian Lord

Ian is a real estate investor, MBA, former health professions educator, and Air Force veteran.

In this lesson, we will go over the foundations of modern portfolio theory. We will also look at how investors can use it to create an appropriate investment mix that optimizes risk.

Modern Portfolio Theory

John gets a bit of anxiety every time he turns on the financial news and sees that oil is down while real estate is up. He's come to the conclusion that he has no idea what the next great investment is. He wants to take an approach in his investing that allows him to achieve enough of a return to meet his retirement goals without having to worry as much about dramatic swings in the market. Modern portfolio theory presents a solution that can help John achieve this goal. Let's take a look at exactly what modern portfolio theory is and how investors can put the ideas into practice.


Modern portfolio theory is a model for maximizing investment returns which allocates a percentage of the total portfolio into different assets so that each one has their own level of risk. Instead of expecting a single asset class to provide a portfolio's returns, having multiple classes spreads out the risk. The concept was developed by Harry Markowitz in an article published in a 1952 issue of the Journal of Finance.

A risk averse investor such as John can implement modern portfolio theory by purchasing stocks and bonds according to different risk classes. The underlying idea is that when an asset class is on a massive downswing the total impact on the portfolio is reduced because other asset classes are still growing or at least not losing money as fast.

An asset class is a group of similar investments that share similar characteristics and as a group may behave differently from other groups. Some years a single class might do fairly well, and other years it loses money. A more risky investment will offer a higher potential return, but the corollary is that the investment could also experience a significant drop in value. Instead of evaluating an investment solely on its own risk and reward prospects, in modern portfolio theory the investment is considered in the context of how it impacts the total portfolio return.


Let's watch as John applies modern portfolio theory into his investments. He decides he wants to use four asset classes: Government bonds, US Stocks, International Stocks, and Real Estate Investment Trusts. Over the last ten years, each class has had the following hypothetical average returns:

  • Bonds - 3%
  • US Stocks - 8%
  • International Stocks -7%
  • Real Estate Investment Trusts - 8%

With his portfolio divided up into 25% chunks, we can determine how much each class contributes to the total portfolio return.

(0.25 X 0.03) + (0.25 X 0.08) + (0.25 X 0.07) + (0.25 X 0.08)

0.0075 + 0.02 + 0.0175 + 0.02

0.065 or 6.5%

The total expected portfolio return is 6.5%. With this knowledge John can modify the percentage placed in each investment if he wanted to make adjustments such as seek a higher rate of return. He could also shift more money into bonds if he would rather have a more consistent rate of return with less risk.

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