Harry Harlow began his studies of rhesus monkeys in 1930. He was inspired by watching the monkeys become attached to terry-cloths and throw tantrums if they were taken away. Harlow sought to find out why baby monkeys became so attached to inanimate towels.
The 30's through the 50's was a time period in which the common approach to raising children was to be unaffectionate and cold. According to an article from the Boston Globe, some researchers were convinced that humans love their mothers because they love their milk. This theory was called "drive reduction" because hunger was thought to be a driving force that needed to be satisfied.
Harlow devised experiments where he constructed two surrogate mothers for the baby monkeys. One mother was made out of wire with a steel nipple that gave milk. The other surrogate mother was softer with a cardboard and terry cloth body, but no milk. When the monkeys were hungry, they would quickly run to the wire mother and then run back to the cloth mother. Harlow concluded that since the monkeys spent more time cuddling with the cloth mother, love was more important to the contact comfort theory than pure desire for milk.
Harlow also performed cruel studies with monkeys that involved a spiked mother that would propel the monkeys off with cold blasts of air. He also created a black isolation chamber that hung animals upside down for up to 2 years and deprived them of contact with the world. These disturbing studies that Harlow performed prompted criticism of his chilling and controversial work.
Slater, Lauren. "Monkey love." The Boston Globe. 21 Mar. 2004