\”We hired a new CEO, but had to let him go after just seven months,\” the chairman of an East Coast think tank complained to me recently. \”His resume looked spectacular, he did splendidly in all the interviews. But within a week or two we were hearing pushback from the staff. They were telling us, \’You hired a first-rate economist with zero social intelligence.\’ He was pure command and control.\”

The think tank\’s work centers on interlocking networks of relationships with the board, staff, donors, and a wide variety of academics and policy experts. The CEO urgently needed to manage those relationships, but lacked the interpersonal skills that organizations increasingly need in their leaders. A CEO who fails to navigate those relationships artfully, the think tank\’s board saw, could torpedo the organization.

Why does social intelligence emerge as the make-or-break leadership skill set? For one, leadership is the art of accomplishing goals through other people.

As I\’ve written with my colleague Richard Boyatzis, technical skills and self-mastery alone allow you to be an outstanding individual contributor. But to lead, you need an additional interpersonal skill set: you\’ve got to listen, communicate, persuade, collaborate.

That was brought home to me yet again reading \”Making Yourself Indispensable,\” by John H. Zenger, Joseph R. Folkman, and Scott E. Edinger, which makes the strong point that a leader\’s competencies are synergistic. The more different competencies a leader displays at strength, the greater her business results.

But there\’s another critically important rule-of-thumb: some competencies matter more than others, particularly at the higher levels of leadership. For C-level executives, for example, technical expertise matters far less than the art of influence: you can hire people with great technical skills, but then you\’ve got to motivate, guide and inspire them.

While Zenger, Folkman, and Edinger make a strong empirical case that competencies matter, it overlooks a crucial point: some competencies matter more than others. Specifically, there are threshold competencies, the abilities every leader needs to some degree, and then there are distinguishing competencies, the abilities you find only in the stars.

You can be the most brilliant innovator, problem-solver or strategic thinker, but if you can\’t inspire and motivate, build relationships or communicate powerfully, those talents will get you nowhere. What Zenger and colleagues call the \”interpersonal skills\” — and what I call social intelligence — are the secret sauce in top-performing leadership.

Lacking social intelligence, no other combination of competences is likely to get much traction. Along with whatever other strengths they may have, the must-have is social intelligence.

So how do you spot this skill set? An executive with a long track record of satisfactory hires told me how his organization assessed social intelligence in a prospect during the round of interviews, group sessions, meals, and parties that candidates there routinely went through.

\”We\’d watch carefully to see if she talks to everyone at the party or a dinner, not just the people who might be helpful to her,\” he said. One of the social intelligence indicators: during a getting-to-know you conversation, does the candidate ask about the other person or engage in a self-centered monologue? At the same time, does she talk about herself in a natural way? At the end of the conversation, you should feel you know the person, not just the social self she tries to project.

I wouldn\’t use such subjective measures alone — you\’re better off to combine them with best practices on hiring without firing. But don\’t ignore your gut.