Born in 1939, George and Charles Finn were both retarded, with IQs around 60. But each twin could remember nearly every event that had ever befallen him, including the weather: “On April 15, 1956, Dr. Williams came to visit me and to ask me questions about dates. It was rainy and windy in the morning, but the sun came out in the middle of the afternoon.”

Both could also almost instantly tell the day of the week for every day on a 40,000-year calendar — and could answer such complex questions as “During what years between 1780 and 1795 did the 7th of August fall on a Wednesday?”

Thus they could have calculated the birthday of Daniel McCartney — who was doing the same thing a century earlier.