Cognitive Biases

We all make decisions everyday about all sorts of things. To make these decisions we need use our understanding of the past and the patterns we perceive in them. Unfortunately we humans are prone to some specific biases that can lead us astray of the correct choices. Listed below are common cognitive biases.

Decision Making, Behavioural and Belief Biases

Ambiguity Effect

The Ambiguity Effect is the tendency to avoid decisions about items that seem incomplete.

This can lead to choosing a complete less good item over a better less complete item. This is driven by the fear of poisoned fruit or of the unknown.

Anchoring Bias

The tendency to anchor our decisions on a single point of reference, frequently the first item presented.

The Anchoring Bias is the tendency to rate the range of information coming based on the assumption that the first item is the prototypical item. That is, all subsequent items of information are compared to the first item as if the first item is correct. This bias is also known as the Focalism Bias.

Consider a bag of apples. The first apple I pull out is about the size of an apricot. This first apple sets up my assumptions for how the rest of the apples should look. The next I pull out is the size of a large orange and so will seem quite large. The next is also the size of a large orange, the fourth is the size of a medium orange. The first small apple means the next three all seem very generous. Yet the average size is between the size of a medium and large orange, so in reality the first apple taken out is undersized. The Anchoring Bias makes it hard for us to look at all the information on its own merits and instead pushes us to base it on the first item’s merits.


The tendency to give human traits to non humans.

We understand people quite well and can empathise with others as if they are ourselves. Imagining

Attentional Bias

Our recurring thoughts affect what we perceive, blinding us to what is actually there.

Automation Bias

The tendency to rely on automated systems to make decisions, giving greater credence to the automatically generated information over a careful review.

Availability Heuristic

The tendency to overestimate the prevalence of most recently remembered items. This memory can be because of recentness, an unusual item or strong emotions.

Availability Cascade

A tendency to overestimate the importance of an item because it is being constantly repeated in the public forum. This can become cyclic as the public forum overestimates the importance of the item because it is in the public forum.

Backfire Effect

Also known as Digging the Heels In, this bias is the tendency to hold your ground in the face of uncomfortable contradictory evidence to your own position.

Bandwagon Effect

Related to Groupthink and Herd Behaviour, this is the tendency to go along with the crowd opinion or action regardless of the evidence.

Base Rate Fallacy

Also known as the Base Rate Neglect, this is the tendency to ignore general information in favour for specific cherry picked information and then assuming the general information is the specific information.

Belief Bias

The tendency to evaluate the logical strength of an argument based on how believable the conclusion of the argument is. That is, if you don’t believe the conclusion is reasonable, you ignore the argument that leads to that uncomfortable conclusion.

Bias Blind Spot

The inability to see one’s own biases and thus the assumption that everyone else is more biased.

Cheerleader Effect

Also known as the Group Attractiveness Effect, is the tendency for an individual to seem more attractive when they are in a group. The individual is attributed additional merit and attributes based on the group that the individual does not actually have on their own.

Choice Supportive Bias

The tendency to remember your choices as being better than they actually were.

Clustering Illusion

The tendency to assign meaning or to overestimate the importance of a random cluster of events.

Confirmation Bias

The tendency to search for, reinterpret and remember information that supports one’s preconceived notions.

Congruence Bias

The tendency to only test one’s hypothesis directly to prove it correct, instead of devising an indirect proof to prove it false.

Conjunction Fallacy

The tendency to overestimate the prevalence of a specific event over a more general event. This is a probabilistic error based on a specificity bias.

For example, the odds of A vs A + B are always higher for A, as B must also be true for the second.

Regressive Bias / Fallacy

The failure to realise expect naturally fluctuating cycles to regress to the mean (average), and thus overestimating the likelihood of exceptional results. For example, the weathers temperature goes up and down, but the bias will overestimate the odds of very hot or very cold weather.


Also known as Bayesian, is the tendency to insufficiently revise one’s opinion when confronted with suitable evidence.

Contrast Effect

The tendency to over or under estimate something based on the exposure of a recent anomalous example.

Curse of Knowledge

The difficulty of well informed people on a subject to understand how less informed people are likely to perceive a subject.

Decoy Effect

The tendency to change one’s choice to another existing choice when presented with a new option that is similar to a previous unselected choice, despite it not effecting the accuracy of that choice.

Denomination Effect

The tendency to underestimate the price of something when it is represented in smaller units (cents instead of dollars).

Disposition Effect

The tendency for someone to sell an item that is increasing in value but not to sell an item that is devaluating.

Distinction Bias

The tendency to over represent the difference between two options when evaluating them together rather than evaluating the on their own individual merits.

Dunning-Kruger Effect

The tendency for experts to underestimate their own ability and the unskilled person to overestimate their own ability.

Duration Neglect

The tendency to judge an experience on it’s peak and rate of drop off rather than how long it endures for.

Empathy Gap

The underestimation of emotions affecting a decision in the self or others.

Endowment Effect

The tendency of a person to want more for an item than they would be willing to sell for that item.


The tendency to categorise something due to a single or collection of traits despite it not truly fitting any set category. Similar to stereotyping. This is a contended issue.

Exaggerated Expectation

The tendency to expect a more extreme reality than is measured.

Experimenter’s Bias

Also known as Expectation Bias, the tendency for an experimenter to inflate or exaggerate the evidence that aligns with the expected outcome and to downgrade the evidence that conflicts.

Focusing Effect

The inflation of importance of the focus of one’s own aspect of an event.

Forer Effect

Also known as the Barnum Effect, the tendency to own general statements as if they applied specifically to the self. Often seen in cold readings or psychic forecasts.

Framing Effect

The change in prediction of static evidence based on how the evidence is framed and presented.

Frequency Illusion

Basically The Blue Car or Pregnant Lady effect. When one’s attention is drawn to notice something that is always around and thinking it has increased in frequency.

Functional Fixedness

The limiting of using an item only in the way it was intended

Gambler’s Fallacy

The illusion that the next random event is related to the previous events. Eg: Coin tossing

Hard-Easy Effect

The tendency to overestimate one’s ability on a task that is hard and to underestimate one’s ability on tasks that are easy.

Hindsight Bias

Often characterised by the phrase “I knew it all along”, the tendency to accurately “predict” previous events which were unknown at the time.

Hot-Hand Fallacy

A belief that a recent lucky experience is going to influence the next event favourably.

Hyperbolic Discounting

The tendency to prefer a choice that has a momentary payoff over a longer term payoff that the future self would not chose, despite thinking that one is using the same system of decision making.

Identifiable Victim Effect

The tendency to empathise better with a single victim than a group of victims who are all at similar risk.

IKEA effect

The increase in value placed on an item that the individual had a hand in making over another identical item.

Illusion of Control

The tendency to overestimate the control one has on external factors.

Illusion of Validity

The tendency to believe that any additional information will be relevant to a decision regardless of its own inherent worth.

Illusory Correlation

The tendency to perceive a pattern between two or more events that is not supported by evidence.

Information Bias

The tendency to want more information to help make a decision, even if that information is not useful.

Insensitivity to Sample Size

The tendency to underestimate the variation effect of a small sample size.

Irrational Escalation

Also known as the Sunk Cost Effect, The tendency to commit more resources to a decision that is not succeeding based on a feeling that prior resources committed will pay off, despite the new evidence indicating that the resources are being wasted.

Less-is-better Effect

The tendency to prefer a more complete smaller set to a incomplete, damaged or less valued larger set.

Loss Aversion

The evaluation that disposing of an item will cost more than keeping the item, or the use that the item might give.

Mere Exposure Effect

The tendency to prefer items that one is familiar with over other items, despite inherent qualities.

Money Illusion

Also known as the Price Illusion, the mistake for the quantity of dollars having an inherent value over what that number can purchase (nominal vs real).

Moral Credential Effect

The decrease in self awareness of one’s own moral misconduct based on the assumption that one has a good moral credential. For example, someone who believes they are gender aware has a greater likelihood of not checking the gender bias in their decisions.

Negativity Effect

The tendency to misattribute the causes of actions in people one does not like. Specifically negative actions are attributed to the disliked person while positive actions are considered to be environmental consequences.

Negativity Bias

The tendency to have greater recall of negative events over positive events

Neglect of Probability

The tendency to not consider probability when making uncertain decisions.

Normalcy Bias

The refusal or inability to plan for or react to an event that has not previously happened aka the unexpected.

Not Invented Here

A form of xenophobia which devalues items or knowledge presented from outside of the group.

Observer-expectancy Effect

Similar to the Experimenter’s Bias, the experimenter unconsciously manipulates or misinterprets data to promote the expected outcome.

Omission Bias

The tendency to overrate an action over an inaction that have a similar level of consequence.

Optimism Bias

Also known as Wishful Thinking or Positive Outcome Bias, related to but opposite to Pessimism Bias, the tendency to expect and overestimate favourable outcomes.

Ostrich Effect

The tendency to ignore an obvious consequence.

Outcome Bias

Also known as 20/20 Hindsight ,the tendency to judge a decision on the outcome rather than the merits of the decision at the time based on the situation at the time.

Overconfidence effect

The tendency to be overconfident about the accuracy of a decision. Example: In one test, subjects who were 99% sure of an answer were wrong 40% of the time.


The tendency to interpret randomness in terms of familiar patterns, such as faces, speech etc.

Parkinson’s Law of Triviality

Also known as Bikeshedding, the tendency of a group to give equal weight to a trivial decision because it seems easier. Example: not working on the nuclear power station, instead designing a better bike shed.

Pessimism Bias

Also called Catastrophising, related but opposite to optimism bias, the tendency to overestimate the probability of a negative outcome.

Planning Fallacy

The tendency to underestimate completion times of a task.

Post-purchase Rationalization

The tendency to justify to oneself the validity of a purchase after the fact.

Pro-innovation Bias

The tendency to overestimate the positive impact of an invention or innovation and underestimate the limits of weaknesses of the invention or innovation. Frequently seen in the media.

Pseudocertainty Effect

The tendency to avoid risk when the outcome is seen as positive, but the increase in risk when the outcome is seen as negative.


Used in Reverse Psychology, the tendency to do the opposite of the recommended action in an effort to express personal power.

Reactive Devaluation

The tendency to devalue a proposal or plan due to the source being with a perceive adversarial person or association.

Recency Illusion

The mistaken belief that things recently noticed must be new, rather than previously unknown.

Restraint Bias

The overestimation of one’s ability to be restrained in the face of temptation.

Rhyme as Reason Effect

The overestimation of accuracy of a statement that includes a rhyme.

Risk Compensation

Also known as the Peltzman effect, the tendency to take greater risks when the perception of safety increases. Example, driving faster because the car contains air bags.

Selective Perception

The tendency of expectations to affect perception. One does not see what one isn’t expecting, but does see what one expects.

Semmelweis Reflex

The tendency to reject new evidence that conflicts with an established paradigm.

Social Comparison Bias

The tendency to chose people to join one’s own group that don’t compete with one’s perceived strengths.

Social Desirability Bias

The tendency to overreport one’s own socially acceptable traits, aspects and behaviours, and underreport socially undesired traits, aspects and behaviours.

Status Quo Bias

The tendency to favour things that promote a continuation of the status quo, that is, keeping things the same rather than promoting change.


The tendency to expect an individual to reflect traits attributed to a group the individual is categorised as.

Subadditivity Effect

The tendency to underestimate the probability of the whole of something compared to the sum of the probability of its parts.

Subjective Validation

The tendency to believe that something is true regardless of the evidence because of a belief system, and or the tendency to believe a connection between things due to a belief system regardless of the evidence.

Survivorship Bias

The tendency to perceive the survivors of something and ignoring those who did not survive.

Time-saving Bias

The tendency people have of estimating time based on the nominal change in speed rather than the ratio of the change in speed. For example, increasing speed from 25 km/h to 50 km/h will save more time in a 100 km journey than increasing speed from 75 km/h to 100 km/h. Both have an increase in speed of 25 km/h, but the first halves the time it takes to get to 100 km (2 hour saving – 2 hours instead of 4 hours travel), while the second has a much smaller gain (20 minutes – 1 hour 20 vs 1 hour).

Unit Bias

The tendency to want to achieve the next unit in a task rather than stopping part way through.

Weber–Fechner Law

Describes the difficulty people have in comparing small differences in large quantities.

Well Travelled Road Effect

The tendency to underestimation of time taken or difficulty travelling familiar routes over the unfamiliar routes, hence a resistance to change even though gains are available.

Zero-risk Bias

Related to the Time-saving Bias, the tendency to want to reduce a small risk to zero rather than a large risk to low risk.

Zero-sum heuristic

The tendency to mistake a situation for a zero-sum situation (finite resources) when it is not (infinite resources). A zero-sum situation is where a finite resource is divided up for all participants, thus the more one has the less another has or the rest have depending on the number of participants.