Hillary Clinton came across as presidential at the Democratic debate because of a key psychological strategy

Many political pundits and critics agree that Hillary Clinton won Tuesday's Democratic presidential debate.

There's one key reason behind her success: She demonstrated a balance of competence and warmth.

In their book "Friend and Foe," Maurice Schweitzer, Ph.D., and Adam Galinsky, Ph.D. note that a combination of warmth and competence is crucial for leadership effectiveness.

"We trust warm people, because we know they care about us," the authors write. And "we trust competent people, because they are credible, effective, and efficient."

According to body language expert Dr. Nick Morgan, Clinton exuded warmth when she laughed and shook Bernie Sanders' hand after he said, "Enough of the emails!" in reference to this year's scandal surrounding the private server Clinton used as Secretary of State.

"That was a nice moment for her, a very human, warm, connected kind of moment," Morgan told Business Insider.

Clinton also flaunted her political competence at several points throughout the debate.

For example, she showed a realistic perspective on policymaking, reminding the audience that she'd worked with Republicans to pass the Children's Health Insurance Program.

"I'm a progressive," she said. "But I'm a progressive who likes to get things done."

It's unclear whether Clinton deliberately sought out ways to act simultaneously warm and competent and elicit her audience's trust. But if she wants to continue to convince Americans that she's the strongest presidential candidate, there's no doubt this strategy could work in her favor.