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.

See the rest of the story at Business Insider

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