Mitch's Mailbox

August 30th, 2021

How Investor Biases Affect Buy and Sell Decisions


Frank H. from Port Arthur, TX asks: Hi Mitch, I’m curious if you have any insights on knowing when to sell? It’s difficult for me anytime I buy something that goes up (can it go up more?) or if I own a loser that I don’t want to sell at a loss. Is there a good rule-of-thumb out there that I’m missing?

Mitch’s Response:

That’s a great question, Frank, and I know that plenty of investors grapple with this issue. I can see from your question that there seems to be a lot of emphasis on price, i.e., how much-unrealized profit or loss you have on a particular position driving whether or not you should sell. There is a hazard with this approach you should watch for, which is known as the “endowment effect.”1

The endowment effect was coined by behavioral economist, Richard Thaler, here at the local University of Chicago (Zacks Investment Management is headquartered in Chicago). It says that the value you place on an asset greatly depends on whether or not you already own it.


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In other words, owning a stock with an unrealized gain will make you feel like a successful investor that is likely to be even more successful while owning a stock with an unrealized loss will make you think it’s only a matter of time before shares rebound. As you can see in both cases, simply owning the stock gets in the way of making an objective decision – investors want to be right no matter what.

Thaler and a group of economists started to close in on this type of investor behavior in the 1980s, when they ran an experiment using coffee mugs. There were three groups in the experiment:

When it came to assigning a price to the mug across the three groups, the value of the mug differed greatly based on whether or not a person already had it in their possession. The first group that had the option to buy the mug thought it should be worth $2.87, while the people who already had the mug thought its value should be $7.12.

When investing, it is important to consider that the endowment effect may be in play for stocks and holdings already in your portfolio. You may be reluctant to sell one of your winners that has gone bad or even to acknowledge that something has gone wrong. On the other end of the spectrum, the endowment effect could cloud your ability to see that one of your investments was never good to begin with.

One of the biggest features of knowing when to sell is getting past the endowment effect and staying true to your thesis to own. When you buy a stock or an asset, there were fundamental reasons for the purchase – do those fundamental factors still apply? Other questions to ask are if you did not already own the holding, would you buy it now? Or is there something else that better aligns with your strategy and thesis to own?

At the end of the day, a major part of successful investing, in my view, is knowing how to ‘get out of our own way’ and creating a repeatable, well-defined investment process that is driven by research and fundamentals—not gut feelings or past performance.

With that being said, now is the perfect time to base your investing decisions on research and fundamentals. Volatility has shifted the market in many ways this year, and for those who are afraid another stock market correction is around the corner, we want you to be prepared for it.

To help you do this, we have created a guide, 4 Keys to Navigating a Stock Market Correction3.. This guide answers question like – Are there any silver linings that come with it? What are some ways to navigate through it?

If you have $500,000 or more to invest and want to learn more, click on the link below to download our latest guide: 4 Keys to Navigating a Stock Market Correction3.


1 Wall Street Journal. August 13, 2021.’

2 ZIM may amend or rescind the free guide “4 Keys to Navigating a Stock Market Correction” for any reason and at ZIM’s discretion.

3 ZIM may amend or rescind the free guide “4 Keys to Navigating a Stock Market Correction” for any reason and at ZIM’s discretion.


Past performance is no guarantee of future results. Inherent in any investment is the potential for loss.

Zacks Investment Management, Inc. is a wholly-owned subsidiary of Zacks Investment Research. Zacks Investment Management is an independent Registered Investment Advisory firm and acts as an investment manager for individuals and institutions. Zacks Investment Research is a provider of earnings data and other financial data to institutions and to individuals.

This material is being provided for informational purposes only and nothing herein constitutes investment, legal, accounting or tax advice, or a recommendation to buy, sell or hold a security. Do not act or rely upon the information and advice given in this publication without seeking the services of competent and professional legal, tax, or accounting counsel. Publication and distribution of this article is not intended to create, and the information contained herein does not constitute, an attorney-client relationship. No recommendation or advice is being given as to whether any investment or strategy is suitable for a particular investor. It should not be assumed that any investments in securities, companies, sectors or markets identified and described were or will be profitable. All information is current as of the date of herein and is subject to change without notice. Any views or opinions expressed may not reflect those of the firm as a whole.

Any projections, targets, or estimates in this report are forward looking statements and are based on the firm’s research, analysis, and assumptions. Due to rapidly changing market conditions and the complexity of investment decisions, supplemental information and other sources may be required to make informed investment decisions based on your individual investment objectives and suitability specifications. All expressions of opinions are subject to change without notice. Clients should seek financial advice regarding the appropriateness of investing in any security or investment strategy discussed in this presentation.

Certain economic and market information contained herein has been obtained from published sources prepared by other parties. Zacks Investment Management does not assume any responsibility for the accuracy or completeness of such information. Further, no third party has assumed responsibility for independently verifying the information contained herein and accordingly no such persons make any representations with respect to the accuracy, completeness or reasonableness of the information provided herein. Unless otherwise indicated, market analysis and conclusions are based upon opinions or assumptions that Zacks Investment Management considers to be reasonable. Any investment inherently involves a high degree of risk, beyond any specific risks discussed herein.

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