An entrepreneur who spent 5 years interviewing 21 self-made billionaires asked every single one how they made their first million dollars - here's what they said

• Self-made billionaires don't get to the three-comma club without first becoming millionaires.
• Rafael Badziag interviewed 21 billionaires for his book, "The Billion Dollar Secret," and they all told him how they made their first $1 million.
• Many earned it through founding their own business or through investments.
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Many of the world's richest billionaires built their wealth from scratch.

Rafael Badziag, an entrepreneur and expert in the psychology of entrepreneurship, spent five years conducting face-to-face interviews with 21 self-made billionaire entrepreneurs around the world (defined as those with a net worth of at least $1 billion) and researching their lives and companies.

In his book "The Billion Dollar Secret: 20 Principles of Billionaire Wealth and Success," he shared how each of those billionaires made their first $1 million. Some earned it through the businesses they founded, while others earned it through investments or by working for someone else's company.

For the purposes of this slideshow, unless otherwise specified, we assumed that any of Badziag's subjects whose net worth we were not able to verify with Forbes' net worth listings is worth at least $1 billion. All information on the billionaires comes from Badziag's book.