The world’s wealthiest people have several things in common, but one of the most prominent is that the 10 richest have more than $100 billion to their name. The very top have $200 billion or more.

As billionaires like Elon Musk, Jeff Bezos, and Mark Zuckerberg grow more powerful, many are asking: Is America still a democracy, or is it turning into an oligarchy? The rich have always influenced ...