People used to send more seasonal greetings cards in times gone by, but in December 1974, Phil and Joyce Kunz received a particularly bountiful crop. Some were simple offerings of “Happy Christmas” but others contained long letters. There was also a complaint from the police.

Phillip Kunz, a sociologist at Brigham Young University in Provo, Utah, was not entirely surprised. A few weeks earlier, he and his colleague Michael Woolcott had posted nearly 600 cards to strangers they had picked out of the phone book, adding a return address for “Phil and Joyce Kunz” or “Dr and Mrs Phillip Kunz”...

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