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U-M PSYCH 240 - Deduction
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PSYCH 240 1st Edition Lecture 21Outline of Last Lecture: Judgement and Decision Making III. Availability HeuristicII. Simulation HeuristicIII. Conjunction FallacyIV. Framing EffectV. ConclusionsOutline of Current Lecture: DeductionI. Intro to DeductionII. Phenomena of Deductive ReasoningIII. Theories of Deductive ReasoningCurrent Lecture: Lecture 21: DeductionI. Intro to Deductiona. We’re trying to go from general to specific and draw conclusions that are guaranteed to be truei. Validity is different from truthb. Classes of deductive i. Quantifier1. Example a. Some businessman are wealthyb. All wealthy people are powerfuli. Therefore some businessmen are powerful2. Quantifier Typesa. Universal Positive: All doctors are richb. Particular Positive: Some lawyers are dishonestc. Universal Negative: No politician is trustworthyd. Particular Negative: Some actors are not handsomeii. Comparative1. Mighty Joe Young is more powerful than Godzilla2. King Kong is more powerful than Mighty Joe Younga. Therefore, King Kong is more powerful than Godzillaiii. Conditional (if-then statements)1. If you write a good research proposal (antecedent), then you will get funded (consequent)2. You write a good research proposalThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.a. You will be fundedII. Phenomena of Deductive Reasoninga. Syllogismsi. Example1. All men are animals. Some animals are aggressive. Therefore, some menare aggressive2. All men are animals. Some animals are female. Therefore, some men arefemale.3. All A are B. Some B are C. Therefore, some A are C*Normatively, content is completely irrelevant. All that matters is the form of the argument. This type of argument will always be invalid.ii. Difficulty Effect1. Syllogism are hard2. Subjects typically get ∼50% wrong3. Some B are A. No C are B. What, if anything, necessarily follows?iii. Validity Effect1. Some A’s are B’sAll B’s are C’sTherefore, some A’s are C’s(valid)2. All A’s are B’sSome B’s are C’sTherefore, some A’s are C’s(invalidiv. Content Effect(makes a big difference,but it shouldn’t)1. All dogs are animalsSome animals are petsTherefore, some dogs are pets.2. All sharks are animalsSome animals are petsTherefore, some sharks are petsv. Atmosphere Effect1. Some A’s are B’sSome B’s are C’sTherefore, some A’s are C’s(invalid)2. Some A’s are B’sSome B’s are C’sTherefore, All A’s are C’s (invalid)3. All A’s are B’s- More likely to accept valid than invalidConclusion matches atmosphere of premises- Particular trumps universalo If one conclusion is universal and the other is particular, the particular premises is preferredo If both premises are universal then ppl prefer universal conclusionsIf both premises are pos, both premises prefer positiveSome B’s are C’sTherefore, some A’s are C’s (invalid)vi. Figural Effect1. Some A’s are B’sSome B’s are C’sTherefore some A’s are C’s (invalid)2. Some B’s are A’sSome C’s are B’sTherefore, some A’s are C’s (invalid)vii. Conversion Effect1. Premise conversionsa. All A’s are B’s  All A’s areB’s and all B’s are A’si. All U of M studentsare smart BUT not all smart ppl are U of M studentsb. Some A’s are not B’s  Some A’s are not B’s and some B’s are not A’si. Converted toc. All A’s are B’sAll C’s are B’sTherefore, all A’s are C’sb. Wason Selection Task:i.1. 4% get the correct answerii. Bar Problem (another form of the Wason experiment)1. You’re a police officer walking into a barMore likely to accept syllogism when terms are in linear orderAll A’s are B’s + all B’s are A’sAll C’s are B’s + all B’s are C’sa. Patron who is drinkingb. Patron who is not drinkingc. Patron who is 21d. Patron who is 192. In a certain state, the legal age for drinking is 213. Rule: If you are drinking alcoholic beverages in a bar, you must be 214. Who do you check? (the patron who is 19)a. 75% get it rightiii. Motisponins: affirming the 1. If A implies B and A is true, then B must be true2. In Wason experiment, this is the E cardiv. Motistolins: denying the consequent1. If A implies B and B is false, then A must be false2. In Wason’s experiment, this is the E cardv. Difficulty constantIII. Theories of Deductive Reasoninga. Formal rules: logical rules that are built into our mindsi. Types1. Intuition: people are logical, but make mistakesa. If A is true, and A implies B, then b must be true2. Assumption: formal rules of logic are built ina. Reason by proof: Apply rules until reach conclusionii. Problems1. Errors occur because: a. Misinterpret premisesi. Assume when someone says all A’s are B’s, we might assume that all B’s are A’sb. Some rule unavailablec. Can’t find proof for conclusion2. Problem 1: if deduction depends on formal content (content-free) rules, then why does content matter?a. To apply motisponins and motistonins, content can’t be applied. b. You’d think that content should be irrelevant but it isn’t3. Problem 2: Why would formal rules be built in, when they only apply to infrequent tasks (deduction)?b. Mental modelsi. People construct mental models (representations) that correspond to the premises, describe them, and then try to falsify conclusion by constructing alternative models1. All of the artists are beekeepers.Some of the beekeepers are clever.Some artists are clevera. Artist – Beekeeper Artist – Beekeeper – clever Beekeeper – clever Beekeeper*some of the artists are clever? Not necessarilyii. Errors arise b/c of WM limitation1. Can’t hold on to multiple modelsiii. Syllogisms that require more models are harderiv. Problem: constructing counterexamples is a very complex process that is specificto deduction. Would subjects with no training use it?c. Verbal reasoning(POLK and Newell)i. Intuition: without training, subjects don’t have deduction – specific strategiesii. They do sophisticated language processesiii. Assumption: repeatedly use linguistic process to extract more info from problemstatement1. Rather than applying formal rules or searching for counterexamplesiv. Errors arise b/c linguistic processes are adapted to demands of communication, not deductionv. In communication, we construct a plausible representation of the gist1. Includes inferred info that may not be deductively valid2.


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