TWO SYSTEMS

The Two-System Model

The mind operates through an uneasy interaction between two modes of thinking, which we can call System 1 and System 2. These are not literal entities in the brain but useful shorthand: "System 1 does X" means "X occurs automatically," and "System 2 is mobilized to do Y" means "arousal increases, pupils dilate, attention is focused, and activity Y is performed."

System 1 operates automatically and quickly, with little or no effort and no sense of voluntary control. It is fast, associative, slow-learning, and emotional, generating impressions, feelings, and inclinations that rise to consciousness without deliberate intention. It works unconsciously and is the source of many cognitive biases.

System 2 allocates attention to effortful mental activities that demand it, including complex computations. It is slow, deliberate, analytical, rule-governed, flexible, and associated with the subjective experience of agency, choice, and concentration. System 2 is responsible for self-control, reasoning, and complex analysis.

The two systems are always active while we are awake: System 1 runs automatically, while System 2 normally remains in a comfortable, low-effort mode, ready to be called upon when needed. This division of labor works remarkably well most of the time, but System 1 can be overconfident and fail to delegate difficult problems to System 2, leading to predictable errors.

What Each System Does

The automatic operations of System 1 include both innate skills we share with other animals and mental activities that become fast and automatic through prolonged practice. Examples include:

• Detecting that one object is more distant than another
• Orienting to a sudden sound
• Completing the phrase "bread and…"
• Detecting hostility in a voice
• Answering simple arithmetic like 2 + 2 or 1 + 1
• Reading words on large billboards
• Driving a car on an empty road
• Finding a strong move in chess (for a chess master)
• Understanding simple sentences
• Recognizing that a "meek and tidy soul with a passion for detail" resembles an occupational stereotype

Several of these mental actions are completely involuntary. You cannot refrain from understanding simple sentences in your own language, nor can you prevent yourself from knowing that 2 + 2 = 4 or thinking of Paris when the capital of France is mentioned. Others, though susceptible to voluntary control, normally run on automatic pilot.

In contrast, System 2 is required for effortful mental work. The moment you face a computation like 37 × 40, System 2 must engage, bringing focused attention and deliberate processing. Its highly diverse operations share one feature: they require attention and are disrupted when attention is drawn away. Examples include:

• Bracing for the starter gun in a race
• Focusing on the voice of a particular person in a crowded room
• Searching memory to identify a surprising sound
• Maintaining a faster walking speed than natural
• Monitoring the appropriateness of your behavior in a social situation
• Counting occurrences of the letter a in a page of text
• Parking in a narrow space
• Comparing two washing machines for overall value
• Filling out a tax form
• Checking the validity of a complex logical argument

Attention and Effort

The phrase "pay attention" is apt: you dispose of a limited budget of attention that you can allocate to activities. If you try to go beyond your budget, you will fail. It is the mark of effortful activities that they interfere with each other, which is why it is difficult or impossible to conduct several at once. You can do several things simultaneously, but only if they are easy and undemanding.

Intense focusing on a task can make people effectively blind, even to stimuli that normally attract attention. In the famous "invisible gorilla" experiment, viewers instructed to count basketball passes by one team failed to notice a woman in a gorilla suit crossing the court, thumping her chest. The counting task absorbed their attention so completely that a striking event went unseen.

Equally revealing is what happens when we read simple text. Consider a sign with three triangles, each containing a short phrase. In the first triangle the word "the" is written twice, in the second "a" appears twice, and in the third "the" is repeated again. Most people read the words flawlessly and completely miss these glaring duplications. Reading is so automatic for skilled readers that System 1 processes the sentence without needing to involve System 2. Confident in its ability, System 1 never flags the task as one requiring additional scrutiny, and the errors slip by unnoticed. This illustrates two important facts: we can be blind to the obvious, and we are also blind to our blindness.

The control of attention is shared by the two systems. Orienting to a loud sound is normally an involuntary operation of System 1, which immediately mobilizes the voluntary attention of System 2. You may resist turning toward an offensive comment at a party, but even if your head does not move, your attention is initially directed there—though it can be moved away by focusing intently on another target.

How the Systems Interact

When we think of ourselves, we identify with System 2, the conscious, reasoning self that has beliefs, makes choices, and decides what to think about and what to do. System 2 articulates judgments and makes choices, but it often endorses or rationalizes ideas and feelings that were generated by System 1. You may not know that you are optimistic about a project because something about its leader reminds you of your sister, or that you dislike someone who looks vaguely like your dentist. If asked for an explanation, however, you will search your memory for presentable reasons and will certainly find some—and you will believe the story you make up.

System 1 continuously generates suggestions for System 2: impressions, intuitions, intentions, and feelings. If endorsed by System 2, these become beliefs and voluntary actions. System 2 is normally in a comfortable low-effort mode, mobilizing only when System 1 encounters difficulty or when rules require deliberate reasoning. Unfortunately, System 1 is not always reliable in determining when a situation truly requires the effort of System 2. It can be overconfident, treating a challenging problem as if it were trivial and failing to pass the baton to the slower, more deliberate system. When active, System 2 can override the freewheeling impulses and associations of System 1. It has some ability to change how System 1 works by programming the normally automatic functions of attention and memory—for instance, setting yourself to look for a white-haired woman at a busy train station.

However, System 2 is not merely an apologist for System 1. It prevents many foolish thoughts and inappropriate impulses from overt expression. The investment of attention improves performance in numerous activities, and it is essential to comparison, choice, and ordered reasoning. Yet System 2 is not a paragon of rationality. Its abilities are limited and so is the knowledge to which it has access. We do not always think straight when we reason, and errors are not always due to intrusive intuitions. Often we make mistakes because our System 2 simply does not know any better.

The Two Faces of System 1

System 1 is the origin of much that we do wrong, but it is also the origin of most of what we do right—which is most of what we do. Our thoughts and actions are routinely guided by System 1 and generally are on the mark. The marvels include the rich, detailed model of our world maintained in associative memory: it distinguishes surprising from normal events in a fraction of a second, immediately generates an idea of what was expected instead, and automatically searches for causal interpretations of surprises as they take place.

Memory also holds the vast repertory of skills acquired in a lifetime of practice, which automatically produce adequate solutions to challenges as they arise—from walking around a large stone to averting a customer's incipient outburst.

Expert Intuition

The acquisition of skills requires a regular environment, adequate opportunity to practice, and rapid and unequivocal feedback about the correctness of thoughts and actions. When these conditions are fulfilled, skill eventually develops, and the intuitive judgments and choices that quickly come to mind will mostly be accurate. A marker of skilled performance is the ability to deal with vast amounts of information swiftly and efficiently. When an individual has acquired relevant expertise, System 1 can produce accurate intuitions. This is recognition-based: the expert sees familiar elements in a new situation and knows how to respond. As Herbert Simon observed, intuition is "nothing more and nothing less than recognition."

Consider the firefighter who sensed danger without knowing why and ordered his crew out of a burning building moments before it collapsed. After escaping, he realized the fire had been unusually quiet and his ears were hot—his System 1 had recognized a pattern from experience, even though he was not consciously aware of the cues. Similarly, after thousands of hours of practice, a chess master looks at a board and immediately sees strong moves. This is recognition, not magic.

Heuristic Intuition

What happens in the absence of skill? System 1 is rarely dumbfounded. Not constrained by capacity limits and profligate in its computations, it simultaneously generates answers to related questions. When engaged in searching for an answer to one question, it may substitute a response that more easily comes to mind for the one that was requested. In this conception of heuristics, the heuristic answer is not necessarily simpler—it is only more accessible, computed more quickly and easily. The heuristic answers are not random and are often approximately correct. And sometimes they are quite wrong.

A broader conception of heuristics includes the affect heuristic, where judgments and decisions are guided directly by feelings of liking or disliking, with little deliberation. A financial executive decided to invest in Ford stock because he liked their cars. The original question—Is the stock underpriced?—was difficult, so he answered an easier one: Do I like the company?

This process of question substitution applies broadly. People rely on a limited number of heuristic principles to reduce the complex tasks of assessing probabilities and predicting values to simpler judgmental operations. In general, these heuristics are quite useful, but sometimes they lead to severe and systematic errors. The subjective assessment of probability resembles the assessment of physical quantities such as distance or size—judgments based on data of limited validity, processed according to heuristic rules. For instance, the apparent distance of an object is determined partly by its clarity. The more sharply an object is seen, the closer it appears. This rule has some validity, because more distant objects appear less sharp. But reliance on this rule leads to systematic errors: distances are overestimated when visibility is poor and underestimated when visibility is good.

The Problem of Cognitive Biases

System 1 registers the cognitive ease with which it processes information, but it does not generate a warning signal when it becomes unreliable. Intuitive answers come to mind quickly and confidently, whether they originate from skills or from heuristics. There is no simple way for System 2 to distinguish between a skilled and a heuristic response. Its only recourse is to slow down and attempt to construct an answer on its own, which it is reluctant to do because it is indolent. Many suggestions of System 1 are casually endorsed with minimal checking, as in the bat-and-ball problem. This is how System 1 acquires its reputation as the source of errors and biases. Its operative features—including WYSIATI (What You See Is All There Is), intensity matching, and associative coherence—give rise to predictable biases and cognitive illusions such as anchoring, nonregressive predictions, overconfidence, and numerous others.

What Can Be Done

What can be done about biases? How can we improve judgments and decisions, both our own and those of the institutions we serve? The short answer is that little can be achieved without a considerable investment of effort. System 1 is not readily educable. Even after making a study of these issues, intuitive thinking remains prone to overconfidence, extreme predictions, and the planning fallacy. We improve mainly in our ability to recognize situations in which errors are likely: "This number will be an anchor…," "The decision could change if the problem is reframed…" And we make much more progress in recognizing the errors of others than our own.

The way to block errors that originate in System 1 is simple in principle: recognize the signs that you are in a cognitive minefield, slow down, and ask for reinforcement from System 2. This is how you proceed when you encounter the Müller-Lyer illusion—you recognize the situation as one in which you should not trust your impressions of length. Unfortunately, this sensible procedure is least likely to be applied when it is needed most. We would all like a warning bell that rings loudly before a serious error, but no such bell exists. Cognitive illusions are generally more difficult to recognize than perceptual illusions. The voice of reason may be much fainter than the loud and clear voice of an erroneous intuition, and questioning your intuitions is unpleasant when facing the stress of a big decision. It is much easier to identify a minefield when observing others wandering into it than when you are about to do so. Observers are less cognitively busy and more open to information than actors.

The Role of Organizations

Organizations are better than individuals when it comes to avoiding errors, because they naturally think more slowly and have the power to impose orderly procedures. They can institute and enforce useful checklists, as well as more elaborate exercises such as reference-class forecasting and the premortem. By providing a distinctive vocabulary, organizations can encourage a culture in which people watch out for one another as they approach minefields. An organization is a factory that manufactures judgments and decisions. Every factory must ensure the quality of its products in the initial design, in fabrication, and in final inspections. The corresponding stages in the production of decisions are: the framing of the problem, the collection of relevant information leading to a decision, and reflection and review. Constant quality control is an alternative to the wholesale reviews that organizations commonly undertake in the wake of disasters.

The Value of Precise Language

Ultimately, a richer language is essential to the skill of constructive criticism. Much like medicine, the identification of judgment errors is a diagnostic task requiring a precise vocabulary. The name of a disease is a hook to which all that is known about it is attached—vulnerabilities, environmental factors, symptoms, prognosis, and care. Similarly, labels such as "anchoring effects," "narrow framing," or "excessive coherence" bring together in memory everything we know about a bias, its causes, its effects, and what can be done about it.

There is a direct link from more precise gossip at the watercooler to better decisions. Decision makers are sometimes better able to imagine the voices of present gossipers and future critics than to hear the hesitant voice of their own doubts. They will make better choices when they trust their critics to be sophisticated and fair, and when they expect their decision to be judged by how it was made, not only by how it turned out.