Harry Frederick Harlow (October 31, 1905 – December 6, 1981) was an American psychologist best known for his maternal-separation and social isolation experiments on rhesus monkeys, which demonstrated the importance of care-giving and companionship in social and cognitive development. He conducted most of his research at the University of Wisconsin–Madison, where humanistic psychologist Abraham Maslow worked for a time with him.
Harlow's experiments were controversial; they included rearing infant macaques in isolation chambers for up to 24 months, from which they emerged severely disturbed. Some researchers cite the experiments as a factor in the rise of the animal liberation movement in the United States.
In a well-known series of experiments conducted between 1957 and 1963, Harlow removed baby rhesus monkeys from their mothers, and offered them a choice between two surrogate mothers, one made of terrycloth, the other of wire.
The studies were motivated by John Bowlby's World Health Organization-sponsored study and report, "Maternal Care and Mental Health" in 1950, in which Bowlby reviewed previous studies on the effects of institutionalization on child development such as René Spitz's and his own surveys on children raised in a variety of settings. In 1953, his colleague, James Robertson, produced a short and controversial documentary film titled A Two-Year-Old Goes to Hospital demonstrating the almost immediate effects of maternal separation. Bowlby's report, coupled with Robertson's film, demonstrated the importance of the primary caregiver in human and non-human primate development. Bowlby de-emphasized the mother's role in feeding as a basis for the development of a strong mother-child relationship. However, his conclusions generated much debate. It was the debate concerning the reasons behind the demonstrated need for maternal care that Harlow addressed in his studies with surrogates.
In Harlow's classic experiment, two groups of baby rhesus monkeys were removed from their mothers. In the first group, a terrycloth mother provided no food, while a wire mother did, in the form of an attached baby bottle containing milk. In the second group, a terrycloth mother provided food; the wire mother did not. It was found that the young monkeys clung to the terrycloth mother whether or not it provided them with food, and that the young monkeys chose the wire surrogate only when it provided food.
Whenever a frightening stimulus was brought into the cage, the monkeys ran to the cloth mother for protection and comfort, no matter which mother provided them with food. This response decreased as the monkeys grew older.
When the monkeys were placed in an unfamiliar room with their cloth surrogate, they clung to it until they felt secure enough to explore. Once they began to explore, they occasionally returned to the cloth mother for comfort. Monkeys placed in an unfamiliar room without their cloth mothers acted very differently. They froze in fear and cried, crouched down, or sucked their thumbs. Some even ran from object to object, apparently searching for the cloth mother, as they cried and screamed. Monkeys placed in this situation with their wire mothers exhibited the same behavior as the monkeys with no mother.
Once the monkeys reached an age where they could eat solid foods, they were separated from their cloth mothers for three days. When they were reunited with their mothers, they clung to them and did not venture off to explore as they had in previous situations. Harlow concluded from this that the need for contact comfort was stronger than the need to explore.
The study found that monkeys who were raised with either a wire mother or a cloth mother gained weight at the same rate. However, the monkeys that had only a wire mother had trouble digesting the milk and suffered from diarrhea more frequently. Harlow's interpretation of this behavior, which is still widely accepted, was that a lack of contact comfort is psychologically stressful to the monkeys.
The importance of these findings is that they contradicted both the then common pedagogic advice of limiting or avoiding bodily contact in an attempt to avoid spoiling children and the insistence of the then dominant 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.
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 the sixty-sixth Annual Convention of the American Psychological Association, Washington, D.C., August 31, 1958.
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