Harry Harlow completed a number of studies in which baby rhesus monkeys were raised with two artificial mothers. One of the mothers was made of wire and delivered food; the other was made of cloth and provided no food. Harlow found that when the infant monkeys were startled or frightened, they:

Harlow found that when the infant monkeys were startled or frightened, they: preferred the cloth surrogate that did not provide food.

Explanation:

In Harlow's experiments with rhesus monkeys, the mother made of wire was only preferred when she provided food. Whenever the monkeys needed comfort - for instance, when they were startled of frightened -, they would cling to the mother made of cloth. Harlow concluded from this experiment that care and love are crucial for child development. Harlow's studies are obviously seen as cruel and unethical nowadays. There was a variation of the study with rhesus monkeys that led to disturbances and even death because the infant monkeys were kept in total or partial isolation.