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

“The optimists have the vision,” Bourla said on the Fortune 500: Titans and Disruptors of Industry podcast. “The pessimists land you to reality and help avoid pitfalls.”

In other words, the top executive doesn’t see optimism and pessimism as opposing forces but rather as complementary—each playing a distinct role in effective leadership.

“Pessimists are usually right,” he told Fortune Editor-in-Chief Alyson Shontell. “But nothing great on this earth has been accomplished without an optimist behind it.” 

The distinction, he suggested, comes down to influence. Being correct isn’t the same as being able to rally people.

“We all appreciate having pessimists around, but no one follows them. Maybe people listen to them, but the pessimist doesn’t have followers. Everybody follows an optimist. That’s the one who can 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 lands you to reality—but be the optimist.”

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

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

“I grew up in a city that had 55,000 Jews when my parents grew up,” he recalled. “There were only 700 of us when I grew up. So it was an almost complete extermination that clearly left a mark in all the Jewish community in Thessaloniki, the city I was coming from.”

At one point, his mom was just seconds away from execution before her life was spared. And she gave her son his first life-long lesson in optimism. 

“From my mom, I got more of her personality drive,” Bourla said. “She was someone that was extremely optimistic in life. She would think that every obstacle is an opportunity to do something better, and she would think that nothing is impossible.”

Bourla’s 30-year journey up Pfizer’s ranks to 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 rising through the ranks.

His first role was in Pfizer’s Greek animal health division, and Bourla had stints in Belgium and Poland moving to the U.S. Ultimately, the 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 made me suited for the CEO position,” he said. “But it certainly shaped who I am. I had the opportunity to live with so many different cultures and work with even more.”

That experience taught him that leadership isn’t one-size-fits-all.

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

“All of that made me have a better understanding of the value of diversity, but also only when you manage diversity the right way.”

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