Before you fast track an employee for promotion, take a moment to ponder the dark side of her positive personality traits.

“Beware of the dark side,” Yoda warned young Luke Skywalker, and apparently the advice holds even in a galaxy far, far away from the Star Wars universe. Namely, ours.

Recently, Adrian Furnham, a professor at University College London, offered a British Psychological Society conference pretty much the same advice in his keynote address. Only his wisdom was directed towards those hoping to become masters of business rather than masters of a mystical, intergalactic knighthood.

First, the BPS Occupational Digest blog reports, Furnham reminded his audience just how hard good leadership is. “There are 70,000 books in the British Library with leadership in the title,” the blog reports Furnham remarking, “but most leaders don’t succeed, they fail, with a base rate of bad leadership collated from various studies of 50 per cent.”

Why do half of would-be leaders falter? Furnham blames, in part, our tendency to look for certain traits when we promote people into leadership positions. Looking for, say, self confidence in candidates may make intuitive sense (we want confident leaders after all), but this method of screening leaves out a truth that later derails many leaders–most positive traits can have a serious dark side.

The personal “force” that made you promote someone to a position of leadership in the first place–whether it’s self-confidence, drive or interpersonal skills–taken to extremes or misused often causes a leader’s downfall. Self-confidence can curdle into unhealthy risk taking, interpersonal abilities sometimes morph into manipulation and showboating, and the dark side of diligence can be perfectionism.

Simply looking for the traits you think a leader should have isn’t enough, according to Furnham, you also need to worry if a candidate has a trait in excess and provide structures that make sure would-be leaders learn how to express those traits for good rather than ill. BPS boils down Furnham’s recommendations:

Furnham distilled some key implications for selection and recruitment. Consider using ‘dark side’ measures; beware excessive self-confidence and charm; do a proper… reference check; and get an expert to ‘select out’ for you.

As for management, the message was to beware fast-tracking wunderkinds, and to seek a mentor, coach or at least a very stable deputy to keep these individuals on the rails.

In other words, a promising young talent with the markers of a potential leader can use their talents to become a force for good, or an office Darth Vader.

So when you’re pondering promotions, select not just by the sheer magnitude of a candidate’s raw ability, but also look at how they channel that ability. A little Yoda-like mentoring certainly can’t hurt either.

Have you ever promoted a wunderkind expecting him or her to become a great leader only to end up with more of a Darth Vader?