Post-decisional justification gives people an excessively optimistic evaluation of their decisions, until they receive very clear and undeniable information to the contrary.

This statement is true. Post-decisional justification is a cognitive bias in which people tend to overestimate the positive aspects of their chosen option and overlook the negative aspects until they receive clear and undeniable evidence to the contrary. This bias helps people feel more confident in their decisions and defend them against possible challenges or doubts.