The subadditivity effect is the tendency to judge the probability of the whole as being less than the sum of the probabilities of its parts. This cognitive bias leads individuals to underestimate overall probabilities when they are broken down into their constituent components.
In a study, participants estimated the probability of dying from cancer as 18%, from heart attacks as 22%, and from other natural causes as 33%. When asked to estimate the probability of dying from any natural cause, they judged it to be only 58%, even though the sum of the individual probabilities was 73%.
To overcome the subadditivity effect, one can consciously aggregate probabilities rather than evaluating them in isolation, ensuring a comprehensive assessment of the total risk.