“Dual process” theories seek to reconcile the traditional dualisms of mind and body (Descartes), conscious and non-conscious mind(Freud). This has provided new perspectives on the practice and teaching of medicine. This article will briefly review dual process theory and an example of its implementation in surgery.
History of Dual Process Theory
In 1975, Wason and Evans suggested two processes of reasoning were needed to explain the results of the “Wason card selection test”. The conscious reasoning of their subjects did not match the unconscious bias that actually directed their behaviour. This was similar to an expert physician who provides an accurate diagnosis while the explanation for that diagnosis suggests post hoc rationalisation.
Daniel Kahneman summarised evolving characteristics of the two systems in his acceptance speech for a Nobel Prize in 2002. The oldest system, in an evolutionary sense, is System 1, which is slow to learn but fast, automatic and effortless to execute. System 1 is context dependent and bound to the emotional centres that activate it. System 1 is outside direct consciousness, its existence inferred from biases and attitudes. System 2 is more recently acquired. At first glance it is the poor cousin, being slower, effortful and relatively easily overwhelmed, particularly when an individual is stressed in other ways. (Hungry, angry, late, or tired). The advantage of System 2 is its flexibility; rules can provide decisions and judgements in new situations where the novel context cannot generate a System 1 solution. System 2 may also contain metacognitive functions such as selecting the most appropriate processing system for a given circumstance.
It is important to understand that these systems work in parallel, not in opposition. System 1 is responsible for the “heavy lifting”, directing the majority of our behaviour. System 2 is the “icing on the cognitive cake”. It is all that we see on first introspection. As a result we make the reasonable, but incorrect, assumption that it is the primary control centre. Like the icing on the cake, System 2 cannot support itself without the lower centres to give it form. Nearly all “rational” decisions require input from emotional centres attributing “weight” to the various alternatives. Even the “feeling of correctness” that we use to accept the final solution rises from the subconscious realms of system 1. 
Slowing down When You Should
Expert practice entails a fluid coordination of system 1 and system 2 processes. A significant proportion of clinical errors in medicine or surgery may be traced to the inappropriate use of system 1 or 2 processes in a given circumstance. These are included in the “human factors” deemed responsible for the bad things that happen to the patients of good practitioners. Examples include “task fixation”, loss of “situational awareness”, and bias. Programs are being developed to provide specific training in the reflective skills needed to avoid these pitfalls. 
Carol-Anne Moulton has characterised the System 2 process in surgery as “slowing down”.  This occurs when the surgeon moves their cognitive emphasis from automatic to effortful. Observed behaviours ranged from increasing attention to the task in hand, decreasing extraneous information by minimising distractions (the “sterile cockpit”), through to stopping altogether and re-evaluating the situation. “Slowing down” moments may be planned preoperatively or occur in response to environmental factors such as unanticipated anatomy or difficulty with exposure. Experienced surgeons identified these behaviours in them-selves even though they used different metaphors.
There is value in making the act of “slowing down” explicit. It is useful for surgical trainees to be briefed on the critical moments in a surgical procedure. It helps other members of the theatre team to optimally support the surgeon, recognising moments requiring a higher level of concentration. During these times of surgical focus the surgeon may lose more general “situation awareness”. The remaining staff need be more vigilant and have agreed methods of escalating concern if other issues emerge.
There are a number of valuable lessons that can be drawn by constructing our understanding of expert medical practice around dual process theory. We can appreciate the distinct and uniquely valuable contributions of both art and science. This can guide our approach to medical education, emphasising the relative importance of didactic teaching, apprenticeship and simulation. It can help delineate vulnerabilities specific to different modes of reasoning and problem solving.
1. Damasio, A.R., Descartes’ error : emotion, reason, and the human brain2005, London: Penguin. xxiii, 312 p.
2. Wilson, T.D., Strangers to ourselves : discovering the adaptive unconscious2002, Cambridge, Mass.: Belknap Press of Harvard University Press. viii, 262 p.
3. Wason, P.C. and J.S.B.T. Evans, Dual processes in reasoning? Cognition, 1975. 3: p. 141–54.
4. Croskerry, P., A universal model of diagnostic reasoning. Acad Med, 2009. 84(8): p. 1022-8.
5. Kahneman, D., A perspective on judgment and choice: mapping bounded rationality. Am Psychol, 2003. 58(9): p. 697-720.
6. Burton, R.A., On being certain : believing you are right even when you’re not. 1st ed2008, New York: St. Martin’s Press. xii, 256 p.
7. Non-Technical Skills for Surgeons (NOTSS). Available from: http://www.surgeons.org.
8. Moulton, C.A., et al., Slowing down when you should: a new model of expert judgment. Acad Med, 2007. 82(10 Suppl): p. S109-16.