If you're a smart leader, you likely hold exit interviews when employees leave. Whether they were one of your strongest employees or your weakest, you want to know what made them decide to cut ties. Something -- your compensation, your benefits, your interesting work, your team -- made them want to come. What made them want to leave?

Was it a better offer from another company? Was it a fight with their closest work buddy? Was it their boss? Harvard Business Review explored this subject last year and concluded that most leaders don't know why their people are leaving -- but they can read between the lines if they look at the evidence.  

In my experience as an executive coach for fast-growing companies, there are two reasons the best employees tend to leave: They didn't get a much-wanted promotion -- or they did.

Promotions Can Be Silent Career Killers

On the surface, that may not make sense. This is a situation where it helps to remember the maxim "Be careful what you wish for." A promotion, after all, seems to carry all pros from an employee's perspective: acknowledgment of one's achievements, increased respect from teammates, a bigger say in what happens, higher pay.

But as you and I both know, there are cons with management. A bigger say in what happens also translates to more responsibility. Higher pay tends to come with longer hours. Relationships with teammates can become more fraught when you're the one in charge. And achievements are harder to come by when you have to rely on other people -- who, you suddenly realize, can't be controlled.

There's an additional consideration for superstar contributors: People who are great at something tend to genuinely love doing it. Once they become managers, they get pulled out of the trenches. That means they're not only dealing with the struggles of being a new supervisor, but they're also missing the thing that made them feel competent. 

A 2018 Gallup study of nearly 7,500 workers found that almost half of employees feel some degree of burnout. One main reason was an unmanageable workload, which is common to mid-level managers. Another was a lack of support from managers, which is common to people working under stressed-out leaders. The wrong promotions have a strong ripple effect.

How to Evaluate an Employee's Fit for a Promotion

While some managers are able to work and lead, that can be less common in smaller businesses that need managers helping multiple teams. Instead, you need to closely evaluate your top employee's likelihood of success in a more visible role. 

1. How does this employee handle stress?

High performers aren't immune to stress. Some handle it better than others -- and they'll encounter a lot more stress as they move up the ranks. When you throw something unexpected at your top employee, does she get anxious? Is she motivated by getting things done or by getting things done perfectly? That distinction can make a huge difference in her potential for success as a manager.

2. Is your top performer focused on details or the big picture?

You actually want both in a great leader. A strong manager can envision the company's long-term goals and then break down the steps it takes to get there. If your potential promotee seems to get too far into the weeds or dismiss important details as minutiae, he may not be happy in a management role that asks him to live in his "weak zone" often.

3. What are this person's work relationships like?

Is your top performer a genius who identifies as a loner? Conversely, is she the type to drop everything to help a co-worker? People lacking people skills -- or those that have a people-pleasing attitude -- tend to struggle when leading others. Making decisions is difficult; they either want to take into account every single perspective or none at all. That's bad for business.

4. What's your employee's true motivation?

If your team has done personality assessments before, you likely have an idea that Jill thrives on solving problems, while Jack wants to be liked. Look closely at what your employee is motivated by. If work-life balance is the thing he values most, will he enjoy a promotion? If she seems absorbed in her actual tasks, will she miss them if she spends most of her day in meetings? A state of flow isn't something to be overlooked.

Some employees are able to overcome tendencies that may mark them as having low promotion potential. But genuinely think about the possibility of your employee's success or happiness in an elevated role. If you make the wrong choice, you may end up needing to fill two spots.