Baby monkeys chose a soft cloth 'mother' over one that actually fed them

Starting in 1958, psychologist Harry Harlow raised infant rhesus monkeys with two surrogate mothers: a bare wire frame that dispensed milk, and a soft cloth-covered frame with no food. The infants clung to the cloth mother for comfort and visited the wire mother only to feed, overturning the assumption that infant attachment is just about who provides nourishment.

— Harry F. Harlow, The Nature of Love — American Psychologist, 1958