Harry Harlow performed experiments on Rhesus Monkeys during the mid-1900s to develop models of social interaction, relationships, and isolation (Wikipedia). His work was very influential and, while criticized by animal-rights advocates, is still studied today.
Harlow's main experiments involved the isolation of baby rhesus monkeys from their mother; the monkeys were instead "raised" by a machine that fed them, either made of exposed wire or covered in soft cloth. Later experiments involved the total isolation of the infant monkeys, resulting in permanent mental damage.
Harlow's work is contraversial to this day, especially among animal-rights groups. The monkeys almost always suffered extreme emotional and mental damage from their isolation, and despite gaining solid scientific results early on, Harlow continued the experiments. There is speculation that the Animal Liberation movement of the 1960s stemmed directly from Harlow's published work. He is most often criticised for his emotional detachment to his subjects; since Harlow considered the monkeys to be entirely animals, he felt no compunction to be empathic towards them and treated them as data points. However, Harlow's work was also the first to draw psychological conclusions from scientific experimentation instead of from unprovable hypotheses.