Key Study: Confirmation bias: why psychiatrists stick to wrong preliminary diagnoses (Mendel et al. 2011)

Travis DixonAbnormal Psychology, Cognitive Psychology

Depression or Alzheimer's? Mendel et al.'s study found that the diagnosis clinicians made of an old man could be influenced by the information they were given beforehand, which demonstrates the effects of confirmation bias on diagnosis.

Clinical bias can affect the validity and reliability of diagnosis. One example of a clinical bias that could have an effect is confirmation bias. While the use of classification systems could reduce the influence of clinical bias, it might also lead to it.

Clinical bias is a general term that refers to any cognitive bias that can affect the diagnosis made by a clinician (e.g. a psychiatrist or psychologist who is treating a patient). There are many ways that bias can affect diagnosis, but one of the most common is confirmation bias – the tendency to focus on and remember information that is consistent (i.e. confirms) our existing beliefs and opinions.

Concept for consultation with psychologist

When making a diagnosis, clinicians should seek information that contradicts their original diagnosis and not just focus on information that confirms it.

In the context of making a diagnosis, confirmation bias can affect the validity (accuracy) of a diagnosis because it can affect how clinicians seek and interpret information. For example, if a clinician has an assumption that a person has a particular disorder, this may affect how they analyze the patient as they may focus on information that is consistent with their original diagnosis.

This was demonstrated in Mendel et al.’s study.

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Key Study: Effects of confirmation bias on diagnosis (Mendel et al. 2011)

The aim of this study was to see if psychiatrists would be affected by confirmation bias when making a diagnosis. They tested 75 psychiatrists by giving them a summary of a case study of an old man (they also ran the same test with 75 medical students). The first summary was written so that the most probable diagnosis was depression, but if all the information was revealed the actual correct diagnosis was clearly Alzheimer’s disease. After an initial diagnosis, participants could then ask to receive additional information related to either diagnosis (depression or Alzheimer’s). The researchers measured accuracy of diagnosis and also the correlation between what further information was asked for and the % correct diagnosis. The results showed that 30% of psychiatrists made an incorrect diagnosis, which suggests by itself that it’s difficult to make a valid diagnosis. A total of 13% of psychiatrists and 25% of students showed confirmation bias when searching for new information after having made a preliminary diagnosis. More specifically, the results showed that confirmation bias can affect diagnosis because the psychiatrists who searched for more new information consistent with their original incorrect diagnosis were more likely to keep that incorrect diagnosis. However, if psychiatrists asked for more information that contradicted their original diagnosis they were less likely to make an incorrect diagnosis. For example, if a clinician first diagnosed the patient with depression and they only asked for more information about depression, they were more likely to keep that original, incorrect diagnosis – in other words, if they showed confirmation bias then their diagnosis would be more likely to be incorrect. However, if they diagnosed depression but asked for more information about symptoms related to other problems like Alzheimer’s they were more likely to get a correct diagnosis.

More results. “Psychiatrists conducting a confirmatory search made a wrong diagnosis in 70% of the cases compared to 27% for a disconfirmatory search or 47% for a balanced information search (students : 63, 26 and 27%). Participants choosing the wrong diagnosis also prescribed different treatment options compared with participants choosing the correct diagnosis.” (Mendel et al. 2011)

One conclusion from this study is that when making a diagnosis, clinicians need to be aware of their own potential for bias and to make sure they are considering all possible options and available information. It also shows the importance of getting the diagnosis right so that the right treatment can be provided.

You can read the full study here.

Critical thinking considerations

  • What are the limitations of Mendel’s research? Think about how they only gave the participants a very limited amount of information. Why is it understandable (and perhaps even desirable) that they would trust the judgement of the initial diagnosis?
  • While the results showed 30% got the diagnosis wrong, it means 70% got it right – is that an acceptable number? How accurate are we expecting diagnosis?
  • What are the possible limitations of having two separate diagnoses to test the reliability of the diagnosis?
  • How could the effects of labelling be avoided and still have two diagnoses to test reliability?

Exam Tips

  • The IB often uses the phrase “the role of (….x…) in…(…y…)” This can be confusing language, so you can just think of any question like this as asking how (….x….) influences (…y…). For this topic, you need to be able to explain how clinical bias can influence diagnosis, using studies to support your answer.
  • You may be asked to evaluate one study related to factors influencing diagnosis. Choose a study that you can apply to multiple topics and make sure you can come up with 2-3 good evaluative points about that study.
  • A good place to start when evaluating studies is thinking about generalizability and ethical considerations, as these are relevant to all studies.
  • ‘Clinician’ is a good term to use because it covers anyone treating a patient, including psychiatrists and psychologists.
  • The example of confirmation bias here could be used as one bias in thinking and decision making in the cognitive approach. It would make for an excellent addition to an essay about biases. 

Mendel R1, Traut-Mattausch E, Jonas E, Leucht S, Kane JM, Maino K, Kissling W, Hamann J. Psychol Med. Confirmation bias: why psychiatrists stick to wrong preliminary diagnoses. 2011 Dec;41(12):2651-9. doi: 10.1017/S0033291711000808. Epub 2011 May 20. (Link)(Full)