Pro Investor Academy
LESSON 8.5
6 min learning time

Confirmation Bias and Overconfidence

Seeking only agreeing evidence, and mistaking a good outcome for a good decision, are two of the most common research traps.

The short answer

Confirmation bias is the tendency to seek and favour evidence that agrees with a belief you already hold, while overconfidence is an inflated sense of the accuracy of your own judgment or control over the outcome. Writing down an explicit anti-thesis, and what would prove you wrong, is a practical way to counter both.

Learning objectives

  • Define confirmation bias and describe how it distorts research
  • Define overconfidence and the illusion of control
  • Explain why a good outcome does not prove a decision was a good one
  • Describe the practice of writing an anti-thesis and disconfirming criteria

Confirmation bias

Once an investor forms a view, it becomes natural to notice supportive articles, agree with supportive comments and skim past anything that disagrees.

The result is a research process that feels thorough but has quietly excluded the evidence most likely to change a decision.

Overconfidence and the illusion of control

Overconfidence is an inflated sense of the accuracy of your own judgment. It often produces an illusion of control — a feeling of steering an outcome that is actually driven by many forces outside anyone's control.

This is common after a string of good outcomes, which can feel like confirmation of skill even when other factors were doing most of the work.

A good outcome is not proof of a good decision

A poorly reasoned decision can still work out, by chance. A carefully reasoned decision can still lose money, because uncertainty never fully disappears.

Judging a decision by its outcome alone, rather than by the quality of the reasoning at the time it was made, reinforces exactly the wrong lessons over time.

Writing the anti-thesis

Before acting, writing down the strongest case against the decision, and specifically what evidence would prove it wrong, forces a fairer look at the other side.

Revisiting that anti-thesis later — regardless of how the decision turned out — is what actually improves judgment over time.

Judging a decision by outcome vs. by reasoning
Outcome-based judgmentUnreliable

Chance plays a large role over short periods

Reasoning-based judgmentMore reliable

Evaluates the process, not the luck involved

Only one of these actually measures the quality of a decision.

Key concepts

Select a term to read its definition.

  • Confirmation bias

    Favoring information that supports what you already believe.

    View in the glossary
  • Overconfidence

    Overestimating your knowledge or your ability to predict outcomes.

    View in the glossary
  • Illusion of control

    Believing you can influence outcomes that are largely down to chance.

    View in the glossary
  • Anti-thesis

    The strongest case against your investment, deliberately written down.

    View in the glossary
See the full glossary

Why this matters

Research improves decisions; it does not remove uncertainty. Building in a deliberate check against your own bias is what keeps research honest.

Common mistakes

  • Reading only sources that agree with a position already taken
  • Treating a past winning decision as proof of skill without examining whether the reasoning was actually sound

Check your understanding

No score is recorded. This is only here to test whether the lesson landed.

1. What is confirmation bias?
2. True or false: a decision that produced a good outcome must have been a well-reasoned decision.
3. An investor writes down, before buying, the strongest case against the purchase and what evidence would prove it wrong. What bias does this practice most directly counter?

0 of 3 answered. Answer every question to see the answers.

Key takeaways

  • Confirmation bias narrows research to whatever already agrees with you
  • Overconfidence can create an illusion of control over outcomes driven by many outside forces
  • A good outcome does not prove a decision was well reasoned
  • Writing an anti-thesis, and what would disprove it, is a practical defence against both biases
Written by
Pro Investor Academy Research Team
Content type
Educational article
Published
2026-09-13
Last reviewed
2026-09-13

Sources

Loss AversionCreating Investment Discipline

Educational content only. This lesson is not investment, tax or legal advice and does not recommend buying or selling any specific investment. All investing involves the risk of loss.