Once you decide someone is unreliable, every late reply proves it and every early one goes unnoticed. Nobody is lying to you. You are simply reading the evidence with your mind already made up — and this happens to doctors and researchers too, which is where it gets serious.
📚 What you need to know
A cognitive bias is a distorted way of understanding the world, and it is a kind of heuristic.
Confirmation bias is the tendency to notice information that agrees with what you already believe, and to overlook information that does not.
It works on preconceived ideas about a person, an event, a situation or a group.
In research it is dangerous: findings that support the hypothesis get recorded, findings that challenge it get ignored.
That leads to low validity and reinforces stereotypes.
The study you need is Pavkov & Lewis (1989), on diagnosis of schizophrenia.
A filter, not a lie
The important thing to understand is that confirmation bias is not dishonesty. Nobody decides to ignore inconvenient evidence. The filtering happens before you are aware of it, which is exactly why it is so hard to correct.
Say you suspect a friend is annoyed with you. Every short message now looks like proof. The three warm messages they sent yesterday do not register at all, because they do not fit the story you are already telling yourself.
The belief does not just affect the conclusion. It affects which evidence ever reaches the conclusion.
Why it matters in research
Researchers are people, and a researcher who badly wants their hypothesis to be right is in a risky position. They may selectively record results that support it and quietly set aside findings that challenge it. The consequence is low validity, and where the research concerns groups of people, the further consequence is that stereotypes get dressed up as evidence.
Key study: Pavkov & Lewis (1989)
Part of the study
What happened
Aim
To investigate whether race and confirmation bias affect clinicians’ decisions when diagnosing schizophrenia.
Participants
Patients from four mental health hospitals in Chicago, USA. Around two thirds were male and around two thirds were aged 18 to 34. The hospitals served both Black-dominant and White-dominant neighbourhoods.
Procedure
Patients were interviewed in hospital. One interview was diagnostic, carried out by an expert who had not been told the aim of the study. The researchers ran a second interview on social and psychological measures, such as how socially integrated the patient was and how aware they were of their condition.
Results
Black patients were more likely than White patients to receive a diagnosis of schizophrenia. This was particularly true in hospitals in Black-dominant neighbourhoods, and it happened even where a patient had previously been diagnosed with a different illness.
Conclusion
Patients may be misdiagnosed partly because of skin colour. Clinicians appear to be working from stereotypes rather than purely from the evidence in front of them.
Notice the blind expert. The diagnostic interviewer was not told what the study was testing. That is a deliberate control against the researchers’ own confirmation bias — which is neatly ironic in a study about confirmation bias, and worth pointing out in an essay.
Evaluating the research
What is strong about it
Understanding confirmation bias should help people avoid labelling and stereotyping others, particularly those outside their own group.
Interviews produce rich, detailed data with strong explanatory power, and give real insight into the patients’ experiences.
The consequences here are serious and real. A misdiagnosis changes someone’s treatment, medication and life.
What is weaker about it
Four separate hospitals means four sets of staff, equipment and local conditions. Those extraneous variables make firm conclusions difficult.
Patients may show social desirability bias in interviews, which weakens validity.
Diagnoses were based on a single interview, which is arguably an unrealistic basis for accurate diagnosis in the first place.
Link to concepts
Causality: confirmation bias is hard to measure because it is a subjective, ill-defined variable. What looks like bias may reflect other things entirely — clinician fatigue, understaffing, limited time and resources. The finding is consistent with racial bias, but it does not on its own prove it caused the pattern.
Bias: the study itself may suffer sampling bias. All the participants came from four hospitals in one city, which limits representativeness. Replication in other states and countries with more diverse samples would be needed before drawing wide conclusions.
EXAM PRACTICE
Discuss one cognitive bias in thinking and decision-making. [22]
Step 1: define and locate it
Confirmation bias: focusing on information that supports an existing belief and ignoring what does not. A heuristic, so it saves effort at the cost of accuracy.
Step 2: explain the mechanism
The filtering happens before awareness, so the person feels they are being objective.
Step 3: the study
Pavkov & Lewis (1989). Four Chicago hospitals, diagnostic interview by a blind expert plus a social-psychological interview.
Step 4: the findingBlack patients were more likely to be diagnosed with schizophrenia, especially in Black-dominant neighbourhoods, and even where a different diagnosis already existed.
Step 5: weigh it up
Serious real-world consequences and rich interview data, but extraneous variables across four hospitals, possible social desirability bias, and alternatives such as fatigue and understaffing.
Bias + mechanism + study + finding + evaluationNaming a rival explanation is what earns the analysis marks here.
💡 Exam tip
Stress that the bias is unconscious. Describing it as deliberate misses the point.
The blind interviewer is a methodological strength most students walk straight past.
Confirmation bias in research is a separate application worth a paragraph of its own.
This topic is socially sensitive. Write about the findings carefully and precisely.
Pairs well with anchoring bias if a question asks you to compare two biases.
⚠ Common mix-up
Saying the study proves racism caused the diagnoses. It shows a pattern consistent with bias; alternative explanations remain.
Confusing it with anchoring bias. Anchoring is about a first figure; confirmation bias is about defending a belief.
Calling it an experiment. It used interviews, so no IV was manipulated.
Treating it as rare. It affects clinicians, researchers and everyone else, constantly.
Claiming awareness cures it. Knowing about a bias does not switch it off.
Up next: Classical Conditioning: Learning by Association — away from how we think and into how behaviour gets learned without any thinking at all.
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