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

  • Recent breakthroughs in behavioral science demonstrate that people make decisions, including treatment adherence decisions, according to two systems of thinking: System 1, which is rapid but intuitive and biased, and System 2, which is more rational and reflective but complex.  Humans tend to favor System 1. 
  • Both System 1 and System 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’ individual cost/benefit analysis. Developments in behavioral sciences can shed light on how adherence decisions are made and how providers may influence those decisions for the better. 

Rational Econs and Irrational Humans do not have the same attitude towards adherence

Until the 1960s, almost all economists relied on the premise that humans are rational and, with the same information, people will 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 disease.1 In the 1970s, the irrational nature of our decision-making processes began to be studied and categorized in detail, and researchers discovered that with the same information but different perspectives and analyses, people often adopt objectively irrational behavior: Econs turn into irrational humans.  Of course, every human believes their behavior is rational, even when rationalizing potentially self-harming behavior like non-adherence to treatment (see article: The impact of non-adherence to therapies: Higher costs and poorer outcomes). 

In recent years, psychology and behavioral sciences as a whole have made significant advances in explaining the way people make decisions. Significant advances were made by 2002 Nobel Prize winner Daniel Kahneman and Amos Tversky, who explained that the human mind has two systems of thinking; System 1 is fast and automatic versus the slower and more deliberate System 2. Combined, 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 introduced his theory on how people make decisions using two 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 made over a person’s entire life.2
  • System 2 requires mental effort, attention, and concentration. System 2 can be considered 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. 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 is a slow process and requires a lot of mental effort, but once a situation is thought through and general conclusions are stored in memory (thanks to the efforts of System 2), those conclusions become the domain of System 1. Kahneman gives the example of a Grand Master chess player who has played tens of thousands of games and has formed the habitof analyzing positions. For such a player, finding a strong move in a chess game can be effortless and left to his System 1, whereas a much less experienced player will have to employ their System 2.3 As another example: Someone from the USA can use System 1 to easily understand a length of 5 feet, while someone from Europe, will have to use System 2 to convert feet into meters before their System 1 can appreciate the actual length.

To the extent it is possible, System 2 will store information learned through applied effort, which System 1 will then 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 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 primary reasons reported by patients for non-adherence are medication side-effects or the complexity of the medication schedule. While these may sound rational, they represent conclusions reached based on System 1 reactions. System 1 constantly and involuntarily performs a cost/benefit analysis between the effort required to take the drug (i.e. remembering to take it, dealing with the side effects, etc.) versus the benefits, which are often not immediate or apparent (especially if the symptoms are gone as in chronic diseases, many of which are asymptomatic much of the time). 

A System 2 reaction, which an econ would embrace, is a rational consideration of the dangers of not properly finishing an antibiotic treatment or of not following the treatment for hypertension. However, System 2 is lazy and requires effort, and irrational humans tend to use shortcuts in the form of System 1 heuristics that allow quick decisions with the least effort. These heuristics include things like snap decisions based on insufficient data and poor assessment of risks; as a result, 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 article, Patient behavior and the drivers of vertigo treatment adherence, discusses patient non-adherence factors and explains that non-adherence, whether prompted by System 1 or System 2, is often intentional; consider the following examples:

  • System 2 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. 
  • System 1 non-adherence may be based on automatically generated irrational beliefs such as when a patient intentionally discontinues treatment simply because they do not experience symptoms despite being told that there will be adverse consequences to stopping treatment. 

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

*Kahneman describes the “econs” as rational in a manner humans never can be because econs, unlike humans, are not subject to any biases. (Kahneman, “Thinking Fast and Slow,” Conclusions: Human and econs, 2011)


  1. Aurel O. Iuga & Maura J. McGuire (2014). “Adherence and Health Care Costs,” Risk Management and Healthcare Policy, (70), pp. 35–44. https://doi:10.2147/RMHP.S19801
  2. Philip Iordanov (2018). “Thinking fast? Slow down,” Neurofied. 26 December, 2018.
  3. Emily Zulz (2018). “Daniel Kahneman: Your intuition is wrong, unless these 3 conditions are met,” Think Advisor,16 November, 2018.
  4. Przemyslaw Kardas (2002). “Patient compliance with antibiotic treatment for respiratory tract infections,” Journal of Antimicrobial Chemotherapy, (49):6, pp. 897–903.
  5. Samantha J. Eells, Megan Nguyen, Jina Jung, Raul Macias-Gil, Larissa May, & Loren G. Miller (2016). “Relationship between adherence to oral antibiotics and postdischarge clinical outcomes among patients hospitalized with Staphylococcus aureus skin infections,” Antimicrobial Agents and Chemotherapy, (60):5, pp. 2941–2948.