Perhaps you have seen pictures or videos from the 1960s of rhesus monkey babies clinging to inanimate surrogate mothers.
In one of Harlow’s experiments, a baby monkey clings to the softer surrogate mother.
These experiments were by Harry Harlow, who eventually went against the psychology mainstream to demonstrate that love—namely caregiver-baby affection—was required for healthy development.
Dr. Harlow created inanimate surrogate mothers for the rhesus infants from wire and wood. Each infant became attached to its particular mother, recognizing its unique face and preferring it above all others. Harlow next chose to investigate if the infants had a preference for bare wire mothers or cloth covered mothers. For this experiment he presented the infants with a cloth mother and a wire mother under two conditions. In one situation, the wire mother held a bottle with food and the cloth mother held no food, and in the other, the cloth mother held the bottle and the wire mother had nothing.
Overwhelmingly, the infant macaques preferred spending their time clinging to the cloth mother. Even when only the wire mother could provide nourishment, the monkeys visited her only to feed. Harlow concluded that there was much more to the mother/infant relationship than milk and that this “contact comfort” was essential to the psychological development and health of infant monkeys and children. 
According to Stuart G. Shanker , various primates reach levels of funtional-emotional development similar to the first 2-3 levels (out of 9) that humans accomplish. Perhaps part of the system is the infancy period is much longer for humans.
Although a baby rhesus doesn’t express its positive affects with the same sorts of wide joyful smiles that we see in human infants between the ages of two and five months, in other respects it behaves in a manner similar to that of a human infant. The rhesus baby spends lots of time snuggling into its mother’s body or looking keenly at her face. It visibly relaxes while being rocked, and vocalizes happily when the mother plays with it. We can even see the baby rhythmically moving its arms and legs and vocalizing in time to its caregiver’s movements and vocalizations.
Shanker said this about Harlow’s experiments:
Although it was clear that the infants were deriving great comfort from the cloth-covered surrogates, they still suffered from striking social and emotional disorders.
One might interject here: Well so what? Who cares about social and emotional disorders? Well, aside from gunshot victims. What about intelligence? What about self awareness? The thing is though, that intelligence and possibly even the capacity for basic symbolic thought—ideas—are developed via emotions and social interactions.
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