Two systems of thought: why rational people make irrational choices

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Two systems of thought: why “rational” people make “irrational” choices


Key messages:

  • Non-adherence, particularly non-adherence to potentially life-saving treatment, can be explained by the fact that people are not rational decision makers
  • Recent breakthroughs in behavioral science demonstrate that people make decisions according to two systems of thinking: an intuitive, biased but rapid System 1, and a more rational, reflective but complex System 2.  Humans tend to favor System 1. Both System 1 & 2 lead to intentional non-adherence


There are many reasons people invoke to explain their medication non-adherence, and often these explanations center around what patients see as their individual cost/benefit analysis. While patients may perceive these decisions as rational, they often are not. Developments in the behavioral sciences can shed light on how adherence decisions are made, and how providers may influence them for the better. 

“Rational Econs” and “Irrational Humans” will not have the same attitude towards adherence

Until the 1960’s almost all economists relied on the premise that humans are rational and that with the same information, all people would act in essentially the same way. For adherence, the implication was that when a life-saving drug is prescribed by a physician, these rational people, or “econs” * would adhere to it diligently. 

However, numerous studies have shown that average adherence to treatment is about 50% for chronic diseases, including cardiovascular disease1. Since the 1970s, the irrational nature of our decision-making processes began to be studied and categorized in detail. With the same information but with different perspectives of analysis, people often adopt objectively irrational behavior: “econs” turn into humans.  Of course, every “human” believes 100% in the rationality of his behavior, rationalizing potentially self-harming behavior like non-adherence to treatment (see the second article in this series on the impact of non-adherence, << The impact of non-adherence to therapies: higher costs and worse outcomes >>.

In recent years, psychology and behavioral science as a whole have made significant advances in explaining the way people make decisions. Significant advances were made by Daniel Kahneman (Nobel Prize, 2002) and Amos Tversky, who proposed that the human mind has two “systems” of thinking, summarized as being “fast” and automatic versus “slow” and deliberate. These two “systems” govern people’s behavior and attitudes.


The two systems of thinking: “fast and slow”

In his book “Thinking fast and slow”, Kahneman introduces his theory of how people make decisions using two fictional constructs; systems 1 and 2. These systems are distinct in nature and each fulfills a distinctive role.

  • System 1 is immediate and spontaneous. It provides assessments without effort and is responsible for generating rapid decisions. It allows people to determine the origin of a noise, detect hostility in a voice or on a face, etc. According to Kahneman, it is responsible for about 95% of all the decisions taken by a human over his/her entire life2.
  • System 2 requires mental effort, attention, and concentration. System 2 can be considered as a person’s rational and conscious self. It allows people to structure complex information, reflect upon it, make rational choices and deal with uncommon situations. In his book, Kahneman describes System 2 as “lazy”, because in most cases it does not change the decisions proposed by System 1. In other words, people tend to respond automatically to most events rather than employ system 2, and even when they do, system 2 tends to favor the conclusions of system 1. It is important to note that the laziness of system 2 has nothing to do with the cognitive ability of the person. Systems 1 and 2 interact quite successfully. System 2 is responsible for learning, which requires a lot of mental effort and is a slow process. But once a situation is thought through and general conclusions are stored in memory (thanks to the efforts of the System 2), those conclusions becomes the domain of system 1. Kahneman brings the example of a Grand Master chess player, who played tens of thousands of games and has the habit of analyzing positions. The ability to find a strong move in a chess game for such player can be effortless and fall under his system 1, whereas a much less experienced player will have to employ his/her system 2.3 Think of another example: if you are from the USA, your system 1 will easily understand the length of 5 feet. But if you are a European, your system 2 will first have to convert them into meters before System 1 could appreciate the actual length. 

To the extent it is possible, System 2 will store information learned through applied effort, which System 1 will use spontaneously in its interactions with new information coming from the environment.

What does our understanding of systems 1 and 2 teach us about patients’ adherence behavior?

Consider the example of an acute disease, which is treated with antibiotics. Studies show that only 16% of patients are fully adherent to antibiotics treatments for an average overall adherence rate ranging from 43-78%4,5. The reasons reported by patients are medication side-effects or the complexity of the medication schedule. While these sound rational, the conclusions reached represent System 1 reactions. System 1 constantly and involuntarily performs a cost/benefit analysis between the effort to take the drug (i.e. remembering to take it, the side effects it causes, etc.) versus the benefits, which are often not immediate (especially if the symptoms are gone). In chronic diseases, many of which are asymptomatic much of the time, the same analysis is constantly being performed. 

A System 2 reaction would be a rational consideration of the dangers of not finalizing the antibiotic treatment correctly, or of not following the treatment for hypertension. However, as System 2 is “lazy” and requires effort, which the patient’s brain might be ready to make.  As such, people tend to use “short cuts”, System 1 “heuristics” that allow us to make quick decisions without making the effort of A System A system 2 reaction would be a rational consideration of the dangers of not finalizing the antibiotic treatment correctly, or of not following the treatment for hypertension. But System 2 is “lazy” and requires effort, which an “econ” might be ready to make, but which a human often will not.  As such, we tend to use “short cuts”, System 1 “heuristics” that allow us to make quick decisions without making the effort of implicating System 2.  These heuristics include things like snap decisions about numbers, and poor assessment of risks. They can drive us to make some very poor decisions… including the decision not to adhere to treatment.

Both systems 1 & 2 lead to intentional non-adherence

The previous article in this series, << Drivers of treatment adherence and the role of patient behavior >>, discusses patient non-adherence factors and indicates that non-adherence is intentional in many cases. It may seem paradoxical that intentional non-adherence can be the result of the “automatic” System 1 processes that govern more than 95% of people’s decisions. This can be clarified by the following examples:

  • Non-adherence may be the result of a deliberate process, which results in a largely informed, intentional decision. For example, a patient who consults several doctors and informs himself about his condition may decide to discontinue treatment. The decision not to adhere will be intended and will result from System 2 activities. 
  • On the other hand, a patient may decide not to adhere to treatment based on beliefs automatically generated by system 1, also leading to intentional non-adherence. For example, a patient who does not experience symptoms may intentionally discontinue treatment despite being told that he should not, without rationalizing the decision.

These new insights about System 1 and System 2 thinking provide important explanation of the patient adherence behavior. Influencing adherence requires a thorough understanding of the way that people make decisions regarding their condition and treatment. The two systems of thinking proposed by Kahneman and Tversky are an important and useful model for understanding adherence behavior. Future articles will examine these systems and the heuristics that drive patient behavior to better influence it.

1. Iuga, Aurel O, and Maura J McGuire. “Adherence and health care costs.” Risk management and healthcare policy vol. 7 35-44. 20 Feb. 2014, doi:10.2147/RMHP.S19801
2. Philip Iordanov. “Thinking fast? Slow down..” 26th of December 2018 (
3. Emily Zulz, “Daniel Kahneman: Your Intuition Is Wrong, Unless These 3 Conditions Are Met”, 16th of November, 2018 (
4. Przemyslaw Kardas, “Patient compliance with antibiotic treatment for respiratory tract infections”, Journal of Antimicrobial Chemotherapy, Volume 49, Issue 6, June 2002, Pages 897–903,
5. Samantha J. Eells, Megan Nguyen, Jina Jung, Raul Macias-Gil, Larissa May, Loren G. Miller, “Relationship between Adherence to Oral Antibiotics and Postdischarge Clinical Outcomes among Patients Hospitalized with Staphylococcus aureus Skin Infections“, DOI: 10.1128/AAC.02626-15 (