Reasoning with Probability
We naturally see patterns everywhere that we then use to make decisions and judgments. As you will be shown in this lab activity, sometimes these decisions defy simple laws of probability.
Choose More Likely Scenario
In this lab activity, you will be asked to choose between two scenarios based on which scenario you think is more likely. There will be 14 experimental trials. At the beginning of each trial, you will see a fixation + that you should focus on and press the spacebar when you are ready to begin. You will then be presented with two possible scenarios. Your task is simply to click on the option for the scenario that you think is more likely. After you complete all trials, you will be shown a summary of your responses.
The Representativeness Heuristic
You were given two options on each and every trial. One option listed a single event (e.g., Mary is a teacher), while the other option listed a conjunction of events (e.g., Mary is a teacher who likes to read). There is a tendency that participants will indicate that the conjunctive option is more likely. But statistically the conjunctive event never is more likely than a single event. The probability of a single event, x, occurring is always greater than the probability of that event co-occurring with another event, y. In other words, x*y is always less than x alone. Tversky and Kahneman (1983) coined the phrase conjunctive fallacy to describe the tendency to choose the conjunctive event as more likely than the single event. They attributed this to individuals using a heuristic, or rule of thumb, termed the representativeness heuristic rather than relying on mathematical probability. The representativeness heuristic is a problem-solving shortcut in which material that is thought to be representative of a population (e.g., teachers liking to read or chefs liking to drink wine) forms the basis of judgments about that population. But think about it: the probability that someone is a chef will always be greater than the probability that they are a chef and like wine. People tend to rely on the representativeness heuristic because it is faster to reach a conclusion than mathematical reasoning.
An Effect of Typicality
Consider these scenarios: “Mary is a teacher” vs. “Mary is a teacher and a professional kickboxer.” Most people would choose the first option because they do not consider being a kickboxer to be representative of being a teacher. In cases where the conjunction is not representative of the population, participants will usually choose the more probable single alternative. This is referred to as an effect of typicality. Now consider these two scenarios: “Mary is a political activist and a feminist” vs. “Mary works at Starbucks.” Most will choose option A because of the conjunctive fallacy that political activists are also feminists despite that far more people work at Starbucks than are both political activists and feminists. The representative option will be chosen despite a sheer advantage in probability due to larger numbers in the population.