A leadership expert says Hillary Clinton is missing one key personality trait - and it could stop her from winning the election

In order to earn the public's trust, psychologists say leaders need to strike a near-perfect balance between two personality traits: warmth and competence.

In other words, they need to find ways to show that they're not only experienced and credible, but also human and friendly.

For Hillary Clinton, who's been accused repeatedly of having a "likability problem," this balance could be virtually impossible to achieve. While Clinton generally has little problem demonstrating her political skill, she's had a considerably harder time conveying the charisma that's necessary to win over voters.

She is largely perceived as competent ... but not warm.

It's an issue that's common among women leaders - but not among their male counterparts - scientists say.

When it comes to warmth and competence, "women in power walk a fine line," said John Antonakis, a professor of organizational behavior at HEC Lausanne, University of Lausanne in Switzerland.

Antonakis' research focuses on the role of charisma in leadership, and he pointed out that, especially compared to her Democratic rival Bernie Sanders and Republican frontrunner Donald Trump, Clinton is a relatively uncharismatic leader.

But if Clinton were to display the warmth and charisma that are characteristic of Sanders and Trump, it's possible that people would perceive her as less competent.

"If she's too warm and too nice, [voters] will go, 'Oh, she's too motherly. She doesn't have the stuff to be a leader," Antonakis said. "She's damned if she does, damned if she doesn't."

Research by psychologists including Princeton's Susan Fiske and Harvard's Amy Cuddy suggests that women often find themselves in a "catch-22" in terms of impression management.

We generally see certain women - think housewives - as warm, while we perceive other women - think political leaders and businesswomen - as competent. Rarely do we see the same woman as both.

To be sure, Clinton has had moments, such as a Democratic debate in October, in which she exhibited both warmth and competence to her benefit. In a display of warmth, she laughed at Sanders when he said, "Enough of the emails!" in reference to the scandal surrounding the private server she used as Secretary of State.

And her campaign has at times gone out of its way to show her warmer side, such as when they released an ad showing her comforting a young girl whose parents might be deported.

Should Clinton go on to become the Democratic nominee, it's unclear whether this gender issue will sabotage her chances at winning the presidential election.

Said Antonakis: "I don't know to what extent the stereotype card is going to come out and mess things up for her."