Concept
Conjunction Fallacy
The conjunction fallacy is the error of judging a specific combination of events as more probable than one of those events alone, because the added detail makes the story more plausible, although a combination can never be more likely than its parts.
Amos Tversky and Daniel Kahneman's best-known demonstration is the Linda problem. Participants read about Linda, a bright, outspoken young woman who had studied philosophy and cared deeply about discrimination and social justice. Asked which was more probable, that Linda is a bank teller or that she is a bank teller active in the feminist movement, most people chose the second. Logically it cannot be: every feminist bank teller is a bank teller, so the combination can be at most as likely as the single event. Kahneman explains the error as representativeness at work. The detail that fits the description makes the story more coherent, and System 1 mistakes coherence for probability. The same trap shapes plans and forecasts. A scenario in which the licence arrives in March, the engineers are hired in April and the key client signs in May feels convincing precisely because it is detailed, yet each added step is one more condition that must hold. Even when every step is fairly likely on its own, all of them together can be much less so. The practical rule is to treat every extra detail in a persuasive story as a cost to its probability, not a gain.
A real-life example
Aslı's startup in Eskişehir pitched investors a launch story: the beta in June, a partnership with a regional supermarket chain in August, two hundred paying shops by December. It sounded compelling. A mentor asked her to judge each step separately. One by one, each looked likely, but all three had to happen, and the whole story was far less certain than any single step. She rewrote the plan with two different routes to the December goal.
How to use it
- 1When a scenario feels convincing, list the separate conditions it needs and judge each one on its own.
- 2Remember that every detail you add to a story can only lower its probability, however much more plausible it makes it sound.
- 3For important plans, prepare at least two routes to the goal rather than one detailed sequence in which everything must go right.