Autism and the Conjunction Fallacy

Background: The conjunction fallacy violates a fundamental rule of probability, that the likelihood of two independent events occurring at the same time (in "conjunction") should always be less than, or equal to the probability of either one occurring alone (P(A) ≥ P(A & B)). People who commit the conjunction fallacy assign a higher probability to a conjunction than to one or the other of its constituents. In the most famous demonstration in the literature (Tversky & Kahneman, 1983) people read a description of Linda, a 31-year-old, smart, outspoken woman who was a philosophy major, concerned with discrimination and social justice, and a participant in antinuclear demonstrations. When asked to judge a number of statements about Linda according to how likely they are, people usually rank the statement “Linda is a bank teller and is active in the feminist movement” above the statement “Linda is a bank teller”, thus committing the fallacy. The conjunction fallacy has been cited as a classic example of the tendency towards the automatic contextualisation of problems (i.e., the fundamental computational bias). This sort of automatic contextualisation, however, is not universal. According to an influential cognitive account of autism, the Weak Central Coherence theory (Frith & Happé, 1994; Happé, 1999), typically developing individuals tend to create global representations, and they process information in context, whereas autistic individuals engage in more detailed, local or piecemeal processing.

Objectives: To date there is virtually no research which has examined the use of reasoning heuristics in autistic individuals. Thus, the aim of the present study is to determine whether the fundamental computational bias, the tendency to automatically contextualise any given input, operates as powerfully amongst autistic individuals as in typical populations. In light of the Weak Central Coherence account, and considering the findings regarding contextual processing of complex verbal materials (e.g., López & Leekam, 2003), we expected autistic participants to be less susceptible to the conjunction fallacy than non-autistic participants.

Methods: In two experiments we compared the performance of a group of high functioning adolescents with autism and a group of typically developing adolescents (between the age of 11 and 16) on a set of conjunction fallacy tasks. The samples were matched on cognitive ability (as measured by the WISC, and the Raven Progressive Matrices) and executive functions (working memory, inhibition and set-shifting).

Results: Experiment 1 showed significantly fewer conjunction errors amongst the autistic sample. Experiment 2 extended these findings to a new set of problems, demonstrating that the difference between the groups did not result from increased sensitivity to the conjunction rule, or from impaired processing of social materials amongst the autistic participants.

Conclusions: Although adolescents with autism showed less bias in their reasoning, they were not more logical than the control group in a normative sense. The fact that autistic participants display less sensitivity to contextual cues than typically developing individuals when they evaluate choice options can have profound consequences to their everyday lives. The compatibility of our findings with the Weak Central Coherence account will also be discussed.