Tens of Thousands of Americans Could Be Billionaires Right Now If Their Wealthy Families Didn't Make This All-Too-Common Financial Mistake Victor Haghani and James White's book 'Missing Billionaires: A Guide to Better Financial Decisions' unpacks what went wrong for the U.S.'s once-richest families.

If the wealthiest American families of centuries past had managed their fortunes differently, tens of thousands of U.S. billionaire heirs would grace today's rich lists. So, what happened?

That's a complicated question, but Victor Haghani and James White's recently published book Missing Billionaires: A Guide to Better Financial Decisions focuses on one critical answer: the poor risk decisions made in both investing and spending.

"Wealthy families tended to have investments which were not well diversified, had high fees and costs, and were overly risky relative to the families' spending policies, which were not typically very flexible or sensitive to the ups and downs of their investment portfolio," White tells Entrepreneur.