In 1965, in the French town of Arles, a 90-year-old woman named Jeanne Calment made a deal that looked like a gift to the other side.

She had no living heirs, so she sold her apartment to her lawyer, André-François Raffray, under a common French arrangement called a viager. The terms were simple. Raffray would pay her 2,500 francs every month for the rest of her life, and in return he would take ownership of the apartment when she died. She got to keep living there until the end.

For Raffray, it seemed like a brilliant investment. He was 47, half her age, and buying a home at a discount from a woman already in her nineties. The math looked easy. Pay for a few years, then move into a nice apartment he had bought for a fraction of its value.