Today's term from psychology is Subadditivity Effect.

Subadditivity Effect: The tendency to judge probability of the whole to be less than the probabilities of the parts. This seems like an unnecessarily esoteric concept to post as a psychology term of the day, but it illustrates something fascinating about how humans think. We are VERY poor at dealing with statistics. Oh, here's an example of the subadditivity effect. Take a random groups of people, ask them 3 questions, what percentage of the local supermarket's sales is candy? What percentage is ice cream? Whut percentage is other junk food? After a bit of a time ask them what percentage of the local supermarket's sales is junk food? Now compare the results. People will almost always estimate the sales of junk food less than the sum of the three types of junk food even though the three questions and the one question ask the same thing.