She died of a stroke after an argument over the price of skimmed milk, and the press remembered her as the Witch of Wall Street, a miser in a rotting black dress. She was also the wealthiest woman in America, a self-taught macroeconomic strategist whose fortune would be counted in billions today, and the person who kept New York City solvent during the Panic of 1907. This episode strips away the tabloid caricature to examine the actual math of Hetty Green's life, from reading stock quotations aloud to her failing-eyed father at age six in whaling-boom New Bedford to keeping the family books at 13.

Sent to New York at 20 with $1,200 to buy a husband-catching wardrobe, she spent $200 on plain clothes and secretly put the rest into high-yield bonds. When her family's millions were locked in trusts run by male executors, she fought back in a la ...