Thinking Fast, Thinking Slow

You are walking home, late at night, after watching a movie at a friend's house.  The street is dark, and dimly lit.  Around the corner, the shape of a large man appears.  He is tall and strongly built, and is walking straight towards you, carrying a metallic, black object in his hands.  Your heart starts beating hard in your chest, and you brace yourself for the worst.

Just as you are about to make a break for it, the man steps under a street lamp, and you get a better view.  You notice that he is dressed in the uniform of an office worker, and the metallic object in his hand was just a cellphone.  Calming yourself down, you tell yourself that he must have been working late at night at the office, and is probably on his way home, just as you are.  Your heart starts slowing down to its regular pace.

What sort of thinking caused you to be afraid at the sight of the stranger, then later on decide that there was nothing to worry about?  Nobel prize winner Daniel Kahneman devoted his life to researching how people think and make decisions.  His research led to the dual process theory of thinking.  According to Kahneman, there are two distinct modes of thinking:  fast, automatic thinking, and slow, deliberate thinking.  To see these two systems in action, try out the activity below.

Try It Out

Complete the quiz below:

1.  What emotion is the woman feeling in the photograph below?

2.  What is 10 x 10 = ?  

3.  John enjoys listening to classical music and solitary walks in nature.  John has always been rather introverted and detail oriented. As a child, John spent much of his time reading books.  Do you think John is more likely to be a librarian or a business owner?

4.  What is 17 x 14 = ?

5.  A bat and a ball costs $1.10 in total.  The bat costs $1 more than the ball.  How much does the ball cost?

Now, think about how you answered these questions.  Were you able to answer some of the questions instantly?  Did other questions require more deliberate thinking and effort before you knew the answer?  Which questions did you answer with fast thinking, and which required slow thinking?

System 1, System 2

As soon as you saw the photograph of the woman, you surely knew that she was feeling sad.  You didn't have to think much about it, or spend time analyzing her facial expression for clues.  You just knew.  Similarly, you surely knew that 10 x 10 = 100 instantly, without much effort.  Kahneman calls this form of rapid, effortless thinking System 1 thinking.  System 1 thinking is automatic, intuitive, and requires no effort.  You don't need to spend any time carefully thinking it over - you just know.

By contrast, you probably didn't know the answer to 17 x 14 right away.  You probably know how to solve it, and could work out the answer, but that would require time and effort.  First, you would have to remember the rules of multiplication, and go through each of the steps involved.  You might work out 17 x 10 first, then 17 x 4, then add the two answers.  Doing so requires manipulating numbers in your head, taking up your working memory, and sapping your concentration.  Kahneman calls this slower, conscious and rational style of thinking System 2 thinking.  System 2 thinking is deliberate and effortful, requiring mental work and concentration.  In fact, when you are engaged in System 2 thinking, your body is also involved - your muscles tense up, your blood pressure rises, and your pupils dilate (get bigger). 

Which system is better?  Well, that depends on the task.  System 1 gives us fast, effortless answers, but it is also  prone to errors.  System 1 uses mental shortcuts to come up with an answer in very little time, but sometimes these mental shortcuts lead to the wrong answer.  For example, did you answer Question 3 with librarian?  Certainly, the description of John certainly matches common stereotypes of librarians.  However, there are far more business owners than librarians in the world.  Taking the raw number of business owners into account, John is much more likely to be a business owner than a librarian, despite his love of reading and introversion.  Similarly, on Question 5, many people quickly come up with the answer of $0.10 for the price of the ball.  The number just seems right.  However, if you think carefully and deliberately, you will arrive at the correct solution of $0.05. 

Take a look at the infographic below for a quick summary of System 1 and System 2:
Video Activity
Watch the video below, and identify as many examples of System 1 and System 2 thinking as you can.

Here are two more problems that illustrate the distinction between fast and slow thinking.  Do your best to solve them.

1.  If it takes 5 machines 5 minutes to make 5 widgets, how long would it take 100 machines to make 100 widgets?

2.  In a lake, there is patch of lily pads.  Every day, the patch doubles in size.  If it takes 48 days for the patch to cover the entire lake, how long does it take for the patch to cover half the lake?

Scroll down to the bottom of the page for the answers.  These two questions (along with the bat and ball problem at the top of this page) make up the Cognitive Reflection Test.  In all three questions, there is an answer that comes to mind almost immediately - an answer from System 1, which also happens to be wrong.  If you can suppress this wrong answer, and engage System 2, you will able to work out the correct answer.

You may have noticed that the first question was written in an easy-to-read font, while the second question was written in a much more difficult font.  Could the font have an effect on how many people get these questions correct?  Read about the surprising answer in the research study below.

Research: Atler & Oppenheimer, 2007

Aim: Investigate how font affects thinking


  • 40 Princeton students completed the Cognitive Reflections Test (CRT).  This test is made up of 3 questions, and measures whether people use fast thinking to answer the question (and get it wrong) or use slow thinking (and get it right)

  • Half the students were given the CRT in an easy-to-read font, while the other half were given the CRT in a difficult-to-read font


  • Among students given the CRT in easy font, only 10% of participants answered all three questions correctly, while among the students given the CRT in difficult font, 65% of participants were fully correct


  • When a question is written in a difficult-to-read font, this causes participants to slow down, and engage in more deliberate, effortful System 2 thinking, resulting in answering the question correctly

  • On the other hand, when the question is written in an easy-to-read font, participants use quick, unconscious and automatic System 1 thinking to come up with the obvious (but incorrect) answer


  • This study provides strong evidence for dual processing theory, providing support for Kahneman's model of fast System 1 and slow System 2 thinking

  • The study only involved Princeton undergraduate students, which are clearly not representative of the general population.  Therefore, the results may not generalize to other groups of participants

  • The CRT is made up of "trick" questions, which rarely come up in everyday life.  Therefore, the ecological validity of this study is low, as the real-world significance of these findings is unclear

IB Psych Matters

We make countless decisions everyday, ranging from inconsequential (like which brand of toothpaste to buy) to potentially life changing (like what major to study in university),

For each of the following decisions, how would System 1 and System 2 make the decision differently?  Do you think you would use System 1 or System 2 thinking if you had to make the decision?  Which System do you think would do a better job of making the right choice?

A.  Deciding which laptop to purchase

B.  Deciding to accept or reject an invitation to be someone's date for the prom

C.  Deciding which drink to buy from a convenience store when you are feeling thirsty

D.  Deciding which university to attend
Evaluating Dual Process Theory

  • The distinction between System 1 and System 2 thinking is well supported by research, such as Atler & Oppenheimer's study on the effects of font on the CRT (cognitive reflections test)

  • The theory can explain why intelligent people can sometimes make poor decisions whenever they rely on System 1 to come up with a fast, effortless (but sometimes wrong) answer

  • Dual process theory is also consistent with evolution.  System 1 is believed to have evolved in the distant past to make quick, potentially life saving decisions, while System 2 is a more modern adaptation to help us think deliberately and carefully.  If a snake is wrapped around your leg, you don't want to think about it for very long - you need to act fast, and this is where System 1 really shines

  • However, System 1 and System 2 must be understood as simply being metaphors for different decision making processes.  There are not actually two different parts of the brain called "System 1" and "System 2".  In fact, many parts of the brain are involved in both System 1 and System 2 thinking

  • Not all researchers agree that there are two systems.  Some alternate models of thinking have suggested that there are four (or more) different systems of thinking

  • I can describe the differences between System 1 and System 2 thinking

  • I can explain why System 1 thinking is prone to errors

  • ​I can explain how the Cognitive Reflections Test (CRT) measures System 1 and System 2 thinking

  • I can describe the Aim, Procedure, Findings and Conclusion of Atler & Oppenheimer's research, and Evaluate the study

  • I can evaluate the strengths and limitations of dual process theory
Quiz Yourself!

1. Which of the following is NOT a feature of System 2 thinking?

(a) Slow and deliberate

(b) Requires concentration and mental energy

(c) Associated with physical changes in the body, such as pupil dilation

(d) Prone to errors

2.  Four friends are going to buy a new smartphone.  Who uses System 2 thinking to choose which phone to purchase?

(a) Sarah buys the I-Phone because Apple is a cool, cutting-edge brand

(b) Mark buys Samsung because he's always bought Samsung in the past, and they were always good phones

(c) Kate buys the Nokia because it has a high megapixel camera, a fast processor and lots of memory, at a lower price than other phones with similar features

(d) Dave buys the Sony phone because the screen resolution and design look great

3.  What do the questions on the Cognitive Reflections Test (CRT) have in common?

(a) The questions can easily be answered using intuition

(b) Using fast thinking tends to result in the wrong answer

(c) Participants experience a spike in blood pressure and pupil dilation as they answer the questions

(d) Logic and reason alone are insufficient to answer the questions correctly

4.  Which statement best summarizes the results of Atler & Oppenheimer's research?

(a) Participants who were given the questions in easy-to-read font used System 1, making more errors

(b) Participants who were given the questions in hard-to-read font used System 2, making more errors

(c) Both A and B are correct

(d) Neither A nor B are correct

5.  From an evolutionary perspective, what is a possible benefit of System 1?

(a) It avoids potentially dangerous errors, increasing the odds for survival

(b) It is based on reason and logic, helping mankind develop tools and civilization

(c) It is rapid, leading to quick reactions in life-or-death situations

(d) It is energy efficient, leading to better circulation and resistance to bacteria


Cognitive Reflections Test

1.  Five minutes (not 100 minutes!)

2.  47 days (not 24 days!)

Quiz Yourself!

1 - D, 2 - C, 3 - B, 4 - A, 5 - C