It's well-documented that hanging around chronically negative people can bring you down. The opposite is also true: Spending time with positive people makes you happier.

In 2013, Princeton researchers found that emotions--both happy and sad--can also spread on social networks, especially on Facebook. They found that "emotional states can be transferred to others via emotional contagion."

Emotional contagion is the study of how emotions spread from one individual to another. Recently, cognitive psychologists have been helping leaders understand contagion to motivate teams, employees, companies and yes, countries.

Remarkably, one of the best examples we have of positive leadership contagion took place decades before scientists began studying the topic. You can see it for yourself in the new movie Darkest Hour.

The title aptly describes the feeling of panic and impending doom during the four weeks of May 1940. Western Europe was falling to Nazi Germany and Hitler's army was on the verge of annihilating the British forces at Dunkirk.

Winston Churchill--played by Gary Oldman in an Oscar-worthy performance--had been named Prime Minister. The big question: Would Churchill bow to public pressure and sign a peace deal with Germany? 

Churchill's words and actions turned around public opinion, motivating the British to hang on and fight to the end. He accomplished the transformation through powerful words and inspiring speeches. But the movie also brings to light an under appreciated leadership technique that Churchill used brilliantly.

Although Churchill felt uneasy in private, he appeared supremely confident in public.

In one critical scene--which historians tell me is accurate--Churchill looked into the sea of press cameras waiting for him to deliver bad news and, instead, he held up two fingers and flashed the "V" sign for victory. Churchill knew instinctively that emotions spread and that leaders set the tone.

In private, Churchill experienced anxiety, isolation, despair, and anger. In public, Churchill held his head high and radiated the look of a winner.

Emotional Contagion as a Leadership Advantage

Wharton management professor Sigal Barsade has spent a decade studying emotional contagion in organizations and companies. She and co-author Olivia A. O'Neill write in a 2016 Harvard Business Review article: "Positive emotions are consistently associated with better performance, quality, and customer service... On the flip side, negative emotions usually lead to negative outcomes, including poor performance and high turnover."

The key is that emotional contagion starts at the top. "The primary mechanism for creating a healthy emotional culture is through leaders," Barsade says.

Barsade recommends that leaders pay attention to how they carry themselves. When you walk into a room, what are you communicating? Are you upbeat and positive or do you look like you're carrying the weight of the world on your shoulders? People are watching you and your attitude is rubbing off on them.

Much of the research on emotional contagion is coming out of the U.S. military, where team unity is everything. This year, I've met with instructors in the Army Special Forces and naval commanders. I also visited a group of officers meeting in a remote desert location whose work is to assess critical threats against the country. They all learn that leaders communicate with their words and their physical presence, especially in uncertain times.

According to a new training manual for navy leaders, "Perhaps the most intangible quality leaders can exercise in taking care of their people is to approach the job and the mission with a contagious enthusiasm." The manual goes on to state that when leaders feel tempted to join "the chorus of complaints" about a particular mission or schedule, they must elevate themselves above the negative attitudes.

Yes, listen to the complaints and let people get it off their chest--but above all, leaders keep their heads high. "Leaders can only inspire others with a sense of contagious enthusiasm coupled with an attitude of relentless optimism," according to the manual. 

You may not feel relentlessly optimistic today. Social media and television news networks are filled with pessimism, anger and division.

Now we know that those negative emotions spread. Leaders, by definition, must be the exception and spread positive emotions.

A relentlessly positive attitude--at least in public-- may very well be your secret weapon to stand out as a leader people want to follow.