|When it’s time to make investing decisions, check your emotions at the door.|
~Bogleheads' Guide To Investing
Understanding and avoiding behavioral pitfalls will ultimately have a greater impact on investing success than any other factor. Since emotions and subsequent behavioral pitfalls are frequently associated with miscalculating risk tolerance and asset allocation, the new investor should be aware of behavioral pitfalls before making asset allocation decisions.
"Your investing brain does not just add and multiply and estimate and evaluate,” says Jason Zweig in his book, Your Money and Your Brain. “When you win, lose, or risk money, you stir up some of the most profound emotions a human being can ever feel.” 
“Financial decision-making,” says psychologist Daniel Kahneman in Zweig’s book, “is not necessarily about money. It’s also about intangible motives like avoiding regret or achieving pride.”
Common behavioral pitfalls
Basing decisions or estimates on events or values already known (the “anchor”), even though these facts may have no bearing on the actual event or value. Investors will tend to hang on to losing investments by waiting for the investment to break even at the price at which it was purchased. Thus, they anchor the value of their investment to the value it once had, and instead of selling it to realize the loss, they take on greater risk by holding it in the hopes it will go back up to its purchase price. [note 1]
A tendency to seek information that confirms one’s existing opinions and overlook or ignore information that refutes them. For example, when researching an investment, an investor might inadvertently look for information that supports his or her beliefs and fail to see information that presents different ideas. The resulting one-sided view can result in a poor investment choice.  “In short, your own mind acts like a compulsive yes-man who echoes whatever you want to believe,” says Wall Street Journal columnist Jason Zweig. [note 1]
The large changes of preferences that are sometimes caused by inconsequential variations in the wording of a choice problem. For example, purchasing an item. The price is advertised as either (1) "$10.00 with a $0.50 cash discount" or (2) "$9.50 and a $0.50 credit card premium." The price is the same. The purchaser feels much better with the first choice (allowing a discount), than with the second (adding a premium).[note 1]
The belief that heads on a coin flip is more likely after a string of tails, red in roulette is more likely after a string of black, and in general good luck is more likely after a string of bad luck, and vice versa. Investors may similarly expect an investment that has recently experienced losses to "revert to the mean" of its former upward trajectory, or anticipate increased risk after a period of steady gains. While examples of such patterns are easy to find in the past, they are of little value in predicting the future.
A human instinct that causes individuals to mimic the actions of a larger group rather than decide independently based on their own information. For example, in a bull market, an investor joins the crowd to avoid being the only one to miss out; in a bear market, he gets out to avoid being the only one to lose. In both cases, he abandons his own reasoning and concludes the majority must be right. Herd investors often don’t have a sound investment plan and they listen to market noise. 
Loss aversion is the emotional tendency to strongly prefer avoiding losses over acquiring gains. As an example, loss aversion implies that one who loses $100 will feel twice the emotional pain as another person will feel satisfaction from receiving $100. Common indications include checking your portfolio on an almost daily basis, selling funds before you intended to lock in profits, or selling when you didn't intend to in order to avoid further losses.
The tendency for people to put their money into separate accounts based on a variety of subjective criteria, like the source of the money and intent for each account. According to the theory, individuals assign different functions to each asset group, which has an often irrational and detrimental effect on their consumption decisions and other behaviors. For example, people often have a special "money jar" or fund set aside for a vacation or a new home, while still carrying substantial credit card debt.
Money illusion is the tendency of people to think of money in nominal terms, instead of inflation-adjusted terms. For example, some people will be happy to get a 1% nominal return on an safe investment during a year when inflation was 2%, yet they would be unhappy with a 1% loss on the same investment in a year with no inflation, despite the two situations representing a 1% annual loss in inflation-adjusted terms. Money illusion can lead people to underestimate the loss to inflation of a fixed nominal income stream over time, or to overestimate the value of a promised nominal income stream that starts far into the future.
Myopic loss aversion
Myopic loss aversion is loss aversion intensified by constant attention to short-term portfolio performance. This behavior leads to focusing on recent losses and increased selling/buying activity without regard to the overall portfolio or the long term view. Myopic loss aversion gives rise to poor portfolio management and lower returns. It also may help explain the equity risk premium.
Being overconfident in your investing abilities can lead to big investing losses. A main reason is that, in the short run, the ups and downs of the stock market are random happenings. Such unpredictable variations mean that intelligence, skill, and knowledge give you no edge, and thinking they do can be “hazardous to your wealth.”  “The only way to achieve everything you’re capable of is to accept what you are not capable of,” says Jason Zweig .
Paralysis by analysis
Investors have thousands of funds to choose from plus an abundance of market “noise” telling them what they should do. The more choices they have the harder it is for them to choose one, making it more likely they won’t make a choice and will fail to invest. For example, employees pass up billions every year in free money offered by their employer’s matching retirement plans. They do this simply because they can’t decide which investment course to take. 
The tendency to draw conclusions about the future behavior of an investment from only the recent past. This leads to investors chasing performance and then buying high and selling low. “When funds go on a streak of high returns, investors tend to get in right before the peak; then, when the hot streak goes cold, too many shareholders bail out at the bottom,” explains Jason Zweig 
A theory that says people anticipate regret if they make a choice that is wrong in hindsight, and take this anticipation into consideration when making decisions. Fear of regret can play a large role in dissuading or motivating someone to do something. In investing, the fear of regret can make investors either risk averse, or, motivate them to take greater risks.
We tend to anticipate greater regret from action than from inaction. For example, we may fear missing out on gains if we rebalance out of stocks (action) and the market keeps going up more than we fear losses if we hold tight (inaction) and the market goes down. However, we may fear those same losses more than the potential gains when deciding to invest new money in the market (action).
Fear of regret may also cause us to act if we will later be faced with the consequences of our inaction, such as being left out while our friend makes money on a stock tip or learning that we would have won a lottery.
Theory and background
Investing is not a science. It is a human activity that involves both emotional as well as rational behavior.
— John Bogle, The Clash of the Cultures
What is behavioral economics?
Behavioral economics explores the emotions and biases that lead investors and consumers to sometimes make irrational economic decisions. It also studies why and how their behavior does not follow the predictions of standard economic models that assume people make rational choices about spending money to “maximize their total satisfaction.” 
Many behavioral pitfalls are based on the finding that investors are risk averse. They dislike losses almost twice as much as they like comparable gains and may take on more risk hoping to avoid a loss than realize a gain. They may also abandon a sound strategy under the stress of a loss. Investors should carefully consider this when choosing their all-important asset allocation.
Investors may not sense risk-aversion bias when an asset allocation is made. It will, however, present itself during a market downturn or crash, when the emotional stress of a loss can cause panic. If the asset allocation was not properly based on their risk tolerance, they are likely to abandon the market and sell low.
Many ideas now accepted in behavioral economics can be traced back to groundbreaking contributions by mathematician Daniel Bernoulli, New Theory on the Measurement of Risk (1738)  and philosopher Adam Smith, The Theory of Moral Sentiments (1759) . The key to these early works was their elements of psychology, but psychology was subsequently rejected by economists at the turn of the 20th century because they thought it provided too unsteady a foundation for economics.
In the second half of the 20th century, several researchers challenged the dismissal of psychology as a branch of economics. George Katona, Harvey Leibenstein, Tibor Scitovsky, and Herbert Simon wrote books and articles suggesting the importance of psychological measures and bounds on rationality. They attracted attention, but did not alter the fundamental direction of economics. Additional work by Allais (1953) , Ellsberg (1961)  , Markowitz (1952)  , and Strotz (1955)  added more data, and the argument for psychology as a valid branch of economics became more convincing and generally accepted.
Enter Prospect theory
The final step in the advancement of behavioral economics as a significant field of economic research is often thought to have started with the work of psychologists Daniel Kahneman and Amos Tversky. Kahneman and Tversky began their joint work in 1969, and the excellent working relationship ultimately led to the publishing of their seminal paper, Prospect Theory, ten years later (1979).[note 2] Prospect Theory offers a framework for how people frame economic outcomes as gains and losses and how this framing affects people's economic decisions and choices. Tversky died in 1996. Kahneman received the Nobel Prize in Economic Sciences for his work in 2002.[note 3]
Prospect theory and the individual investor
The investor’s chief problem - and even his worst enemy - is likely to be himself.
— Ben Graham
Prospect theory is important because it brings awareness to the common and costly pitfalls that frequently trip up investors. There are two ways pitfalls arise: heuristics and cognitive biases.
Heuristics are mental shortcuts people use for processing complex information. The process falls back on experience by trial and error, rule of thumb, or common sense. While heuristics provide a fast decision, often those decisions are faulty when applied to investing. Anchoring is a heuristic.
Cognitive bias describes inherent thinking errors that humans make in processing information. When investors act on a bias, they do not explore the full issue and can be ignorant of evidence that contradicts their initial opinions. Confirmation bias is an example of cognitive bias.
- An example of framing effect (a form of anchoring) and confirmation bias on purchasing homeowners insurance is in this Boglehead forum post: Re: Sometimes Safer NOT to Carry Homeowners Insurance (!?).
- Kahneman, D., & Tversky, A. (1979). Prospect theory: An analysis of decision under risk. Econometrica, 47 , 263-291. Obtained from Daniel Kahneman's Publications, Woodrow Wilson School of Public and International Affairs, Princeton University.
- Much information is available directly at NobelPrize.org.
- Daniel Kahneman - Biographical A complete biography, including the background and development of prospect theory.
- The name "prospect theory" was deliberately chosen to be meaningless. If the theory ever became well known, having a distinctive label would be an advantage.
- Prize Lecture - Maps of Bounded Rationality Daniel Kahneman holds his Prize Lecture December 8, 2002, at Aula Magna, Stockholm University.
- Prize Lecture by Daniel Kahneman (Video), an overview of prospect theory presented by Kahneman, 38 minutes.
- Maps Of Bounded Rationality: A Perspective On Intuitive Judgment And Choice (PDF), Prize Lecture, Daniel Kahneman, December 8, 2002.
- Daniel Kahneman - Biographical A complete biography, including the background and development of prospect theory.
- Jason Zweig, Your Money and Your Brain: How the New Science of Neuroeconomics Can Help Make You Rich, Simon & Schuster (August 1, 2007) ISBN 978-0743276689
- Anchoring, investopedia
- Confirmation bias, investopedia
- Daniel Kahneman, Thinking, Fast and Slow, Chapter 7
- Taylor Larimore,Mel Lindauer, and Michael LeBoeuf, The Bogleheads' Guide to Investing : Chapter 19, Mastering Investments Means Mastering Emotions, Wiley; 1 edition (September 28, 2007), ISBN 978-0470067369
- Myopic Loss Aversion and the Equity Risk Premium Puzzle, Larry Swedroe, MutualFunds.com, October 12, 2016, viewed July 26, 2017.
- English translation in Bernoulli, D. (1954). "Exposition of a New Theory on the Measurement of Risk". Econometrica 22 pp. 23–36. JSTOR DOI:10.2307/1909829
- The Theory of Moral Sentiments, Adam Smith
- Allais, M.(1953). "Le comportement de l’homme rationnel devant le risque: critique des postulats et axiomes de l’école Américaine". Econometrica 21. pp. 503–546. JSTOR DOI:10.2307/1907921
- Risk, Ambiguity, and the Savage Axioms, Daniel Ellsberg
- Portfolio Selection, Harry Markowitz
- Strotz, R. H. (1955–56). "Myopia and Inconsistency in Dynamic Utility Maximization". Review of Economic Studies 23. pp. 165–180. JSTOR DOI:10.2307/2295722.
- Diego Zunino, History of "Prospect Theory": Birth and evolution of a successful paper, 2009/2010.
- Ariely, Dan (April 27, 2010). Predictably Irrational: The Hidden Forces That Shape Our Decisions. Harper Perennial. ISBN 978-0061353246.
- Belsky, Gary; Gilovich, Thomas (January 12, 2010). Why Smart People Make Big Money Mistakes and How to Correct Them: Lessons from the Life-Changing Science of Behavioral Economics. Simon & Schuster. ISBN 978-1439163368.
- Bendle, Neil (2013). Behavioural Economics For Kids: Second Edition. Fehr Advice. ISBN 978-3-033-04315-2. Free download from Behavioural Econ For Kids.
- Kahneman, Daniel (April 2, 2013). Thinking, Fast and Slow. Farrar, Straus and Giroux. ISBN 978-0374533557.
- Montier, James. The Little Book of Behavioral Investing: How not to be your own worst enemy. ISBN 978-0470686027.. Reviewed by forum member Robert T: Behavioral Investing - James Montier
- Barry Ritholtz, 10 Behavioral Economics/Psychology Books for Investors, blog article containing a list of 10 recommended books, July 30th, 2012.
- Swedroe, Larry (June 17, 2002). Rational Investing in Irrational Times: How to Avoid the Costly Mistakes Even Smart People Make Today. Truman Talley Books. ISBN 978-0312291303.
- Zweig, Jason (August 1, 2007). Your Money and Your Brain: How the New Science of Neuroeconomics Can Help Make You Rich. Simon & Schuster. ISBN 978-0743276689.
- Prospect theory, on Wikipedia
- Behavioral Finance - Investopedia
- Behavioural Finance, articles and papers
- Behavior and U.S. Investors: Behavioral Patterns and Pitfalls of U.S. Investors, U.S. Library of Congress, August 2010.
- How Behavioral Economics Differs from Traditional Economics, Richard H. Thaler and Sendhil Mullainathan, from the Library of Economics and Liberty.
- Wiki - Emotions and Investing (new investors - read this), forum discussion