Memory & Cognition

, Volume 30, Issue 2, pp 191-198

First online:

On the reality of the conjunction fallacy

  • Ashley SidesAffiliated withDepartment of Psychology, Rice University
  • , Daniel OshersonAffiliated withDepartment of Psychology, Rice University Email author 
  • , Nicolao BoniniAffiliated withDepartment of Psychology, Rice University
  • , Riccardo VialeAffiliated withDepartment of Psychology, Rice University


Attributing higher “probability” to a sentence of formp-and-q, relative top, is a reasoning fallacy only if (1) the wordprobability carries its modern, technical meaning and (2) the sentencep is interpreted as a conjunct of the conjunctionp-and-q. Legitimate doubts arise about both conditions in classic demonstrations of the conjunction fallacy. We used betting paradigms and unambiguously conjunctive statements to reduce these sources of ambiguity about conjunctive reasoning. Despite the precautions, conjunction fallacies were as frequent under betting instructions as under standard probability instructions.