Rosabeth Kanter wrote a book a few years ago with the simple title "Confidence". Kanter is theformer editor of the Harvard Business Review and knows where of she speaks.
Confidence is the expectation of positive outcomes. Think Muhammad Ali. I don't know how long it took to internalize the belief that 'I am The Greatest'. However once internalized, the fight was over before the opponent entered the ring.
Working with Managers and Leaders these past twenty years, I have noticed different outcomes from those groups led by less confident people. So, confidence is not optimism, nor is it an attribute of character. It's an expectation of a positive outcome.
It was said of General Robert E. Lee, that he was worth at least two Northern Divisions. On paper the American Civil War should have been over within a year. The difference was the quality of leadership exhibited by Confederate officers. The closest to Lee on the Union side was Grant. Not a very bright man, nobody would have predicted his rise to greatness before the war. Yet, when push came to shove, it was Grant who rescued the Union.
Kanter says, "Talent,intelligence, knowledge are nice, but confidence is essential."
Confidence is not arrogance, conceit, or complacency. The confident leader or manager believes that persistance and hard work will win out most every time over talented but insecure people. Confidence is contagious.
Secure, successful people (i.e. excellent leaders/managers) can be emotionally generous. Because they are generous, people want to be part of their team. People who are not secure take their frustration out on others and engender no loyalty or desire on the part of others to help them succeed.
A personal note: when I began my business career I was not as confident as I was a few years later. Once I gained confidence, I let no obstacle stand in the way of getting the outcomes I wanted to attain.
When you promote, I suggest you test for confidence and not just know-how or experience. A confident person will help the organization achieve its goals on a regular and consistent schedule.