Previous studies have shown that people’s memories are changeable, and systematic incorrect memories (e.g., false memory) can be created. We hypothesize that people’s beliefs about the real world can be changed similarly to the way systematic incorrect memories and systematic incorrect beliefs (which we call memory-based false belief) are generated. We also predict that since memory-based false beliefs are consistent with abstract knowledge that is consisted with prototypical patterns and organization found in the real world, false beliefs work adaptively in making inferences about environmental information in the real world. We conducted behavioral and simulation studies in order to examine our hypotheses on people’s beliefs and inferences about the real world. The results showed that participants had systematic false beliefs about cities’ attributes (e.g., whether they have a professional baseball team), and that such false beliefs worked adaptively in making inferences about population size.