That is the conclusion of a study published in this month's issue of the Journal of Experimental Psychology, General conducted by Woo-kyoung Ahn, associate professor of psychology at Vanderbilt, and Nancy Kim, visiting faculty at Wesleyan University.
"For the last 22 years, clinical psychologists have been told that they should make diagnoses based solely on a checklist of symptoms. But our results indicate that individual theories still play a surprisingly strong role. Clinicians are significantly more likely to diagnose patients with a mental disorder when the person exhibits symptoms that are central in the clinician's own theories of the disorder. Similarly, they are far less likely to make the same diagnosis for a patient with symptoms that they consider to be peripheral," says Ahn.
To determine the impact that personal theories have on the diagnosis of mental disorders, the two researchers had 35 clinicians and 25 clinical trainees perform four basic tasks. First, they determined the participants' theoretical views by having them draw the relationships between the symptoms of some disorders. For example, one clinician might consider the symptom "excessive social anxiety" as a key indicator of one type of personality disorder and "unusual perceptual experiences" as peripheral, while another clinician may list "unusual perceptual experiences" as a central symptom and "excessive social anxiety" as peripheral for the same disorder.
Next, the researchers asked the subjects to identify the relative importance of the symptoms associated with these disorders. Then they asked them to diagnose some hypothetical cases. Finally, several hours after their diagnoses, they tested participants' memories of the symptoms of the patients that they diagnosed.
The researchers found that both the practicing clinicians and the graduate students that they tested held complicated theories about various disorders - ranging from schizophrenia, major depression and anorexia nervosa to a variety of personality disorders - and the relative importance of various symptoms. They also discovered that there was not a lot of agreement among the individual theories.
According to Ahn, such theorizing appears to be part of human nature and is not necessarily bad. In the case of mental disorders, however, there is no basic understanding of the underlying causes of these conditions. As a result, expert theorizing can be idiosyncratic and is likely to lead to conflicting diagnoses.
Not only did the study find that the theories held by individual clinicians about a given disorder affected his or her diagnosis, it also found that the theories influenced the clinician's recollection of a patient's symptoms. Kim and Ahn found that the experts were more likely to remember symptoms correctly if they judged them to be central to a given disorder. They were far more likely to forget symptoms they considered peripheral. Even more striking was how clinicians' theories affected their memory of patients' symptoms. When they diagnosed patients with a specific disorder, who did not have some of symptoms that the clinicians considered central, they were likely to remember that the patients had these key symptoms when, in fact, they had not.
Because the study is based on hypothetical cases, its relevance to actual clinical practice remains unclear. But Ahn predicts these problems will be even more pronounced when clinicians are dealing with real patients. "If anything, I think this effect may actually be stronger because there are many more ambiguities when working with actual patients. For instance, clinicians' theories may influence their interpretation of patients' symptoms or characteristics, such as mood or level of hygiene," Ahn says.
She and her co-author hope that their results will have an impact on the next issue of the Diagnostic and Statistical Manual of Mental Disorders, the document that contains the definitions of more than 300 mental disorders that clinicians use in making formal diagnoses. The current edition, number four, was issued in 1994 and describes each disorder in terms of checklists of symptoms.
"DSM-IV did as good a job as possible in trying to objectify these diagnoses," says Ahn. "But, if that is not possible, then we have to do a better job of categorizing the disorders."
The study was made possible by funding from the National Science Foundation and the National Institute of Mental Health.
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