This research work analyzes human behavior in complex situations and explains how decisions makers act in ambiguous situations.

The objective of this research work is to study the sunk cost effect and the completion percentage effect of an investment project in a decision-making process. This research work uses a "retrospective rationality" approach to justify irrational behaviors such as the sunk cost effect, the completion percentage effect of an investment project and the irrational escalation since decision-makers are repeatedly affected by the decisions on past irreversible investments.

This research work evaluates three sunk cost levels, and three completion percentage levels of an investment project, besides three neutral situations in a business environment and a personal decision situation. Graduate students in three Portuguese Management Schools responded to the questionnaires.

Model results show that the value of resources invested is crucial for understanding the students' rational behavior, who participated in this research work. These results disclose statistical evidence that the information on sunk costs and completion percentage of an investment project determines human behavior under irrational escalation in ambiguous situations. As a consequence, decision makers have the opportunity to interpret their decisions, since the scenarios do not allow a unique definition of rational choice, it is not correct to judge the irrational decision makers that decide to continue to invest in ambiguous situations