As a young child, the ex-PepsiCo CEO developed a crucial leadership skill through debating everything.

Sibling disadvantage can be felt by the youngest, with hand-me-downs and unjust blame.

At the Aspen Ideas Festival on Tuesday, Indra Nooyi, the former PepsiCo CEO who retired in 2018, stated that being the youngest of three siblings provided her with a professional advantage. This is because it taught her how to debate and assert herself.

"My childhood unknowingly honed my core leadership abilities through debates at home, as we were forced to discuss every topic and make decisions, even over a simple bar of chocolate."

Nooyi explained that she and her siblings frequently posed questions such as "If you were president of this country, what would you do?" and "If you were chief minister of the state, what would you do?" and would argue about whose ideas were superior.

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"As a child, you often feel the urge to argue more because you feel like you are often overlooked. However, now that you've achieved success, you realize that you've won the lottery in life."

Nooyi observed how leaders like Vernon Jordan and Bill Clinton used different techniques to build strong arguments. Jordan emphasized his key points, while Clinton showed his emotional commitment to a topic by biting his lip.

PepsiCo's stock gained cumulatively 79% under Nooyi's leadership, which began in 2006. The company's annual net revenue increased from $35 billion in 2006 to $63.5 billion in 2017.

How to get better at speaking up for yourself

Nooyi learned to speak up for herself and express her opinions with courage and confidence through practice.

To build confidence, start by training your brain to dismiss self-doubt, advised millionaire investor Matt Higgins to graduating students at Molloy University. When presented with an opportunity at work, disregard the inner voice that questions your worthiness.

Instead, Higgins recommended, ask yourself: "Why not me?"

When speaking, maintain a straight posture, advised Stanford University lecturer and communication expert Matt Abrahams in a CNBC Make It interview last month. Avoid swaying physically, as it can make you appear nervous.

According to social psychologist Adam Galinsky, when you're knowledgeable or passionate about a topic, it's easier to speak confidently about it.

"According to Galinsky's 2017 TED blog post, "Expertise gives us credibility." She further stated that we can come across as an expert by tapping into our passion. When we tap into our passion, we give ourselves the courage to speak up and get permission from others to do so."

The Aspen Ideas Festival is a media partner with NBCUniversal News Group, which includes CNBC.

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