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Confirmation bias can lead mental health professionals to find information that confirms their beliefs, overlook evidence that contradicts their beliefs, and fail to look for evidence that challenges their beliefs.
For mental health professionals conducting forensic evaluations, this bias can lead to incorrect conclusions. The consequences can change the lives of people who have been charged with a crime, says Dr. Phillip J. Resnick in a presentation for the International Forensic Psychiatry Lecture Series.
But forensic mental health professionals can take steps to reduce the likelihood of confirmation bias.
Why People Experience Confirmation Bias
Confirmation bias is one of many cognitive biases — systematic errors in thinking, which affect how we process information, perceive others, and make decisions.
It means people seek or interpret evidence to support their existing beliefs, expectations, or a hypothesis. They are unaware that a confirmation bias is influencing their views. This is different from conscious choices that people make when building a case.
People think in two ways: a fast, automatic, and intuitive way, as well as a slow, analytical, and deliberate way. The fast way of thinking often involves mental decision aids or general rules to arrive at answers quickly, especially when the answers are not immediately clear.
As a result, people may quickly form views that may be incorrect. If they later get conflicting information, they may experience discomfort and resist changing their mind. So they may find evidence that supports their original beliefs and explain away information that does not fit.
Serious Consequences
Dr. Resnick discussed how confirmation bias has led to incorrect conclusions in investigations of crimes, leading to charges against innocent people. Investigators have experienced confirmation bias even in investigations of evidence, such as fingerprints, bite marks, and fire debris.
Confirmation bias can affect mental health professionals in clinical practice. Once clinicians have about six years of experience, they start to rely on their intuition, making diagnoses and choosing treatments too quickly. One study showed that mental health professionals make a diagnosis within 30 to 60 seconds in an interview and do not change the diagnosis if they later get contradictory information. Another study showed that mental health professionals look for information that confirms their view of a case, instead of information that would change their minds.
When conducting forensic assessments, mental health professionals must review complex data that may be unclear. They must interpret the data, decide which data are relevant, and understand the person’s thoughts at the time of the crime. Confirmation bias can affect any of these steps. This risk increases when the data are complex or unclear.
Types of Cognative Biases
Confirmation bias is one type of cognitive biases. Others include anchor bias, adversarial bias, observer bias, and disgust bias.
Anchor bias, also called the primacy effect, means the professional relies too much on the first information they get. Lawyers may first give mental health professionals information that supports the side that the lawyer represents — defence or prosecution — to influence the professional’s view.
In adversarial bias, mental health professionals are influenced by whether they are working for the defence or prosecution. In one study, mental health professionals working for the defence or prosecution gave people who had committed sexual violence different scores on the Psychopathy Checklist–Revised. Mental health professionals may wish to please the lawyer, the lawyer may provide biased information, or the mental health professional may spend time with the lawyer and form a good relationship.
In observer bias, the professional’s existing thoughts, feelings, and beliefs influence their perceptions and conclusions. Political views may affect a mental health professional’s attitude toward the defence of not guilty by reason of mental illness. This may affect the likelihood that the professional will evaluate the person charged with a crime as having a mental illness.
Disgust bias involves a professional’s emotional response to details or images of the crime. This emotional response may affect their view of information they receive afterward.
Steps to Reduce Bias
Forensic mental health professionals can reduce, but never eliminate, confirmation bias in assessments. A survey found that most forensic mental health professionals think, incorrectly, that they can reduce bias through willpower. In fact, forensic professionals rarely recognize their own biases, although they identify the biases in experts on the other side of a case.
To be as objective as possible, professionals can take steps to reduce bias.
- They should not form an opinion until they have reviewed all the data and evidence.
- They should be open-minded when new information is found.
- They should find and review evidence for and against their first opinion.
- They should not learn the opinions of other professionals, which could influence their own views.
Consulting with a mentor or colleague can provide fresh insights and reveal the professional’s own biases.
For more details, watch the recording of the March 12, 2026, lecture, “Confirmation Bias: Enemy of Truth in Forensic Mental Health Assessments” on our website.
Written by Carolyn Brown and approved by Dr. Phillip J. Resnick.
