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Chapter 11

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McGill University

Psychology

PSYC 213

Jelena Ristic

Winter

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Chapter 11
Reasoning, judgement and choice
Reasoning
Syllogistic reasoning
Reasoning: thought process that yields a conclusion from premises (percepts, thoughts, assertions).
Syllogistic reasoning: a syllogism consists of 2 premises and a conclusion. Each of the premises specifies a relationship
between 2 categories. Sometimes called categorical reasoning.
1. Universal affirmative: all As are Bs.
2. Universal negative: no Bs are As.
3. Particular affirmative: some As are Bs.
4. Particular negative: some As are not Bs.
Possible to interpret the premises in a syllogism in a variety of ways. More complicated when premises are combined to
arrive at a conclusion.
Logicism
Logicism: The belief that logical reasoning is an essential part of human nature.
Practical syllogism: occurs when the conclusion drawn from 2 premises becomes an action.
Common feature of everyday life.
Problem= untrained participants make logical errors when asked to evaluate the validity of syllogistic arguments BUT not
always.
The effect of content on syllogistic reasoning
Truth/falsehood of a premise = irrelevant when judging its validity.
Validity depends only on whether or not the conclusion necessarily follows from the premises.
Effect of participants’ beliefs greater if they believe the conclusion is true in the real world.
Participants first determine whether the conclusion is believable or not. Then they try to find some way of
thinking about the premises that renders the conclusion invalid (unbelievable). Or they try to determine if there
is not some way of thinking about the premises that renders the conclusion acceptable (believable).
The interpretation of some
Great difficulty= premises often are open to alternative interpretations.
Must consider only inferences that are consistent with all possible interpretations of a set of premises BUT do
not always work out all the possible interpretations = reason according to the specific way they interpret the
premises.
“some” usually means “at least one, but not all”. Can also mean “not all”.
Beggs (1987): participants given a description of people+their occupations.
Subsets: artists, writers, men, women.
1. asked to evaluate statements in which the word “some”= minority of the group
2. asked to evaluate statements in which “some” = majority of the group.
Result: For ++ people some = less than the whole amount under consideration.
Mental models and deductive reasoning
Jonhson-Laird (1988): most influential theory of syllogistic reasoning.
1. Construct a mental model (mental structure) of the situation. 2. Draw conclusions consistent with the model.
Details of the model are irrelevant.
Good explanation for the sources of difficulty in deductive reasoning, but challenged by other approaches. Relational reasoning.
Relational reasoning: Reasoning involving premises that express the relations between items.
Transitive relation: typically come in pairs, items opposite of each other. A is taller than B.
Three-term series problem: Linear syllogisms consisting of two comparative sentences from which a conclusion must be
drawn. B is smaller than A. B is larger than C. Which is smallest?
Iconic: the relations between the parts of the model correspond to the relations between the parts of the situation it
represents.
Emergent consequences: you can get more out of a mental model than you put into it.
Parsimony: people tend to construct only the simplest mental model if possible.
Pseudotransitive relations logically incorrect conclusions most of the time.
An alternative to the mental models approach
Natural deduction systems: A reasoning system made up of propositions and deduction rules to draw conclusions from
these propositions.
P and Q entail P,Q
P or Q and not P entails Q
Number of errors and time taken to solve problems depend on the number of inferences required.
No evidence that people construct mental models to solve reasoning puzzles.
Wason’s puzzles
The generative problem
Generative problem: Participants are told that the three numbers 2, 4, 6 conform to a simple relational rule that the
experimenter has in mind, and that their tas

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