Watch for
Probabilities that do not add up when they logically should.
Belief & ConfidenceBias #106
Subadditivity Effect
The whole seems less than the sum of its parts.
The tendency to judge the probability of an entire event as less than the combined probabilities of its individual component parts violating probability theory.
Why it matters: A violation of the extension rule in probability. Shows intuitive probability is not additive.
Try this
Check whether your probability estimates are internally consistent.
Real-world example
Judging the probability of dying from a heart attack as lower than the summed probabilities of dying from specific heart conditions.
Key researchers
Daniel Kahneman, Amos Tversky
First described in 1984
Psychological mechanism
Differential availability and unpacking. Unpacking an event into specifics makes each component more vivid and available increasing its judged probability but the unpacked whole loses the additive force.
Seminal research
Discussed in Kahneman and Tversky (1984) on the extension rule and subadditivity.
Related Biases
Debrief
Biases are not character flaws. They are recurring patterns in how minds compress uncertainty, save energy, and narrate reality. Once you recognise the pattern, you can slow the decision down, test the assumption, and make space for a better explanation.
Sources
Original ResearchTversky and Koehler (1994) — Subadditivity
EncyclopediaWikipedia overview