The clue was in plain sight, clutched in the monkeys’ hands, until finally a grad student, Bill Mason, noticed it: diapers. The cages in the baby hatchery were sometimes lined with old diapers to provide bedding material and protect the babies from the cold floor. The monkeys clung to the diapers, especially when they were afraid, and took them along when they were carried to new cages. Mason proposed a test to Harlow: Let’s expose some young monkeys to a bundle of cloth and a bundle of wood. Let’s see whether the monkeys just need to hold on to something, anything, or whether there’s
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