EffectReasoning & flexibility · Logical Fallacy · Entry 69
Subadditivity Effect – when parts seem more probable than the whole
Do events seem more likely when broken into parts than when considered as a whole? That’s Subadditivity Effect – the tendency to judge the probability of separate components as higher than the probability of the entire event.
Where it can show up
- Weather forecasts – people think “30% chance of rain in the morning or 30% in the evening” means rain is more likely than the overall “50% chance of rain today.”
- Risk assessment – asking about the probability of a “terrorist attack at an airport,” “at a subway,” and “on a train” separately results in higher total perceived risk than asking about a “terrorist attack in the country.”
- Business evaluations – the likelihood of individual project stages succeeding seems higher than the probability of the entire project’s success.
A practical countermeasure
- Remember that parts can’t be more probable than the whole – if an event includes all options, its probability can’t be lower.
- Don’t trust fragmented forecasts without context – broken-down probabilities can distort perception.
- Double-check logical inconsistencies – if the sum of probabilities exceeds 100%, there’s an error in judgment.
