Deduction

Psy315h - Class 26

(This page last updated 25 November, 2005.)
  

Confirmation Bias

Most people tend to seek confirming evidence rather than disconfirming evidence.

See pp. 412-413 in textbook

 

Wason Card Task (Four-Card Task)

• Each card has a word on the front and a single character on the back.
• True or false: All cards with a place name on the front have a letter on the back.
• Which of these four cards must you turn over to determine whether the rule above is true?

Enforcing the Drinking Age

You are working in a bar. Your job is to prevent people under 18 from consuming alcohol. You come to a table with four people.

#1 is drinking a gin and tonic.

#2 is drinking orange juice.

#3 looks about 45 years old.

#4 looks about 13 years old.

Which of these people do you need to check more closely to be sure the drinking law is not being violated?

We can format this problem in the same way that we did the Wason Card Task.

• Each card has the name of a beverage on the front, and an age on the back.
• True or false: All cards with an alcoholic drink on the front have an age over 21 on the back.
• Which of these four cards must you turn over to determine whether the rule above is true?

Errors in Logic

In the basic Wason task, you are testing a rule of the form "If P, then Q"

"If there is a place name on the front, there is a letter on the back."

P = place name on front

Q = letter on back

The four cards correspond to the four different types of information you can have in solving this problem.

The rules of logic tell us that in order to properly test this type of rule, we must test "P" and "not Q". The other two categories, "Q" and "not P", are not relevant to the truth of the rule.

In the basic version of this task, 25% or less usually respond correctly.

Many subjects leave out the "not Q"response, and many include the "Q" response.

The structure of the drinking age problem is exactly the same. Yet 75% usually respond correctly to this problem. Why do so many get the drinking problem correct while getting the basic problem wrong?

Why do we Make these Errors in Reasoning?

One explanation: Permission Schemas

• To make the connection to the appropriate schema, we need to recall situations similar to the problem.
• This idea is similar to the Availability heuristic.

Alternative explanation: Detecting cheaters

• We do better whenever we can understand the problem in terms of a social contract, because we have cognitive mechanisms to detect cheaters.
• Social cooperation key for survival.
• These mechanisms could be shaped by evolution.

Testing Availability and Social Contract Explanations

Experiment by Leda Cosmides

True or false: If a man eats cassava root, then he must have a tattoo on his face.

Which of these men must you check to determine whether the rule is true?

#1 is eating cassava root.

#2 is eating molo nuts.

#3 has a tattoo on his face.

#4 has no tattoo on his face.

In terms of the Wason task, this problem looks like this:

In this form, only 21% give the correct response. Not a surprise, because both the Availability and the Social Contract explanations predict poor performance.

But what if subjects are told this before solving the problem?

• A tattoo means that a man is married.
• Cassava root is an aphrodisiac that makes men irresistable to women.

  This makes it a social contract situation. Now 75% correct responses.

Cosmides' experiment suggests that we perform better at reasoning when we understand problems in terms of a social contract. It still leaves open the question of whether our understanding of social contracts is rooted in genes that were shaped by evolution.

next class: Consciousness
  
Psych 315H: Cognitive Psychology Kyle Cave Psychology Dept. U. Mass.