test", infants were presented with a fearful stimulus, often a noise-making teddy bear. Without the mother, the infants cowered and avoided the object. When the surrogate mother was present, however, the infant did not show great fearful responses and often contacted the device—exploring and attacking it. Another study looked at the differentiated effects of being raised with only either a wire-mother or a cloth-mother. Both groups gained weight at equal rates, but the monkeys raised on a wire-mother had softer stool and trouble digesting the milk, frequently suffering from diarrhea. Harlow's interpretation of this behavior, which is still widely accepted, was that a lack of contact comfort is psychologically stressful to the monkeys, and the digestive problems are a physiological manifestation of that stress. The importance of these findings is that they contradicted both the traditional pedagogic advice of limiting or avoiding bodily contact in an attempt to avoid spoiling children, and the insistence of the predominant behaviorist school of psychology that emotions were negligible. Feeding was thought to be the most important factor in the formation of a mother–child bond. Harlow concluded, however, that nursing strengthened the mother–child bond because of the intimate body contact that it provided. He described his experiments as a study of love. He also believed that contact comfort could be provided by either mother or father. Though widely accepted now, this idea was revolutionary at the time in provoking thoughts and values concerning the studies of love. Some of Harlow's final experiments explored social deprivation in the quest to create an animal model for the study of depression. This study is the most controversial, and involved isolation of infant and juvenile macaques for various periods of time. Monkeys placed in isolation exhibited social deficits when introduced or re-introduced into a peer group. They appeared unsure of how to interact with their conspecifics, and mostly stayed separate from the group, demonstrating the importance of social interaction and stimuli in forming the ability to interact with conspecifics in developing monkeys, and, comparatively, in children. Critics of Harlow's research have observed that clinging is a matter of survival in young rhesus monkeys, but not in humans, and have suggested that his conclusions, when applied to humans, overestimate the importance of contact comfort and underestimate the importance of nursing. Harlow first reported the results of these experiments in "The Nature of Love", the title of his address to