Yet it is the leadership mindset that guides Bourla as he runs nearly $150 billion pharmaceutical giant not just about science or strategy. It’s about psychology—specifically, balancing optimism with realism.

“Optimists have a vision,” Bourla said Fortune 500: Titans and Industry Disruptors podcast. “Pessimists bring you to reality and help avoid pitfalls.”

In other words, the top executive does not see optimism and pessimism as opposing forces but as complementary—each has a different role to play in effective leadership.

“The pessimists are usually right,” he said luck Editor-in-Chief Alyson Shontell. “But nothing remarkable on earth has been accomplished without an optimist behind it.”

The difference, he suggests, comes down to influence. Righteousness is not the same as rallying the people.

“We are all thankful that there are pessimists around, but no one follows them. Maybe people listen to them, but a pessimist has no followers. Everyone follows an optimist. That will inspire them.”

His formula for leadership success is simple: Surround yourself with people who keep you grounded—but be the person who carries the torch when it matters most.

“You want to be a successful leader? Bring a team around you that expects you to be realistic—but optimistic.”

Bourla’s penchant for optimism was born from his mother, who survived the Holocaust

Bourla was born in Thessaloniki, Greece, to a middle-class Jewish family. His parents survived the Holocaust, which killed six million Jews worldwide and more than 80% of the population of Greece before the war.

“I grew up in a city with 55,000 Jews when my parents were growing up,” he remembers. “There were only 700 of us when I grew up. So it was an almost total extermination that clearly left a mark on all the Jewish communities in Thessaloniki, the city I came from.”

At one point, his mom was seconds from being killed before her life was saved. And he gave his son his first lifelong lesson in optimism.

“From my mom, I got her personality drive,” Bourla said. “He is a person who is very optimistic in life. He thinks that every obstacle is an opportunity to do something better, and he thinks that nothing is impossible.”

Bourla’s 30-year journey through Pfizer’s ranks in the C-suite

Beyond Bourla’s family history, the places his career has taken him have helped shape his leadership style—and guide his success.

After becoming a doctor of veterinary medicine, he completed his PhD in reproductive biotechnology. In 1993, he joined Pfizer—where he would spend more than three decades climbing the ranks.

His first role was in the animal health division of Pfizer, and Bourla had stints in Belgium and Poland before moving to the U.S. Finally, constant exposure to different cultures, markets, and teams shaped his personal leadership style as much as any title did.

“I don’t know if that makes me a good fit for the CEO position,” he said. “But it’s definitely shaped who I am. I’ve had the opportunity to live with so many different cultures and work with so many more.”

That experience taught him that leadership is not one-size-fits-all.

“I learned to be respectful of the differences of others. I learned to be sensitive about how you behave if you want to achieve results, if you want to motivate people,” said Bourla.

“All that made me have a better understanding of the importance of diversity, but also if you manage diversity in the right way.”

Watch the full episode on YouTube. The transcript of the episode can be found here.