Subadditivity effect
The subadditivity effect is judging the chance of a whole event to be less than the sum of the chances of its parts. Breaking an event into detailed pieces makes each piece feel more likely, so the pieces add up to more than the whole.
In everyday life
People rate the chance of dying from 'natural causes' as lower than the total they get when they separately rate heart disease, cancer and other specific natural causes.
You rate the chance that your trip gets delayed as low, but when asked separately about a late train, a missed connection and a strike, your three estimates together come to far more than your first answer.
Why your mind does this
Our brains handle small, concrete quantities well but struggle with large numbers, fractions and odds. Rounding, grouping and labeling let us make fast decisions without doing the math. For most daily choices, a rough answer is good enough.
Its family, in 30 seconds: Simpler numbers
How to spot it
Ask whether your estimate for the overall event matches the total of your estimates for each way it could happen.
How to counter it
List the specific ways an event could happen, estimate each, and use the total to check your overall guess.