Harlow’s Rhesus Monkey Experiments
Imagine a world where the very essence of love and attachment is put to the test, stripped down to its rawest components. In a dimly lit laboratory in the late 1950s, psychologist Harry Harlow embarked on an audacious journey that would forever alter our understanding of emotional bonds. Through his controversial experiments with Rhesus monkeys, Harlow unearthed startling truths about maternal care and social development—truths that challenged established theories and ignited fierce debates within both scientific communities and society at large.
As these innocent creatures were subjected to heart-wrenching separations from their mothers, what emerged was a powerful narrative about affection’s critical role in shaping not just primate behavior but also our own human experience.
Harlow’s findings revealed that love is more than mere survival; it is a fundamental need woven into the fabric of existence itself. His iconic surrogate mother experiment illustrated how infant monkeys gravitated toward soft comfort over food, emphasizing that emotional reassurance often outweighs physical needs when forming attachments. This shocking revelation resonates deeply today as we grapple with questions surrounding child development and the consequences of neglect in various forms.
As we delve into Harlow’s groundbreaking research, prepare to explore not only the ethical dilemmas posed by his methods but also the profound implications for our understanding of relationships—the very connections that bind us together as individuals seeking love, security, and belonging in an ever-complex world.
Key Definition:
Harry Harlow’s rhesus monkey experiments were a series of controversial studies on maternal separation and social isolation conducted in the 1950s and 1960s. Using rhesus monkeys, Harlow investigated the effects of maternal deprivation by separating infant monkeys from their mothers and subjecting them to varying degrees of social isolation.
Introduction: The Developmental Impact of Social Isolation
Harry Harlow, a prominent American psychologist, conducted a series of groundbreaking and controversial experiments on Rhesus monkeys beginning in the late 1950s. His research has become a foundational topic in introductory psychology courses due to its profound implications for our understanding of attachment theory and social development (van Rosmalen et al., 2020).
By exploring the effects of maternal separation and social isolation, Harlow’s work highlighted the essential role that affection and emotional bonds play in the healthy psychological development of primates. His findings not only mirrored many theories proposed by John Bowlby regarding attachment (Horst et al., 2008a) but also initiated discussions about the significance of maternal care during early stages of life.
Through his innovative yet ethically contentious methods, Harlow sought to unravel complex questions surrounding love, security, and emotional nurturing within familial relationships. By using infant rhesus monkeys as subjects and providing them with surrogate mothers—one made from wire that dispensed food and another covered in soft cloth—he demonstrated that comfort often supersedes mere physical sustenance when forming attachments.
The results from these experiments continue to resonate today, influencing current perspectives on child-rearing practices while raising vital ethical considerations regarding animal welfare in research settings. Ultimately, Harlow’s contributions have significantly shaped our comprehension of human behavior by illuminating how early experiences can impact lifelong social interactions and emotional health.
Background
In 1930, Harlow received his Ph.D. in psychology from Stanford University. He spent the remainder of his academic career as a professor at the University of Wisconsin—Madison (Horst et al., 2008). Harlow’s education was during a time that behaviorism dominated the college classrooms.
Harlow’s research aimed to investigate the nature of love, attachment, and social development in primates. Harlow saw love and affection as a neglected facet of human behavior. Harlow theorized that “the initial love responses of the human being are those made by the infant to the mother or some mother surrogate.” He further stated that “from this intimate attachment of the child to mother, multiple learned and generalized affectional responses are formed” (Harlow, 1958).
Harlow’s view of learned affectional responses coincides with John Bowlby’s theory of internal working models of relationship attachment. Robin Karr-Morse and Meredith Wiley explains this concept clearly in their writing, stating that “the child’s first relationship, typically with the mother, acts as a template for the imprinting of circuits in the child’s developing, emotion-processing right brain” (Karr-Morse & Wiley, 2014).
The basic underlying question these studies aimed to answer was “do infants have an innate need for mother love, or did they learn to love their mothers because mothers provided them with food?” (Vicedo, 2010).
See Internal Working Models for more on this topic
The Rhesus Monkey
Harlow sought to understand the significance of early maternal separation and the effects of social isolation through experiments with infant rhesus monkeys. He explained that the macaque (rhuses) infant differs from the human infant in that the monkey is more mature at birth and grows more rapidly; but the basic responses relating to affection, including nursing, contact, clinging, and even visual and auditory exploration, exhibit no fundamental differences in the two species” (Harlow, 1958).
Harlow chose to raise the monkeys from birth, he proclaimed that his bottle raised monkeys were physically healthier and heavier than the monkeys raised by their mothers. However, the experiments of varying social isolation had severe impacts on the behaviors of the rhesus monkeys. His findings on attachment, affection, and socialization are staples of early psychology. Modern ethics and moral standards disallow current studies to subject animals to these extremes.
Experiment Details
Harlow separated sixty infant rhesus monkeys from their mothers six to twelve hours after birth. They suckled the infant monkey’s on tiny bottles. harlow and his colleagues noticed that the infant monkeys showed strong attachment to the soft pads on the bottom of the cage. If experimenters pulled the young monkeys from the pads, the monkey would engage in a violent temper tantrum. Harlow therosed that the soft pads provided a sense of security. Perhaps, similar to a young child holding a security blanket or a stuffed animal to regulate stress.
Harlow designed experiments around offering the infant monkeys two surrogate “mothers.” They made one of wire and equipped it with a feeding bottle, and the other surrogate mother they covered in soft tan terry cloth. Harlow tried a variety of different variations of these two surrogate mothers. He tried the soft warm surrogate mother with and without a feeding apparatus. He observed the monkeys’ reactions and behaviors towards these surrogate mothers, aiming to discern the influence of comfort and food on the attachment behavior of the infants.
Surrogate Mother Preferences
A surprising and interesting early finding was that the young monkeys preferred the non-feeding, soft terry cloth surrogate mother over the wire feeding surrogate mother. “The monkeys spent most of their time with soft mother, regardless of which mother provided milk”(Vicedo, 2010). This opposed some early theories that suggested that infants bond to their mothers because the mother satisfies the basic needs of hunger and thirst. Basically, these theories posit that mom’s presence was associated with sustenance. The child bonded with mother through classical conditioning.
However, the infant monkey’s preference suggested that the soft surrogate mother provided an affectional need, independent of the milk. His studies provided support for the attachment need, underlying the need for both nursing and contact comfort.
Surrogate Mother and Security
To further the experiment, Harlow introduced the infant rhesus monkeys to a similar experiment as Mary Ainsworth’s strange situation. The infant monkeys responded in a similar fashion to the toddlers in the strange situation studies, utilizing their surrogate mothers as a secure base. “The baby monkeys reared with cloth mothers used the surrogate mother as ‘a source of security, a base of operations’” (Vicedo, 2010). The monkeys with only a wire surrogate mother would also use the mother as a secure base. However, they never would leave her to explore objects.
To conduct this experiment Harlow placed several novel objects (a small artificial tree, a crumpled piece of paper, a folded gauze diaper, a wooden block and a doorknob) in a six feet by six feet room. Harlow reports that when the cloth mother was later placed in the room the baby monkeys would seek comfort from it.
He wrote:
“The infant would rush wildly to her, climb upon her, rub against her, and cling to her tightly.” However, if the mother was not placed back into the room the overwhelmed infants “would rush across the test room and throw themselves face downward, clutching their heads and bodies and screaming their distress” (Harlow, 1959, p. 78).
Harlow commented their conduct resembled “the autistic behavior seen frequently among neglected children in and out of institutions” (Vicedo, 2010).
Early in Harlow’s studies he surmised that a real mother was unnecessary for emotional development. He saw the soft surrogate mother as an “eminently satisfactory mother.”
Adolescent Monkeys
While some of the early findings suggested that a surrogate mother could adequately replace a real mother, the adolescent and adult monkeys reared in isolation from other monkeys told a different story. Harlow that the adult monkeys raised by surrogate mothers were “without question socially and sexually aberrant.” He continues to explain that “the nourishment and contact comfort provided by the nursing cloth covered mother in infancy does not produce a normal adolescent or adult” (Harlow, 1966, p. 231).
The female monkeys that were artificially impregnated did not know how to mother their own babies. These monkeys were punitive mothers. “They rejected, abused and killed their infants.” Harlow recanted his early suggestions that a real mother was dispensable. He wrote that “their early social deprivation permanently impairs their ability to form effective relations with other monkeys” (Harlow, 1966, p. 231).
Two Stages of Development
Harlow concluded that real mothers guided their children through two stages of development. During the first stage of development, the feral rhesus provided their offspring with sustenance and security (comfort contact). During the second phase, the mother literally pushes the child away so they can interact with other monkeys.
The surrogate mother adequately provided the infant with the developmental needs in stage one. However, these monkeys were deprived of the necessary developmental tasks encountered during phase two. The cage raised monkeys were “deprived of fathers, siblings, friends and all other family members of a social group” (Vicedo, 2010).
Key Findings
The experiments yielded compelling results, demonstrating that the infant monkeys primarily sought comfort and security from the soft, cloth-covered surrogate mother, even when the wire mother provided nourishment. This emphasized the significance of emotional reassurance and physical contact in the formation of social attachments, challenging prevailing beliefs about the importance of nourishment in mother-infant relationships.
Contact Comfort
The need for human touch and physical contact is an essential aspect of our well-being. From the moment we are born, the comforting sensation of being held and touched plays a crucial role in our development. Research has shown that without adequate physical contact, especially during infancy and early childhood, individuals may experience impairments in their emotional, social, and cognitive development.
The act of being held and receiving physical affection from caregivers and loved ones creates a sense of security and attachment, allowing individuals to develop healthy relationships and emotional resilience. Through touch, our nervous system is activated, promoting the release of oxytocin, often referred to as the “love hormone,” which can reduce stress and anxiety while fostering a sense of trust and connection.
Furthermore, studies have indicated that the absence of physical touch can lead to adverse effects on both mental and physical health. Individuals who experience chronic deprivation of touch may be more susceptible to feelings of loneliness, depression, and anxiety. Additionally, the absence of touch has been linked to weakened immune function and heightened stress responses.
Ultimately, acknowledging and addressing the significance of physical touch in human development and well-being is critical. Creating environments and relationships that afford opportunities for meaningful touch and embraces can contribute to overall health and vitality, supporting individuals in leading fulfilling and balanced lives.
Affectional Systems
Harlow extrapolated from his research that primates have innate affectional systems. He posits that an individual’s different relationships do not simply arise from an extension of an infant’s love for its mother, but from developmental appropriate social exposures. These different systems translate into different forms of relationships. Such as mother-child, child-peer, infant-mother, and father-infant.
Affectional systems refer to the complex networks of relationships, emotions, and interactions that individuals have with others in their lives. These systems can include family members, friends, romantic partners, and even pets, and they play a crucial role in shaping an individual’s emotional well-being and social development. Affectional systems are essential for providing support, love, and a sense of belonging, and they contribute significantly to an individual’s overall happiness and resilience. Understanding and nurturing these different systems is important for maintaining healthy and fulfilling relationships.
Socialization
Harlow emphasized that proper emotional development requires more than a mother. Deborah Blum wrote “there’s a simple name for the next phase in building relationships. It’s called play and it’s one reason why it is so important that parents encourage their children to form friendships with peers.”
Children need play with peers for healthy development is a fundamental principle that underscores the importance of social interaction in the formative years of a child’s life. Engaging in play with peers not only fosters the development of crucial social skills such as communication, cooperation, and conflict resolution, but it also contributes to the overall well-being and emotional intelligence of a child.
Through play, children learn to navigate various social dynamics, understand boundaries, and cultivate empathy and understanding towards others. Furthermore, play with peers provides an avenue for children to express themselves, develop their creativity, and enhance their cognitive abilities through imaginative and collaborative activities. As such, creating opportunities for children to engage in play with their peers is essential in laying the foundation for healthy social, emotional, and cognitive development.
In reference to the monkeys in Harlow’s lab, Blum wrote:
“As the monkeys grow older and play harder, they get better at sending and at reading the kind of messages that we call nonverbal communication. Peers tend to reinforce behaviors—reciprocating when they like an activity, ignoring or turning away when they don’t. So during play, you can also learn what makes your friends leave and how to coax them back” (Blum, 2002).
Child Neglect
The Harlow’s monkey experiments are a haunting reminder of the profound effects of neglect on child development. These experiments, conducted by psychologist Harry Harlow, involved separating infant monkeys from their mothers and exposing them to two artificial surrogate “mothers” – one made of wire and the other covered with soft cloth. The monkeys consistently preferred the comforting cloth mother, even when the wire mother provided sustenance. This suggests that emotional comfort and attachment are essential for healthy development, and not just basic physical needs.
The implications of these experiments are indeed deep and unsettling, shedding light on the crucial nature of emotional nourishment and care in early childhood. It serves as a stark reminder that children require not only physical sustenance, but also emotional support and nurturing to thrive and develop into healthy individuals.
Neuroscientists at Harvard University studied the cortisol levels of orphans who were raised in the dreadfully neglected child Romanian orphanages established during the Ceausescu regime. These institutions had a caregiver/child ratio of one caregiver for every twenty children. The children only received the basic the rudiments of care, seldom physically picked up or touched (comfort contact).
Gabor Maté reports that they displayed “the self-hugging motions and depressed demeanour typical of abandoned young, human or primate.” Further research tested these orphaned children’s saliva discovering that “their cortisol levels were abnormal, indicating that their hypothalamic-pituitary-adrenal axes were already impaired” (Maté, 2008).
Basically, emotional neglect impacts the development of the brain.
Ethical Controversy
The experiments conducted by Harlow undeniably ignited intense ethical debates due to the evident psychological distress inflicted on the monkeys, sparking deep concerns and raising serious questions about the treatment of animals in research settings. The profound impact of these studies on the moral and ethical considerations of animal research cannot be overlooked. However, amidst these ethical controversies, it is crucial to recognize that Harlow’s research significantly contributed to our understanding of the critical role of early maternal care and social interactions in primate development.
These insights have had a lasting influence on the field of psychology and continue to shape our understanding of human and animal behavior.
Associated Concepts
- Attachment Theory: This theory explains how human beings form emotional bonds and connections with others, particularly in early childhood.
- Attachment Styles: These are patterns of behavior in close relationships that develop in early childhood and continue into adulthood. They are influenced by the quality of the bond between an infant and their primary caregiver.
- Piaget’s Stages of Cognitive Development: This theory describes how children develop intellectually as they interact with the world around them.
- Enriched Environment: This refers to a stimulating and intellectually engaging setting that provides a wide range of experiences for an individual, particularly during critical periods of development.
- Separation-Individuation Theory of Child Development: This process proposed by Margaret Mahler, describes the stages through which a child develops a sense of individual identity and separates from their primary caregivers.
- Still Face Experiment: This experiment involved instructing a mother to maintain a neutral facial expression and unresponsive demeanor while interacting with her infant, simulating a ‘still face’ devoid of emotional expression.
- Lorenz’s Imprinting Theory: This concept introduced by ethologist Konrad Lorenz, refers to the rapid and relatively permanent learning process that occurs during a brief critical period in early life. This phenomenon involves the attachment of a young animal to a specific individual or object, usually the parent. Imprinting is common among birds and some mammals.
A Few Words by Psychology Fanatic
Despite the ethical concerns surrounding his work, Harlow’s experiments revolutionized the field of psychology, highlighting the profound impact of social and emotional support on primate development. However, his findings have left an enduring legacy, shaping modern theories of attachment and influencing child-rearing practices.
Harlow’s Rhesus Monkey Experiments remain a contentious yet impactful chapter in the history of psychological research, underscoring the complex interplay of emotional, social, and physical needs in the formation of social bonds and relationships.
Last Updated: March 3, 2026
References:
Blum, Deborah (2002). Love at Goon Park: Harry Harlow and the Science of Affection. Basic Books; 2nd edition. ISBN: 9780465026012; APA Record: 2002-18511-000
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Harlow, Harry (1958). The Nature of Love. American Psychologist, 13(12), 673-685. DOI: 10.1037/h0047884
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Harlow, Harry (1959). The Development of Affectional Patterns in Infant Monkeys. In: D.M. Foss (ed.), Determinants of Infant Behavior: Proceedings of a Tavistock Study Group on Mother-Infant Interaction Held in the House of the CIBA Foundation. London: Methuen. APA Record: 1971-00215-000
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Harlow, Harry (1966). Social Deprivation in Monkeys. Haimowitz, Morris L., (ed.), Human Development Selected Readings. New York, Crowell. ASIN: B0018GG01I
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Horst, Frank; Veer, René (2008). Loneliness in Infancy: Harry Harlow, John Bowlby and Issues of Separation. Integrative Psychological and Behavioral Science, 42(4), 325-335. DOI: 10.1007/s12124-008-9071-x
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Horst, Frank; LeRoy, Helen; Veer, René (2008a). “When Strangers Meet”: John Bowlby and Harry Harlow on Attachment Behavior. Integrative Psychological and Behavioral Science, 42(4), 370-388. DOI: 10.1007/s12124-008-9079-2
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Karr-Morse, Robin; Wiley, Meredith S. (2014). Ghosts from the Nursery: Tracing the Roots of Violence. Atlantic Monthly Press; 1st edition. ISBN-10: 0802196330
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Maté, Gabor (2008). When the Body Says No. Trade Paper Press; 1st edition. ISBN-10: 0470349476
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van Rosmalen, Lenny; van der Veer, René; van der Horst, Frank (2020). The nature of love: Harlow, Bowlby and Bettelheim on affectionless mothers. History of Psychiatry, 31(2), 227-231. DOI: 10.1177/0957154×19898997
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Vicedo, Marga (2010). The evolution of Harry Harlow: from the nature to the nurture of love. History of Psychiatry, 21(2), 190-205. DOI: 10.1177/0957154X10370909
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