A bat and a ball cost $1.10 together. The bat costs $1 more than the ball. Most people answer ‘ten cents’ within a second, and most people are wrong. The answer arrives before you have thought about it — which is the whole idea behind this model.
📚 What you need to know
The dual process model (DPM) explains thinking and decision-making. It was proposed by Wason & Evans (1975).
Decision-making cannot be separated from thinking, because you can only choose after considering the options.
System 1 (S1) is fast, intuitive and automatic.
System 2 (S2) is slow, rational and effortful.
S1 runs on heuristics and biases, so it can be wrong — but it saves enormous time and energy.
Humans are cognitive misers: we avoid using S2 unless we have to.
The study you need is Alter et al. (2007).
Two gears, not two brains
Neither system is the good one. S1 is why you can cross a road without calculating anything, and why an experienced driver goes into automatic mode. S2 is why you can buy a house or prepare for an interview without disaster. The problems come when S1 answers a question that needed S2.
Being a cognitive miser is sensible, not lazy. Running everything through S2 would use up energy you need elsewhere.
Back to the bat and ball. The bat costs $1.05 and the ball costs 5 cents. Most people say $1 and 10 cents because that answer arrives instantly and looks tidy. The question is not hard — it just needs S2, and S1 gets there first.
Key study: Alter et al. (2007)
The clever move here is that the researchers did not change the questions. They changed the font, on the theory that making text harder to read would force people out of S1.
Part of the study
What happened
Aim
To test the dual process model by seeing whether disfluency — a hard-to-read font — shifts people from S1 to S2 thinking.
Participants
40 undergraduate students at Princeton University, USA, obtained by self-selecting (volunteer) sampling.
Procedure
An independent measures design. All participants answered the same cognitive reflection test — questions that are not truly difficult but need some effort, such as the bat and ball problem. They were randomly allocated to a fluent condition (black, easy-to-read 12-point) or a disfluent condition (grey, italic, 10-point). The dependent variable was the number of correct answers.
Results
Participants in the disfluent condition answered more questions correctly than those in the fluent condition.
Conclusion
A hard-to-read font increases the chance of System 2 processing, because it forces more cognitive effort. External factors can therefore shift which system we use.
This finding is genuinely counter-intuitive: making something harder to read made people better at it. If you can explain why — the difficulty interrupts the automatic answer — you are demonstrating exactly the understanding the mark scheme rewards.
Evaluating the model
What is strong about it
It gives a convincing account of thinking fast and thinking slow, and it applies across a huge range of situations, which gives it good validity.
It has practical use for improving decision-making in business, education and policy-making — slow the process down where accuracy matters.
Alter’s design was tightly controlled, with the same questions in both conditions and only the font varied.
What is weaker about it
The model describes what may lie behind thinking, but it is not good at explaining how S1 and S2 actually work. It lacks explanatory power.
Operationalising the two systems is difficult, so research cannot say with certainty which system a participant was using at any moment.
40 self-selected students from one elite university is a narrow sample.
Link to concepts
Measurement: DPM research is predominantly lab-based and uses artificial, contrived tasks, so it tends to be low in both mundane realism and ecological validity. Studying the model in real conditions that mirror real decisions would strengthen it considerably.
Change: technology may be altering how people reach for each system. Some research suggests computer games improve teenagers’ performance on computer-based tasks that use S1. Other work argues that heavy social media use damages critical thinking, or S2, possibly through shorter attention spans and a habit of reacting with emotion rather than logic.
EXAM PRACTICE
Explain one model of thinking and decision-making. [9]
Step 1: name the model and its two parts
Dual process model (Wason & Evans, 1975). S1 fast, automatic, heuristic-driven. S2 slow, effortful, analytical.
Step 2: add the key idea
People are cognitive misers, so S1 runs by default and S2 is only recruited when necessary.
Step 3: the study
Alter et al. (2007). 40 Princeton students, same cognitive reflection test, fluent versus disfluent font.
Step 4: the result and what it means
The disfluent group scored higher. Difficulty interrupted the automatic answer and triggered S2.
Step 5: evaluate
Well controlled and widely applicable, but low mundane realism, a narrow volunteer sample, and the systems are hard to operationalise.
Model + mechanism + study + result + evaluationSay what the disfluent font did, not just that it worked.
💡 Exam tip
Use cognitive miser. It explains in two words why S1 dominates.
The bat and ball problem is worth including — it makes the point in one line.
Say disfluency and explain how it was operationalised: grey, italic, 10-point.
Link back to anchoring and confirmation bias. Both are S1 shortcuts, so the pages connect.
Independent measures with random allocation is a design strength worth naming.
⚠ Common mix-up
Saying S1 is bad and S2 is good. S1 is efficient and usually right; S2 is expensive.
Getting the result backwards. The harder font produced better scores.
Saying the questions were difficult. They were easy but needed reflection.
Calling the sample random. It was self-selecting; allocation to conditions was random.
Treating the systems as separate brain regions. They are modes of thinking, not locations.
Up next: Schema Theory: The Mind’s Shortcuts — the mental templates that let S1 work at all, and the price they charge in accuracy.
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