What Is Bias?

A bias is an illogical preference or prejudice. It's a uniquely human foible, and since investors are human they can be affected by it as well. Psychologists have identified more than a dozen kinds of bias, and any or all of them can cloud the judgment of an investor.

Understanding Bias

Bias is an irrational assumption or belief that warps the ability to make a decision based on facts and evidence. Equally, it is a tendency to ignore any evidence that does not line up with that assumption.

Key Takeaways

  • Bias clouds decision-making judgment.
  • Investors are as vulnerable as any humans to making decisions based on irrational prejudices.
  • Understanding the types of bias can help you avoid falling victim to them.

A bias can be conscious or unconscious. When investors act upon them, they fail to absorb evidence that contradicts their assumptions.

Smart investors avoid two big types of bias: emotional bias and cognitive bias. Controlling them can allow the investor to reach an impartial decision based solely on the available data.

Relying on bias rather than hard data can be costly.

Common Biases in Investing

Psychologists have identified a number of types of bias that are relevant to investors:

  • Representative bias leads to a snap judgment on a question based on its apparent similarity to an earlier matter.
  • Cognitive dissonance leads to an avoidance of uncomfortable facts that contradict one's convictions.
  • Home country bias and familiarity bias lead to an avoidance of anything outside one's comfort zone.
  • Mood bias, optimism (or pessimism) bias, and overconfidence bias all add a note of irrationality and emotion to the decision-making process.
  • The endowment effect causes people to over-value the things they own just because they own them.
  • Status quo bias is resistance to change.
  • Reference point bias and anchoring bias are tendencies to value a thing in comparison to another thing rather than independently.
  • The law of small numbers is the reliance on a too-small sample size to make a decision.
  • Mental accounting is an irrational attitude towards spending and valuing money.
  • The disposition effect is the tendency to sell investments that are doing well and hang onto losers.
  • Attachment bias is a blurring of judgment when one's own interests or a related person's interests are involved.
  • Changing risk preference is the gambler's disease. A small risk, no matter what the outcome, creates a willingness to take on greater and greater risks.
  • Media bias and internet information bias represent uncritical acceptance of widely-reported opinions and assumptions.

Example of Bias

All of these common types of bias can be seen in the way some people invest. For example, endowment bias can lead investors to overestimate the value of an investment simply because they bought it. If they bought it for more than it is now worth, they think they're right and the market will surely correct its error. They may reinforce this belief by reviewing all of the reasons it was worth what they paid for it. They choose to ignore the reasons its value fell.

The rational investor would review all of the data, positive and negative, and decide whether it's time to take the loss and move on.