The psychologist who proved that affection is a biological need said he never loved the monkeys he studied. Born Harry Israel in Fairfield, Iowa, in 1905, Harlow switched from English to psychology at Stanford and took the name Harlow on the advice of mentor Lewis Terman. Denied lab space at the University of Wisconsin, he and his students renovated a vacant building into a primate lab. At a time when behaviorists said infants bonded only for food and doctors warned against touching babies, his nursery raised monkeys clung desperately to cloth diapers.

His surrogate experiment offered infant rhesus macaques a wire mother with milk and a cloth mother with none. They spent up to 18 hours a day on the cloth mother, used her as a secure base, and he called the result contact comfort in his 1958 address The Nature of Love. After his wife Margaret died of cancer in 1971, Harlow sank into depression and turned to isolation chambers he called the pit of despair, keeping some infants alone for up to two years. The backlash fed the American animal liberation movement.

• Learning sets, his early finding that monkeys learn how to learn
• The drumming teddy bear fear test and the cloth mother as a secure base
• Chronic diarrhea in wire mother infants that drank as much milk
• Peer therapy that paired isolated monkeys with younger, gentler ones
• Student Stephen Suomi’s nightmares and William Mason’s account of Harlow’s mindset