The main goal of our research was to investigate relationships between decision making, personality traits (extraversion, agreeableness, conscientiousness, emotional stability, and openness), and resistance to sunk cost. Additionally, we examined the incremental validity of resistance to sunk cost in addition to personality traits when predicting decision outcomes. We used self-reports for measuring all variables. In our research, there were 222 participants (57% of which were women) aged from 19 to 63 years. All the participants were employed, were working at least half of the standard working hours, and had at least half a year in the length of service working at their current job. The results showed that the dimensions of extraversion, emotional stability, and openness were significantly positively correlated with resistance to sunk costs. As expected, agreeableness and conscientiousness were unrelated to resistance to sunk costs. According to the hypotheses, emotional stability and conscientiousness were negatively associated with decision outcomes, and there were no significant relations between extraversion, agreeableness, and openness to experiences with decision outcomes. Unexpectedly, our results showed that decision outcomes and resistance to sunk cost were unrelated. Based on this finding the incremental validity of resistance to sunk costs in predicting the outcome of decisions over basic personality traits was not confirmed. The results of our study show that personality traits which include good self-regulation and cognitive flexibility such as conscientiousness and emotional stability are associated with rational thinking and that individuals with more pronounced these traits will more often are more likely to avoid negative decision outcomes and be more resistant to irrational, harmful investments.