Dual Process Theory

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The Dual Process Theory has been adapted from the psychology literature to describe how clinicians think when reasoning through a patient’s case. 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 a reflexive, intuitive, efficient process, and based on pattern recognition. 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. 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.

System 1	Intuitive	Fast Implicit/automatic Draws on prior experience Based on pattern recognition and mental shortcuts
System 2	Analytical	Slow Explicit, based on knowledge and logic Deliberate/rational, less susceptible to bias Careful analysis to avoid diagnostic errors in complex cases Requires considerable cognitive work

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. 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 utilising System 1 reasoning unless triggered to switch to System 2 when a patient does not neatly match one of their stored illness scripts and there is a diagnostic dilemma. And so they activate System 1 or System 2 thinking depending on the clinical scenario.

Example: Differential Diagnosis for Chronic Proximal Posterior Thigh Pain
Local causes	Referred pain
System 1
Soft tissue Hamstring tendinopathy	Referred pain Sacroiliac joint referred pain
System 2
Soft tissue Hamstring tendinopathy Semimembranosus or ischiogluteal bursitis Adductor tendinopathy Myositis ossificans of the hamstring muscle	Referred pain Sacroiliac joint Lumbar spine
Compartment syndrome Posterior thigh compartment syndrome
Neuropathic causes "Hamstring syndrome" Nerve entrapment of the posterior cutaneous or sciatic nerves
Vascular causes Iliac artery endofibrosis
Bony causes Pelvic bone tumours Ischial tuberosity apophysitis/avulsion fracture Femoral neck or shaft stress fracture

Like most models, the Dual Process Theory oversimplifies reality. 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. 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, dyspnoea on exertion, and worsening bilateral lower extremity oedema—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.

On review, Norman et al conclude that "Limited evidence suggests that strategies directed at encouraging both kinds of reasoning will lead to limited gains in accuracy."^[1]