The conjunction fallacy is credited to the work of Amos Tversky and Daniel Kahneman. It is a mistake that people make when they assume that a specific condition is more likely than a more general one. A specific event will have many conjoined events, while a single general condition will have only one event. The probability of any two events occurring together is always much less than either one happening apart.
An example similar to one used in the original research by Tversky and Kahneman may be helpful:
Lucas is 28 years old, single, outspoken, and very bright. He majored in humanities. As a student, he was deeply concerned with issues of discrimination and social justice. Which statement about Lucas is more likely?
1. He is a barista.
2. Lucas is a barista and is active in the anti-war movement.
People who make the conjunction fallacy chose option 2. However, mathematically, the probability of two events occurring together (in “conjunction”) will always be less than or equal to the probability of either one occurring alone. That is, Lucas could be a barista and he could be involved in the anti-war movement, but it is much less likely that he is both.
Tversky and Kahneman might argue that people make the conjunctive error because option 2 seems more “representative” of Lucas based on the description of him, even though it is mathematically less likely.