Harry F. Harlow, “Love in Infant Monkeys,” 1959

Source:  Photograph by Gordon Coster in Scientific American

An infant monkey clinging to its terry cloth “mother.”

Source: Courtesy of National Primate Research Center, University of Wisconsin

After long periods of complete isolation and maternal deprivation, which produced disturbed behaviors, Harry Harlow experimented with monkey “group psychotherapy.” After being placed in a zoo, the monkeys began to play together and groom one another, but they reverted to their abnormal behaviors when they were returned to Harlow’s laboratory.

The first love of the human infant is for his mother. The tender intimacy of this attachment is such that it is sometimes regarded as a sacred or mystical force, an instinct incapable of analysis. No doubt such compunctions, along with the obvious obstacles in the way of objective study, have hampered experimental observation of the bonds between child and mother.

Though the data are thin, the theoretical literature on the subject is rich. Psychologists, sociologists and anthropologists commonly hold that the infant’s love is learned through the association of the mother’s face, body and other physical characteristics with the alleviation of internal biological tensions, particularly hunger and thirst. Traditional psychoanalysts have tended to emphasize the role of attaching and sucking at the breast as the basis for affectional development. . . .

Now it is difficult, if not impossible, to use human infants as subjects for the studies necessary to break through the present speculative impasse. . . . Clearly research into the infant-mother relationship has need of a more suitable laboratory animal. We believe we have found it in the infant monkey. For the past several years our group at the Primate Laboratory of the University of Wisconsin has been employing baby rhesus monkeys in a study that we believe has begun to yield significant insights into the origin of the infant’s love for his mother. . . .

We have sought to compare the importance of nursing and all associated activities with that of simple bodily contact in engendering the infant monkey’s attachment to its mother. For this purpose we contrived two surrogate mother monkeys. One is a bare welded-wire cylindrical form surmounted by a wooden head with a crude face. In the other the welded wire is cushioned by a sheathing of terry cloth. We placed eight newborn monkeys in individual cages, each with equal access to a cloth and a wire mother. Four of the infants received their milk from one mother and four from the other. . . .

The monkeys in the two groups drank the same amount of milk and gained weight at the same rate. But the two mothers proved to be by no means psychologically equivalent. . . . Records made automatically showed that both groups of infants spent far more time climbing and clinging on their cloth-covered mothers than they did on their wire mothers. . . .

These results attest to the importance—possibly the overwhelming importance—of bodily contact and the immediate comfort it supplies in forming the infant’s attachment for its mother. . . .

The time that the infant monkeys spent cuddling on their surrogate mothers was a strong but perhaps not conclusive index of emotional attachment. Would they also seek the inanimate mother for comfort and security when they were subjected to emotional stress? With this question in mind we exposed our monkey infants to the stress of fear by presenting them with strange objects, for example, a mechanical teddy bear which moved forward, beating a drum. Whether the infants had nursed from the wire or the cloth mother, they overwhelmingly sought succor from the cloth one; this differential in behavior was enhanced with the passage of time and the accrual of experience. . . .

Thus all the objective tests we have been able to devise agree in showing that the infant monkey’s relationship to its surrogate mother is a full one. Comparison with the behavior of infant monkeys raised by their real mothers confirms this view. Like our experimental monkeys, these infants spend many hours a day clinging to their mothers, and run to them for comfort or reassurance when they are frightened. The deep and abiding bond between mother and child appears to be essentially the same, whether the mother is real or a cloth surrogate. . . .

The depth and persistence of attachment to the mother depend not only on the kind of stimuli that the young animal receives but also on when it receives them. . . . Clinical experience with human beings indicates that people who have been deprived of affection in infancy may have difficulty forming affectional ties in later life. From preliminary experiments with our monkeys we have also found that their affectional responses develop, or fail to develop, according to a similar pattern.

Early in our investigation we had segregated four infant monkeys as a general control group, denying them physical contact either with a mother surrogate or with other monkeys. After about eight months we placed them in cages with access to both cloth and wire mothers. . . .

In the open-field test these “orphan” monkeys derived far less assurance from the cloth mothers than did the other infants. The deprivation of physical contact during their first eight months had plainly affected the capacity of these infants to develop the full and normal pattern of affection. . . . The long period of maternal deprivation had evidently left them incapable of forming a lasting affectional tie. . . .

The effects of maternal separation and deprivation in the human infant have scarcely been investigated, in spite of their implications concerning child-rearing practices. . . .

Above and beyond demonstration of the surprising importance of contact comfort as a prime requisite in the formation of an infant’s love for its mother—and the discovery of the unimportant or nonexistent role of the breast and act of nursing—our investigations have established a secure experimental approach to this realm of dramatic and subtle emotional relationships. The further exploration of the broad field of research that now opens up depends merely upon the availability of infant monkeys. We expect to extend our researches by undertaking the study of the mother’s (and even the father’s!) love for the infant, using real monkey infants or infant surrogates. Finally, with such techniques established, there appears to be no reason why we cannot at some future time investigate the fundamental neurophysiological and biochemical variables underlying affection and love.


Source: Harry F. Harlow, “Love in Infant Monkeys,” Scientific American 200 (June 1959):68, 70, 72-73, 74.

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