Harry Harlow (1905 - 1981) is known for his experiments on maternal separation and social isolation of rhesus monkeys. His work emphasized the importance of care-giving and companionship as vital to normal social and cognitive development.
In his surrogate mother experiment, Harlow demonstrated the importance of contact comfort. Baby rhesus monkeys were separated from their mothers and given two surrogate mothers - one made out of wire, and another made of terry cloth. He found that the baby monkeys preferred to cling to the terry cloth surrogate even when food was provided by the wire surrogate.
In his social isolation experiments, he again separated baby rhesus monkeys from their mothers and subjected them to partial or total isolation of varying duration. He found that those who experienced partial isolation exhibited abnormal behaviors such as blank staring, going in circles, and self mutilation. Those who experienced total isolation exhibited severe psychological disturbance, and experienced emotional shock upon being released from isolation. He also found that subsequent attempts to socialize monkeys who were isolated were only partially successful.
Harlow's work revealed the importance of contact comfort and social interaction to healthy development, and influenced child-rearing practices, particularly in orphanages and other institutions that provided care to children.