A self-made millionaire who interviewed 1,200 wealthy people noticed one behavior they all have in common

After interviewing more than 1,200 of the world's wealthiest people over the past three decades, author and self-made millionaire Steve Siebold found that they shared a single behavior: they're selfish, and they're OK with it.

Rich people think selfishness is a virtue, he explains in "How Rich People Think," while average people think selfishness is a vice.

"The rich go out there and try to make themselves happy. They don't try to pretend to save the world," he tells Business Insider. "If you're not taking care of you, you're not in a position to help anyone else. You can't give what you don't have."

In this selfish quest to make themselves happy, they have an active mindset - they go after what they want and they problem solve until they achieve it.

Siebold puts it bluntly: "You're not going to be discovered, saved, or made rich by an outside force. If you want a lot of money, build your own ship. No one is coming to the rescue."

The rich not only actively pursue their ambitious financial goals until they're met, they expect to make money.

"Wealthy people ask, 'Why not me? I'm as good as anyone else and I deserve to be rich. If I serve others by solving problems, why shouldn't I be rewarded with a fortune?'" Siebold writes. "And since they have this belief, their behavior moves them toward the manifestation of their dreams."

While the rich are busy putting themselves first and embracing selfishness, the average person sees it as a negative - and it's keeping them from wealth, the self-made millionaire says.