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PSYC 2650 Chapter Notes -Modus Tollens, Modus Ponens, Confirmation Bias

Course Code
PSYC 2650
Anneke Olthof

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Chapter 13: Reasoning: Thinking Through the Implications of
What You Know
-often we draw conclusions from claims that seem solidly established
-this relies on Deduction - a process through which we start with claims or assertions
that we could as “given” and ask what follows from these premises
-deduction allows us to make predictions about upcoming events
-the same predictions can keep our beliefs in touch with reality
-if deduction leads to a prediction based on our beliefs, and the prediction turns out to
be wrong, this tells us that something is off track in our beliefs, and that things we
thought to be solidly established aren’t so solid after all
Confirmation and Disconfirmation
-each of us regards that many of our beliefs are solidly established e.g. water is wet
-we are far less certain about some other beliefs e.g. would he make a good secretary?
-how do we proceed in cases where we are uncertain?
-there are concerns with the ambiguity of confirming evidence and the great value of
disconfirming evidence
Confirmation Bias
-in order to evaluate a belief, it’s often useful to seek out both evidence that supports
that belief and also evidence that might challenge the belief
-despite the usefulness of disinformation, people generally don’t do it
Confirmation Bias - a strong tendency to seek out confirming evidence and to rely on
that evidence in drawing their conclusions
-first, when people are assessing a belief or hypothesis, they are far more likely to seek
evidence that confirms the belief than evidence that might disconfirm it
-second, when disconfirming evidence is made available to them, people often fail to
use it in adjusting their beliefs
-third, when people encounter confirming evidence, they take it at face value but when
they encounter disconfirming evidence, they reinterpret the evidence to diminish its im-
-people regularly fail to consider alternative hypotheses that might explain the available
data just as well as their current hypothesis does

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Memory for Disconfirming Evidence
-since people scrutinize disconfirming evidence, this mental activity sometimes makes
people more likely to remember disconfirming evidence
-e.g. in a study people who had places bets on a football game believed they had good
strategies for picking winning teams
-their faith was undiminished by a series of losses
-gamblers don’t remember their losses as “losses”, they remember them as mere flukes
or oddball coincidences
-winning bets were remembered as wins, but losing bets were remembered as “near
Belief Perseverance
-a phenomenon where even when disconfirming evidence is undeniable and out in
plain view but some people do not use it
-in a study, participants were asked to read a series of suicide notes and asked to find
out which ones were authentic
-as they offered their judgments, they were provided feedback
-the feedback was predetermined and had nothing to do with their judgments
-later participants were debriefed and told that the feedback they received was bogus
-they were then asked additional questions where they had to assess their own “social
sensitivity” and their actual ability on the previous task
-participants were influenced by the feedback - those who received the above average
feedback continued to think of their social sensitivity as being above average
-their beliefs persevered even when the basis for the belief had been completely dis-
-this has an important implication - maybe debriefing is not effective
Reasoning About Syllogisms
-a number of theorists have proposed that thought does follow the rules of logic
-if we make reasoning errors, it is not because of some flaw in our thinking
-instead, the errors must come from other sources e.g. carelessness, misreading
-errors in logical reasoning are ubiquitous
-if we are careless or misread problems, we do so with great frequency
-this is shown in studies of Categorical Syllogisms - a type of logical argument that
begins with 2 assertions (the problem’s premises) each containing a statement about a

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-the syllogism can be completed with conclusions that follow these premises
-e.g. all plumbers are mortal, all sadists are mortal, but all sadists are not plumbers
-participants who are asked to reason about syllogisms do remarkably poorly
Sources of Logical Errors
-errors in logical reasoning are extremely common
-errors don’t look at all like the product of carelessness, they tend to fall into a few sim-
ple categories
Belief Bias - if a syllogism’s conclusion happens to be something people believe to be
true anyway, they are more likely to judge the conclusion as following logically from the
-if the conclusion happens to be something they believe to be false, they are likely to re-
ject the conclusion as invalid
-logic is concerned with moral “local” issues of reasoning, specifically whether a partic-
ular conclusion is warranted by a particular set of premises
-when people show belief-bias, they are failing to distinguish between good arguments
and bad ones
-other logical errors seem to be the result of people relying on a low-level “matching
strategy” - endorsing conclusions if the words “match” those in the premise
-e.g. if a participants sees a premise such as “all A are B” and then another premise
such as “all D are B” the participant is likely to accept a conclusion like “all A are D be-
cause this conclusion “matches” the wording and structure of the premises
-this “matching strategy” is illogical and will lead to errors
Reasoning About Conditional Statements
Conditional Statements - statements of the familiar “if X then Y” format, with the first
statement providing a condition under which the second statement it guaranteed to be
- the rules of logic specify how one should reason about conditional statements
-one logical rule, called Modus Ponens justifies conclusions in this case: If P is true,
then Q is true. P is true. Therefore, Q must be true.
-another more difficult rule of Modus Tollens which justified the conclusion in this case:
If P is true, then Q is true. Q is false. Therefore, P must be false.
-other logical errors are Affirming the Consequent (modus ponens) and Denying the
Antecedent (modus tollens)
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