Cosmides (1989)

Used the Wason card selection task, and various invented scenarios involving the breaking of social rules.

In the original Wason expt:
The rule was “if there is a vowel on one side, then there is an even number on the other side.”

Cards showing E, K, 2, 7, - which should be turned over to check for rule violations?

The answer
E to see if there is a vowel with an odd number: and 7 to see if vowel — the rule of logic is that if P leads to Q, there is only a violation if P and NOT_Q occur together (sometimes written P & ~Q). In this case we need to look to see if a vowel and an odd number co-occur, so we should only turn over vowels and odd numbers (the rule allows for having even numbers on the back of everything: that's why it's only P and ~Q which matters).

But 80% of undergraduates get it wrong. Simplified examples (not used by Cosmides) are given below.

Bus pass 1


1st overhead. Logically this means that you should turn over having the pass and not going on the bus. It is not “if and only if” and therefore logicians would say the rule allows people to travel on the bus without a pass. But most people would ‘look for cheaters’.

Bus pass 2


2nd overhead. This way around almost everyone would get it right, since the logically correct answer for exception to the rule is someone travelling on a bus without a pass.

Cosmides (1989) argued that detailed experimental results of this general kind should be explained in terms of the influence of natural selection on human reasoning — the idea is that human mental capacities are adaptations to the demands of living in hunter-gatherer societies, and not naturally suited to the more abstract rules of the propositional calculus found in textbooks of logic. The phrase used in the online “Primer” on Evolutionary Psychology is that “Our modern skulls house a stone age mind”.

But, although there is support for the finding that in experimental contexts human subjects tend to be bad at logic, not many psychologists are convinced that this makes the stone ages relevant (e.g. Fodor, 2000). Empiricists can argue that most human subjects have had a lifetime of learning about rules and rule-breaking, but little experience of abstract logic.


Cosmides, L. (1989) The logic of social exchange: Has natural selection shaped how humans reason? Studies with the Wason selection task. Cognition, 31, 187-276.

Duchaine, B., Cosmides, L., & Tooby, J. (2001). Evolutionary psychology and the brain. Current Opinion in Neurobiology, 11(2), 225-230. (online in ScienceDirect).

Fodor, J. (2000). Why we are so good at catching cheaters. Cognition, 75, 29–32.

Wason, P.C. (1966). Reasoning. In B.M. Foss (Ed.), New horizons in psychology. Harmondsworth: Penguin. pp. 135-151.

See also a discussion with examples in The Philosophers Magazine on the Internet.

Some other external links refering to the Wason card selection task –

Wason's obituary in the Guardian of April 25, 2003

A description in Cosmides and Tooby's web pages