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Clinical Reasoning Exercises

Dual Process Theory Overview


Reza Manesh, Denise M. Connor, Jeff Kohlwes


The Dual Process Theory has been adapted from the psychology literature to describe how clinicians think when reasoning through a patient’s case.1 The dual processes, or System 1 and System 2, work together by enabling a clinician to think both fast and slow when reasoning through a patient's presentation.

System 1 is intuitive, efficient, and based on pattern recognition.2 Reasoning using System 1 often occurs so quickly that we do not explicitly recognize it as a distinct cognitive process. For example, a post-operative patient with sinus tachycardia, asymmetric lower extremity edema, and hypoxia is recognized immediately as having a pulmonary embolus by an experienced clinician. This rapid thinking draws on prior clinical experience, and is invaluable in helping busy clinicians accurately assess and treat patients with straightforward presentations.

In contrast, System 2 is an analytical cognitive process that is time intensive and deliberate.3 It involves the conscious, explicit application of an analytical approach to arrive at the correct diagnosis. An HIV positive patient with a CD4 count of 50 with fevers, weight loss, headaches, diarrhea, and recent travel to South Africa would likely activate System 2 reasoning given the myriad diagnostic possibilities. Complicated or atypical patient presentations that do not closely match known patterns require clinicians to slow down and systematically consider multiple potential etiologies to avoid making diagnostic errors.  

Which system is activated at a given time depends on two factors: the providers’ prior experience with a particular clinical presentation, and their ability to activate the appropriate illness script that sufficiently explains the patient’s clinical syndrome.3,4 Medical students tend to utilize System 2 thinking more often than System 1 thinking since they have insufficient clinical experience to accurately reason through a case using pattern recognition alone. In contrast, seasoned clinicians practicing in a familiar setting spend more time utilizing System 1 reasoning unless triggered to switch to System 2 when a patient does not neatly match one of their stored illness scripts.

Like most models, the Dual Process Theory oversimplifies reality.5 In real-world practice, a clinician’s reasoning process is unlikely to fall exclusively into either category, but rather oscillates between the two, even within a single case.6 Choosing the right analytic frameworks to use, and selecting the appropriate clinical features to consider are difficult tasks and require practice. To avoid mistakes, experts often check a diagnosis they arrived at quickly through System 1 reasoning by applying System 2 reasoning to the case.

Diagnostic errors can occur with System 1 or System 2 thinking. However, the types of errors clinicians are most at risk for making differ depending on which end of the spectrum they are operating in. For example, exclusively utilizing System 1, and being too reliant on fitting a patient into a previously stored pattern, may lead clinicians to unconsciously ignore key aspects (history, exam, labs, or imaging) of a patient’s presentation that do not fit with their initial diagnosis, leading them to anchor on an incorrect diagnosis. An elderly man with jaundice, weight loss, and a pancreatic mass should make a clinician reflexively consider pancreatic adenocarcinoma. However, if the endoscopic biopsy results are benign, then presuming the biopsy results are false and proceeding with a Whipple procedure without first pausing to consider other causes of this clinical picture (e.g., IgG4-related disease) would represent a failure to slow down and switch to System 2 thinking. In contrast, System 2 may place undue emphasis on a particular finding from history, exam, or diagnostic testing. For example, a patient with coronary artery disease, dyspnea on exertion, and worsening bilateral lower extremity edema—but normal pro-BNP—still likely has heart failure. Undue emphasis on the normal pro-BNP may unnecessarily broaden the differential to include cryptogenic organizing pneumonia, resulting in unnecessary tests and delays in appropriate treatment.

Illustrative Teaching Case:

[JGIM article available for free download from PubMed Central]

In “NSTEMI or Not: A 59-Year-Old Man with Chest Pain and Troponin Elevation,”7 the discussant initially utilizes System 1 reasoning and intuitively prioritizes acute coronary syndrome (ACS) as the most likely cause of the patient’s chest pain. However, the clinician notices that the patient has radicular pain to his leg which causes the discussant to pause, as it is not part of his illness script for ACS. When there is a mismatch between the problem representation and a physician’s leading  illness script, expert clinicians pause and switch from System 1 to System 2 thinking.8 In this case, this pause allows the discussant to slow down and utilize an analytic approach to consider other possibilities, eventually arriving at the correct diagnosis.

The slides for this case, which include an embedded teaching guide, provide a didactic approach for teachers interested in developing their trainees’ understanding of the clinical reasoning process.  The slides can be viewed online here but for best results, download the side deck from Slideshare.


For additional resources to learn more about the dual process theory visit:, click on the link—Dual Process Model of Reasoning.


  1. Klein G. Naturalistic decision making. Hum Factors. 2008;50:456–60.
  2. McCormic JS. Diagnosis: the need for demystification. Lancet 1986;2:1434-5.
  3. Elstein AS, Schwarz A. Clinical problem solving and diagnostic decision making: selective review of the cognitive literature. BMJ 2002;324:729-32.>
  4. Schmid HG, Norman GR, Boshuizen HP. A cognitive perspective on medical expertise: theory and implication. Acad Med 1990;65:611-21.
  5. Monteiro SM, Norman G. Diagnostic reasoning: where we’ve been, where we’re going. Teach Learn Med. 2013;25:26-32.
  6. Eva KW. What every teacher needs to know about clinical reasoning. Med Educ 2005;39: 98–106.
  7. Schleifer JW, Centor RM, Heudebert GR, Estrada CA, Morris JL. NSTEMI or Not: A 59-Year-Old Man with Chest Pain and Troponin Elevation. J Gen Intern Med 2012;28(4):583-90.
  8. Moulton CA, Regehr G, Mylopoulos M, MacRae HM. Slowing down when you should: a new model of expert judgment. Acad Med. 2007 Oct;82(10):S109-16.


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