Brain Development: Harlow’s Experiment with Rhesus Monkeys

Brain development begins with genes and other biological factors but hundreds of epigenetic factors affect brain development. Brain plasticity and growth depend on genes and maturation but even more so on experience (Siles & Jernigan, 2010)

In Harlow’s experiment with Rhesus Monkeys, it demonstrated the nature of “affection”. His results attest to the importance of bodily contact and the comfort and security it provides which forms the basis of emotional attachment to the mother. He reasoned that contact comfort is a “variable of overwhelming importance in the development of affectional response, whereas lactation is variable of negligible importance.” Motherly love was emotional rather than physiological. As demonstrated by his experiment, sense of touch is acute in infants. Wrapping, rubbing, massaging and cradling are soothing to many new babies. There have also been many further researches that demonstrate the key role that early nurturing plays in shaping a child’s emotional security. In early development, social touch has both beneficial neurophysiological and epigenetic effects.

But what interests me more so than the emotional attachment experienced in response to the touch of the cloth surrogate mother is the neurobiology that may be shaping affective touch. This would further delve into the understanding of parental love and affection. In the study of mice, it is found that when the mother mouse licks her newborn babies, it reduces the methylation of gene Nr3c1, allowing increased serotonin to be released by the hypothalamus. The serotonin increases momentary pleasure attributed from licking and also starts a path of epigenetic responses that reduces stress hormones from many parts of the brain and body, including adrenal gland. This is equated to a mother’s gentle stroking in babies (Kathleen Stassen Berger). Animals like humans, are social creatures, and there is a need for physical contact in both species whether it is a baby needing to suckle and cling to their caregivers, or primates who groom each other. Physical touch increases levels of dopamine and serotonin and oxytocin. For example, oxytocin released in the brain, as a response to activation of sensory nerves from interactive behaviors contributes to the ability to handle stress. In the latter part of the experiment, the monkey was able to handle stress and adapt to a foreign environment more readily with the presence of the cloth mother. Aside from the emotional social aspect, there may also be a biological aspect where the sensory touch induced hormones that help the monkey adapt

In addition, what was interesting in regards to the experiment is that the surrogate monkeys were just an inanimate object whether it is the cloth or the metal. Therefore, no reciprocated behavior is exhibited, yet the monkey was able to establish a relationship, an attachment. Attachment does not need to be reciprocated and can be one sided. But I would assume that in the bond of a caregiver and child, a reciprocated relationship would be detrimental to cognitive and social development (i.e. dismissive mothers). Another take on the experiment would be if Harlow monitored the growth of a monkey raised with a surrogate cloth mother vs a caregiver monkey. I wonder whether this experiment can be fully reflected in human studies. Unlike animals, human babies are a lot more helpless and more dependent on their caregiver. But in terms of human studies, the relationship between the cloth surrogate mother with the monkeys may be different than the relationship between a human mother and child. Maybe this is can help us understand why in western culture, babies usually develop an attachment to a soft blanket or cuddly toy especially when sleeping separately from their parents in a crib.   

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