Key Study for the Dual Processing Model (Thinking and Decision Making)

Travis DixonCognitive Psychology

The study in this blog post provides evidence for how people are prone to using system one processing when making decisions.

The following is adapted from: “IB Psychology: A Revision Guide.”

Tversky and Kahneman spent many years studying people’s thinking and decision making. As a result, they devised a dual processing model that attempts to explain two systems people use when processing information: system one and system two. The system used to process information can affect our decision making.

  • System one processing is intuitive (automatic), fast, unconscious, and based on experiences.
  • System two processing is rational (controlled), slow, conscious, and based on consequences.

Relying on system one can lead to making errors in judgement and incorrect decisions, as can be seen in the study below.

The following terms could be used in IB Psychology SAQs about thinking and decision making: rational (controlled) and intuitive (automatic).


In order to properly describe the dual processing model, you should aim to be able to use at least 3-4 adjectives to describe each system, especially rational (controlled) and intuitive (automatic).

Key Study: Judgement Under Uncertainty (Tversky and Kahneman, 1974)

The following study is one example of the numerous tests Kahneman and Tversky devised to test people’s mental shortcuts and biases when making decisions. In one task, they had 95 participants and they gave them the following scenario:

  • A certain town is served by two hospitals. In the larger hospital about 45 babies are born each day, and in the smaller hospital about 15 babies are born each day. As you know, about 50 percent of all babies are boys. However, the exact percentage varies from day to day. Sometimes it may be higher than 50 percent, sometimes lower. For a period of 1 year, each hospital recorded the days on which more than 60 percent of the babies born were boys. Which hospital do you think recorded more such days?
    • The larger hospital
    • The smaller hospital
    • About the same (that is, within 5 percent of each other)

The researchers then recorded the participants responses. The correct answer is the smaller hospital, because statistical probabilities suggest that the larger the sample size (e.g. the more babies born) the closer it will get to the average, so it is more likely that a small hospital will have more disproportionate days.

However, 78% of participants got this wrong and most (56%) chose option three – about the same, “…presumably because these events are described by the same statistic and are therefore equally representative of the general population.” In other words, because birth-rate is 50/50, they assumed the probability in this scenario would also be 50/50. They were using system one and were going with intuition, rather than using system two and using logic and reasoning to deduce the correct answer. They were also using a representative heuristic (see below).

This is a simple study that can be used to demonstrate the dual processing model. Here are two others:

Exam Tips

If asked about “thinking and decision making” in an exam question, describing the model and then summarizing the above study should be enough. However, if you have time available I would also recommend writing a couple of sentences suggesting that studies have shown (e.g. Bechara et al.) that damage to the brain (e.g. vmPFC) is a factor that can affect how we process information and making decisions.

If the question is: “Explain one study related to thinking and decision making,” then I think either Kahneman and Tversky’s or Bechara et al.’s studies are equally suited. However, you may want to go with Kahneman and Tversky’s study since the dual processing model is their model.

More Info: Heuristics

The problem above is an example of what Kahneman and Tversky call “heuristics.” These are cognitive biases (or shortcuts) that people use when making decisions. There are different types of heuristics and the test above is an example of people using the “representativeness heuristic.” This is used when people are making judgements based on probabilities. Instead of assessing the problem logically (system two), they think of an object or event as being representative of other similar objects or events, so they use the same probability across multiple scenarios. In other words, one probability is used to represent (and to calculate) other similar probabilities.

For example, in the above problem they are using the average birth rate of 50/50 boys/girls to represent the other probabilities about birth rates in specific hospitals, even though this is incorrect.

Heuristic: “rules of thumb that are cognitively undemanding and often produce approximately accurate answers.” (Eyesenck and Keane, 2010)

Critical Thinking Considerations 

  • Does the above study show that all people use heuristics (system one) when processing information?
  • When participants provide answers to the above problem, there are not consequences for their decisions. Does this affect the generalizability of these results to other situations? Can you think of situations where people might be less likely to use system one (heuristics) to make decisions because of consequences?
  • What factors might affect individual differences in processing information? (See Bechara et al.)
  • Does the dual processing model and the associated studies suffer from cultural biases? (These were studied mainly in Western countries). Can you think of any reasons to think why we might not expect the same results in some cultures?
  • Why is understanding how we process information a relevant field of study? Can you think of any potential applications of this research?

Eyesenck and Keane, Cognitive Psychology: A Student’s Handbook. 2010
Tversky, Amos and Kahneman, Daniel. Judgment under Uncertainty: Heuristics and Biases. Science, New Series, Vol. 185, No. 4157. (Sep. 27, 1974), pp. 1124-1131 (Link)