Q.Why did Harlow and Harlow's experiment with baby monkeys challenge the earlier belief that attachment is based purely on feeding?

Harlow and Harlow's experiment showed that baby monkeys preferred a non-feeding cloth surrogate over a feeding wire surrogate, directly contradicting the idea that attachment is driven simply by who provides food. …

Before Harlow and Harlow's (1962) study, a common assumption (rooted in learning-theory accounts of attachment) was that infants become attached to their caregivers essentially because the caregiver is associated with the satisfaction of a basic biological need — feeding. On this view, an infant would be expected to become most strongly attached to whichever adult (or object) actually provides food, since attachment was assumed to develop through this repeated association between the caregiver and hunger relief.

Harlow and Harlow's experiment tested this assumption directly, and its results challenged it decisively. Baby monkeys were separated from their biological mothers about 8 hours after birth and reared for six months by two artificial, surrogate 'mothers' built in an experimental chamber — one made of bare wire, the other covered in soft cloth. Critically, the researchers varied which surrogate actually provided feeding: half of the baby monkeys were fed by the wire mother, and half were fed by the cloth mother. If attachment really depended purely on feeding, the monkeys fed by the wire mother should have shown a preference for her, since she was the source of nourishment for them.

Instead, the study found that regardless of which surrogate had actually fed them, the baby monkeys consistently showed a strong preference for the cloth mother, and spent significantly more time with her than with the wire mother — even the monkeys who had been fed exclusively by the wire mother still preferred spending their time clinging to the soft, cloth-covered surrogate. This result clearly demonstrated that feeding, by itself, was not the crucial factor driving attachment. Instead, what mattered was contact comfort — the physical comfort and reassurance provided by a soft, warm, huggable surrogate — regardless of whether that surrogate was actually the source of food. …