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Chapter 13

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University of Toronto St. George
Christine Burton

Chapter 13: Reasoning and Decision Making  Decisions: Making choices between alternatives  Reasoning: The process of drawing conclusions  Two specific types of reasoning: Deductive and inductive  Deductive reasoning: involves syllogisms (sequence of statements) in which a conclusion logically follows from premises; Rationalism  Inductive reasoning: in which a conclusion follows from a consideration of evidence, this conclusion is stated as being probably true, observe the world, put observations together to get the probable truth; Empiricism Deductive Reasoning: Syllogisms and Logic  Syllogism includes two statements called premises followed by a third statement, called the conclusion  Categorical Syllogisms: premises and conclusion describe the relation between two categories using statements that begin with all, no, or some  Ex. P1: All birds are animals. P2: All animals eat food. Conclusion: Therefore, all birds eat food.  Sometimes we cannot draw a logical conclusion from a syllogism, so it is indeterminate  Ex. Some birds can fly. Some things that can fly are airplanes. Therefore, some birds are airplanes  Mental models used to solve syllogisms are limited by the same types of things that limit other cognitive tasks: Working memory (if you don’t have enough WM capacity, cannot create multiple models and hold them), prior knowledge, visual imagery Validity and Truth in Syllogisms  A syllogism is valid when its conclusion follows logically from its two premises  EX. P1: All birds are animals (all 1 are 2). All animals have four legs (all 2 are 3). Conclusion: All birds have four legs (All 1 are C).  Syllogism 2 has the same form as syllogism 1, therefore both are valid because the conclusion follows from two premises  However validity and truth are two different things  Validity depends on the form of the syllogism and truth depends on the content of the premises Conditional Reasoning  Have two premises and a conclusion but the first premise has the form “IF…then…”  4 major types of conditional syllogisms, they are presented in abstract form (using p and q)  The symbol p, the first or “if” term is called the antecedent. The symbol q, the second or “then” term, is called the consequent  We can perform 1 of 2 actions in a conditional reasoning task:  Affirm part of the statement (say it’s true – did happen)  Deny part of the statement (say it’s false – didn’t happen)  Only two can lead to a valid point, others are invalid  Reasoning tasks are more difficult if they involve abstract concepts  We can use pragmatic reasoning schemas to help reduce resources required to solve a task  If we have concrete situation we recognize, schema is activated, in abstract situations, don’t have access to schema, have to go through logic, schemas reduce resources because we don’t really have to think about the situation  Concrete example: If I study, then I’ll get a good grade. PART OF THE STATEMENT Action Taken Antecedent Consequent Affirm Valid Invalid Deny Invalid Valid Conditional Reasoning: The Wason Four-Card Problem  4 cards, each card has a letter on one side and a number on the other side, your task is to indicate which cards you need to turn over to test the following rule: if there is vowel on one side, then there is an even number on the other side  53% said E directly tests the rule (if there is an E, then there must be an even number, so if there is an odd number on the other side, this would prove the rule to be false).  However, another card needs to be turned over to fully the test the rule, 46% said that in addition to E, the 4 would need to be turned over. Problem with this is that if a vowel is on the other side of the card, this is consistent with the rule, but if a consonant is on the other side, turning over the 4 tells us nothing about the rule, because having a consonant on one side and a vowel on the other does not violate the rule  Only 4% came up with the correct answer, that the second card needs to be 7, turning 7 over is important because revealing a vowel would disconfirm the rule  Key to solving the card problem is to be aware that it is necessary to look for situations that would falsify the rule  People have tendency to look for information that supports claim but tend not to look for information that refutes it, this is known as confirmation bias  Task in real world experiment  Task: each card has an age on one side and the name of a beverage on the other side, imagine you are a police officer who is applying the rule “if a person is drinking beer, then he or she must be over 19 years old”, which card must be flipped over to determine whether the rule is being followed  The beer/drinking-age version of Wason’s problem is identical to the abstract version except that concrete everyday terms (beer, soda, and age) are substituted for letters and numbers.  Griggs and Cox found that 73% provided the correct answer; it is necessary to turn over the “beer” and the “16 years” cards.  In contrast, none of the participants answered the abstract task correctly  Why is the concrete task easier than the abstract? Being able to relate the beer task to regulations about drinking make it easier to realize the “16 years” card must be turned over  Context makes huge difference Inductive Reasoning: Reaching Conclusions from Evidence  In deductive, premises are stated as facts, however, in inductive, p
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