KembaraXtra – Psychology: Conjunction Fallacy

Definition

The conjunction fallacy is the error of judging a combination of two events or attributes as more probable than one of its components alone. This violates a basic principle of probability because the probability of A and B occurring together cannot be greater than the probability of A by itself.

Classic Demonstration

The fallacy was famously demonstrated by Amos Tversky and Daniel Kahneman using the Linda problem. Participants read a description of a socially engaged young woman and then judged it more likely that she was both a bank teller and active in the feminist movement than that she was simply a bank teller.

Cognitive Mechanism

The error is commonly explained by the representativeness heuristic. The more detailed description appears to match the personality sketch better, so it feels more plausible. People substitute similarity or narrative coherence for formal probability and overlook the fact that the conjunction is a smaller subset of the broader category.

Role in Judgement and Decision-Making

The conjunction fallacy appears in medical diagnosis, forecasting, legal reasoning, risk assessment, and everyday prediction. It is more likely when added detail produces a vivid or causally coherent story. Training in set inclusion, frequency formats, and explicit probability rules can reduce the error.

Summary

The conjunction fallacy occurs when A and B together are judged more likely than A alone. It is driven largely by representativeness and narrative fit. The error reveals a conflict between intuitive judgement and probability theory. It has broad implications for reasoning under uncertainty.