Overconfidence can be a manager’s biggest downfall.

Researchers from Harvard Kennedy School, the University of Gothenburg, the University of Warwick, and the Lahore University of Management Sciencesexamined how advancing to a managerial position through either self-promotion or external promotion affects the promotee’s job performance and leadership ability. The study found that managers who are self-promoted perform worse due to hubris, especially over their social skills.

While traits like extraversion and self-confidence increase are lauded among those who want to become leaders—and for good reason—the study found that they can also create a blindspot for managers. As a result, C-suite leaders should take a holistic approach to promoting managers and consider those who have yet to raise their hand for the role. As for could-be managers, they should be mindful of how their confidence can narrow their vision and hamper their leadership performance.

The study split 555 managers into two groups: those who proactively expressed interest in becoming managers and those who were told they would become managers. Managers were then asked to lead four different teams of three to solve puzzles over three hours.

After completing the puzzles, managers rated how well they felt they performed. Fifty-five percent of self-promoted managers described their performance as “better” or “much better” than all managers participating in the study, but in actuality they performed worse than lottery managers. Only 38% of lottery managers however rated themselves as “better” or “much better” than their peers.

In examining both groups of people, the study found that lottery managers did a better job at reading other people and had better social skills, in comparison to those who were self-promoted and overconfident in how well they were doing as a leader.

The next step for this research is to bring it into the real world to see the extent to which the study is predictive of managerial performance, measuring it against other ways in which companies might assess their managers, such as their 360 reviews and other performance metrics.

Ben Weidmann, corresponding author for this study and director of research at Harvard Kennedy School’s Skills Lab, says the typical management selection is suboptimal and riddled with biases because leaders often appoint managers with whom they share similarities or they erroneously believe that those who are eager to be managers make for good leaders; that isn’t the case.

“I think it would be a step in the right direction if people were able to cast the net much more widely and do these broad skill assessments to see prospectively who might be good managers,” Weidmann says. A better metric for promoting managers, he says, is to seek out individuals who display strong economic decision-making skills, such as their ability to smartly allocate time and team resources.