Deduction

• biases and content effects

• bias = whenever there is a systematic deviation in performance from the normative approach

Belief Bias

• make conclusions based on personal beliefs about the world (you are supposed to use logical rules to draw conclusions)

• Selma Hayek looks best sporting the “uni-brow” that she had in “Frida”

• Ben Affleck and Jennifer Lopez will have a longlasting marriage

Validity of the silly statement

• sometimes being logical, means providing reasons for your conclusions

• Women look their best when sporting a “unibrow”

• The actress that played “Frida” sported a unibrow

• Selma Hayek is the actress that played “Frida”

• Selma Hayek looked best sporting the “unibrow” that she had in “Frida”

Laboratory evidence

• Believable conclusions = are consistent with your personal beliefs

• Unbelievable conclusions = are inconsistent with your personal beliefs (e.g., unibrow statement)

• also, valid and invalid conclusions

• believable/valid, believable/invalid, unbelievable/valid, unbelievable/invalid

Results

• Evans, Newstead, & Byrne (1993)• % of conclusions accepted as valid

• Valid Invalid• Believable 86% 66%• Unbelievable 62% 13%

• conclusion: the believability of a statement has a strong influence on people’s critical thinking (i.e., belief-bias effect)

content effects

• def.: whenever the “content” (i.e., the specific words) influences your critical thinking performance

• normative approach content is irrelevant and should not influence your performance

Wason selection task

• Manchester Leeds Train Car

• (imagine that these are four cards, each having a city on one side and a mode of transportation on the other)

• “Every time I go to Manchester I travel by train”

• answer: turn over Manchester and Car

results in laboratory

• Wason & Shapiro (1971)

• showed that students did a good job with this version of the task compared to an abstract version

• conclusion adding realistic words (content) to an abstract problem improved performance (thus, it’s a content effect)

guaranteed solution to the selection task (algorithm)

• D B 3 5 7• If a card has a D on one side, then it has a

3 on the other side.• answer: D, 5, 7

• p not p q not q not q• If p, then q• answer: p, not q

Test 1 Study Guide

• normative approach, descriptive approach• logic, def. of rational• logic vs. real-life• syllogisms, deduction, parts of the

syllogism, validity of conclusion, algorithm, Euler’s circles

• conditional reasoning problems, conditional, content of a problem vs. logical structure

Test 1 (cont.)

• logical rules related to conditional reasoning, affirming the antecedent, etc. (including modus ponens, modus tollens)

• Wason selection task, abstract vs. realistic content of a problem

• logical operators, if…then, and, or, not; two types of “or” (exclusive, inclusive)

• disjunctive reasoning; deontic content or deontic reasoning

Test 1 (cont.)

• bias, belief-bias, personal beliefs vs. logic, content effects