Behaviour
Confirmation bias works hardest on things you already own
Once you hold something, your reading changes. Supporting evidence becomes obvious and contrary evidence becomes questionable.
These are listed in the order worth acting on, which with confirmation bias in a portfolio is not the order they are usually presented in.
What matters most
- Ownership changes how evidence about a holding is evaluated.
- Search behaviour shifts toward sources likely to agree.
- Deliberately seeking the strongest opposing case is the only practical counter.
Ownership changes reading
After buying, the same article that would have looked balanced beforehand reads as either supportive or biased. This is not a failure of intelligence; it operates below the level of deliberate reasoning and affects experts too. It is strengthened by anything that increases commitment, such as having recommended the holding to someone.
The result is a slowly hardening position supported by an increasingly selective evidence base.
Search is where it starts
Most of the effect happens before evaluation, in which sources you consult and which questions you type. Searching for reasons a holding is good returns exactly that, and the results feel like research.
Communities organised around a particular approach compound this by filtering what you see. The fix begins with the query rather than with the judgement.
Seek the strongest opposing case
Look specifically for the best argument against a holding, made by someone competent rather than by an obvious opponent. A weak opposing argument is worse than none, because dismissing it increases confidence. If you cannot state the strongest case against your position, you do not yet understand the position.
This is uncomfortable and is the entire value of the exercise.
Pre-commit to what would change your mind
Write down, at the time of purchase, what evidence would cause you to sell. A position that nothing could falsify is a belief rather than an investment decision. Written criteria are evaluated by rules rather than by your current reading of the evidence.
That is the same protection that pre-set sell criteria provide against every other bias.
It affects strategy as much as holdings
Confirmation bias applies to approaches as well as to products: indexing, active management, particular allocations. People generally read the literature that supports the approach they already follow, which is why disagreements persist. The evidence in this field is genuinely mixed in places, which gives selective reading plenty of material to work with.
Put simply, acknowledging where the evidence is weak on your own side is a good discipline and a rare one.
Limits of self-correction
You cannot detect this bias by introspection, since it operates on the material introspection uses. Structural fixes work better: written criteria, scheduled reviews, deliberately reading opposing sources.
A second opinion from someone with no stake helps, which is one legitimate function of regulated advice. For anything specific to your circumstances, that advice should come from someone qualified where you live.
Everything above, in order of what to do first
- Ownership changes reading. After buying, the same article that would have looked balanced beforehand reads as either supportive or biased.
- Search is where it starts. Most of the effect happens before evaluation, in which sources you consult and which questions you type.
- Seek the strongest opposing case. Look specifically for the best argument against a holding, made by someone competent rather than by an obvious opponent.
- Pre-commit to what would change your mind. Write down, at the time of purchase, what evidence would cause you to sell.
- It affects strategy as much as holdings. Confirmation bias applies to approaches as well as to products: indexing, active management, particular allocations.
- Limits of self-correction. You cannot detect this bias by introspection, since it operates on the material introspection uses.
The takeaway
Search for the best argument against what you own. If you cannot find one, you have not looked.
The version you keep doing is the version that works.
Questions readers ask
How do I read about my own holdings fairly?
Search for the strongest criticism rather than for information, and write down in advance what would count as evidence against the position.
Does this apply to index investing too?
Yes. Any approach can become a position you defend rather than a decision you review. The evidence in favour of low-cost diversification is reasonably strong, which does not exempt anyone from checking.
