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Sources of Bias and Evidentiary Standards in Forensic Psychology Expert Reports: Best Practices Forensic Psychology Guidelines

By: Dr. Michael J. Perrotti, Ph.D.
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Voir dire of experts as to credentials and experience needs to be viewed as only one aspect of the vetting process of forensic work product. Experts, as much as anyone else, are prone to conscious and unconscious sources of bias. This rather significant problem area in forensic reports is discussed by (Stuld and Simon, 2013) under the rubric of heuristic and cognitive biases. Heuristics addresses how people arrive at decisions or reach conclusions in complex family law, civil, or criminal matters. Errors in this area are of concern when experts focus or overly attend to only a part of the problem or afford overemphasis to certain factors and minimize the contribution of other factors. One can also see this problem in medicine. A patient may complain of pain in a particular body region whereas the provider focuses only another medical issue.

The sources of cognitive biases are unfamiliarity with Best Practice Standards, ignorance of the research literature and use of methods not supported by the literature, eg, play therapy, anatomical dolls in assessment of child sexual abuse allegations.

Anchoring Bias

This heuristic is characterized by focusing on only selected information during the evaluation at the expense of other information. This results in not being able to see the forest through the trees. Once an expert is settled on a particular "set" of assumptions, all other assumptions are interpreted consistent with the initial assumptions. This writer has heard a litany of complaints from parents concerning child custody evaluators being biased against them in reports. This particular bias is also seen in the police investigation wherein investigators have already arrived at conclusions about a defendant before the investigative interview concludes. This then leads to comments to investigators who "contaminate" the interview. One also comes across rebuttal experts who "always" seem to opine on the "side" of retaining counsel. Thus, individuals engage in "doctor shopping" to find these individuals.

Confirmatory Bias

A confirmatory bias is a tendency to form an opinion or impression prior to collecting all data and not considering other data at odds with the expert's initial opinion and impressions.


This type of bias is seen when an evaluator is affected by characteristics of the individual being evaluated to the detriment of focus on all of the data. One frequently observes the effects of this type of bias on an evaluator's perception of a particular parent in a child custody evaluation. Data unfavorable to the evaluator's position with respect to the individual being evaluated is not given due consideration. Standards for experts for the Courts dictate objectivity by an expert with opinions grounded in data that are research driven not driven by the evaluator's biases or preconceptions. One sees this type of bias in cases where the evaluator disregards data from other third party sources or, in the case of Jodi Arias, the defense expert apparently did not interview anyone other than the defendant. This procedure is suspect as it depends solely on self-report with a defendant invested in presenting positively skewed information in order to extricate themselves from untenable legal situations.

Data Gathering Biases

A source of bias, viz, data gathering bias, results in evaluators placing more credence on certain types of data and procedures than others, viz, play therapy as opposed to objective psychological testing in assessment of child sexual abuse allegations. This writer has seen evaluators draw conclusions on parenting and bonding in child custody evaluations solely on the basis of a scale score on the MMPI-2-RF.

Research Bias

This writer has seen experts who cite only research to support their position while giving no attention to research not supporting their position. The research they present is cherry-picked to support their opinion and point of view.

Third Party Observers (TPOs)

Some evaluators have third parties in the examination room during forensic assessments. The literature speaks in unison on this point, viz, forensic assessments are not standardized with respect to other individuals being in the examining room with the evaluator and subject. The result of this is a non-standardized forensic assessment, resulting in questionable reliability. This arrangement distorts the examiner-subject field. A third-party, especially a party who has antipathy towards another party in the room with the child being interviewed, has a significant adverse influence on the response set of the subject being interviewed.

Additional Factors Constituting Forensic Frye and Daubert Evidentiary Standards

It is important to vet an expert not just according to credentials and experience, but also conduct a critique of their methodology. Whereas the Frye "General Acceptance" Standard requires teaching based upon methodology generally accepted in the scientific community, Daubert requires the theory and/or techniques utilized by the expert to have been tested (falsifiability), whether techniques utilized have been suggested to peer review, error rate(s) of the technique and criteria governing application of techniques.

Concluding Comments

Attorneys and consumers of forensic psychological assessments would be wise to view forensic assessments through the lens of guidelines (Forensic Psychology Speciality Guidelines, American Psychological Association Practice Guidelines for Child Custody Evaluations) as well as the other criteria discussed in this paper. Clients and the Courts deserve no less!

Dr. Perrotti received his PhD in Clinical Psychology from Alliant University in San Diego, CA. He is a licensed psychologist in California and Pennsylvania. Dr. Perrotti is a member of the National Register of Health Service Provider in psychology and the National Academy of Neuropsychology. He was an Assistant Professor of Psychiatry and Behavioral Sciences at the Keck School of Medicine, USC from 2005-2006. Dr. Perrotti is the author of numerous publications in forensic psychology and assessment, traumatic brain injury in college, professional sports and military populations, and child trauma and complex PTSD.

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