Harry Harlow’s Monkey Experiments: Wire vs Cloth Mothers

A baby monkey clings desperately to a cloth-covered surrogate mother while ignoring the wire mother that actually provides its food—revealing that love isn’t about meeting basic needs but about comfort, security, and emotional connection.

Introduction

In 1958, a revolutionary psychology experiment would forever change our understanding of love, attachment, and human relationships. Harry Harlow’s groundbreaking research with rhesus monkeys challenged the prevailing scientific wisdom that babies loved their mothers simply because mothers provided food. Instead, Harlow discovered something far more profound: that comfort, warmth, and emotional security were more important than nourishment in forming deep emotional bonds.

These experiments didn’t just reshape psychology—they transformed how we understand attachment theory and laid the foundation for modern approaches to child development, therapy, and even parenting practices. The image of a baby monkey clinging desperately to a cloth-covered surrogate mother while ignoring a wire mother that provided food became one of psychology’s most iconic and influential findings. For psychology students and anyone interested in understanding human nature, Harlow’s work remains as relevant today as it was groundbreaking nearly seven decades ago.

Who Was Harry Harlow?

Early Life and Career Development

Harry Frederick Harlow was born Harry Israel in 1905 in Fairfield, Iowa, to parents who ran a small farming operation. The antisemitism he encountered during his early academic career led him to change his surname to Harlow—a decision that reflected both the social challenges of his era and his determination to succeed in the academic world (Blum, 2002).

Harlow’s path to psychology wasn’t immediately obvious. He initially struggled academically and was advised by a professor that he wasn’t suited for graduate work. However, his persistence paid off when he was accepted into Stanford University’s psychology program in 1924, where he earned his PhD in 1930 under the supervision of Calvin Stone, a prominent comparative psychologist who studied animal behavior.

After completing his doctorate, Harlow joined the University of Wisconsin-Madison faculty in 1930, where he would spend the majority of his career. It was here that he established the Primate Laboratory in 1932, initially studying learning and cognition in rhesus monkeys. What began as research into monkey intelligence would eventually evolve into the attachment studies that made him famous.

Pre-Attachment Research

Before his famous attachment experiments, Harlow was already making significant contributions to psychology through his work on learning and problem-solving. He developed the Wisconsin General Test Apparatus (WGTA), an innovative device that allowed researchers to study complex learning processes in primates under controlled conditions.

His early research focused on “learning sets”—the ability of monkeys to learn how to learn, essentially developing problem-solving strategies that could be applied to new situations. This work was groundbreaking because it demonstrated that monkeys could engage in what appeared to be insight learning, challenging behaviorist theories that reduced all learning to simple stimulus-response associations.

However, it was Harlow’s observations of the young monkeys in his laboratory that sparked his interest in emotional relationships. He noticed that baby monkeys formed strong emotional attachments to the cloth diapers used to line their cages, clinging to them and showing distress when the diapers were removed for cleaning. This seemingly simple observation would lead to one of psychology’s most significant research programs.

The transition from studying learning to studying love wasn’t accidental. Harlow became increasingly interested in what he called “the forgotten variable”—affection and emotional attachment, which had been largely ignored by the behaviorist psychology dominant in his era.

The Scientific Context: Why These Experiments Mattered

The Behaviorist Dominance

To understand the revolutionary nature of Harlow’s findings, it’s essential to grasp the scientific landscape of the 1950s. Psychology was dominated by behaviorism, a theoretical approach championed by researchers like John B. Watson and B.F. Skinner that focused exclusively on observable behavior while dismissing internal emotional states as unscientific.

According to behaviorist theory, all behavior could be explained through conditioning—the process by which organisms learn to associate certain stimuli with rewards or punishments. Applied to infant-mother relationships, this meant that babies learned to “love” their mothers simply because mothers provided food, warmth, and other basic needs. This was called the “cupboard love” or “secondary drive” theory—the idea that attachment was merely a learned response based on the satisfaction of primary biological needs.

Watson himself had famously declared in 1913 that psychology should focus solely on observable behavior, rejecting concepts like emotions, thoughts, or feelings as too subjective to study scientifically. The Little Albert experiment, conducted by Watson and Rayner in 1920, had demonstrated how emotional responses like fear could be conditioned in infants, reinforcing the behaviorist view that all emotional bonds were simply learned associations.

This mechanistic view of human relationships dominated psychological research for decades. Infants were seen as passive recipients of conditioning rather than active participants in emotional relationships. The prevailing wisdom suggested that as long as basic physical needs were met, healthy development would naturally follow.

Emerging Attachment Questions

While behaviorism dominated American psychology, parallel developments in Europe were beginning to challenge these assumptions. John Bowlby, a British psychiatrist and psychoanalyst, was developing his own theories about the importance of early relationships based on his observations of children who had been separated from their parents during World War II.

Bowlby noticed that children who experienced prolonged separation from their mothers showed consistent patterns of distress that couldn’t be explained simply by unmet physical needs. These children often displayed what he termed “anaclitic depression”—a profound sadness and withdrawal that persisted even when their physical needs were adequately met by institutional caregivers.

The post-war period also brought increased attention to child development and welfare. The horrors of war had highlighted the psychological impact of separation and loss on children, creating a social context that was more receptive to theories emphasizing the importance of emotional bonds and security.

It was against this backdrop that Harlow began to formulate the questions that would drive his research: Was the behaviorist explanation of infant-mother attachment accurate? Did babies truly love their mothers primarily because mothers provided food? Or was there something deeper—something about comfort, security, and emotional connection—that formed the foundation of these crucial early relationships?

These questions would lead to experiments that fundamentally challenged the scientific establishment and revolutionized our understanding of love itself.

The Surrogate Mother Experiments

Experimental Design and Methodology

Harlow’s most famous experiments began in 1957 with a deceptively simple question: Do baby monkeys prefer a mother who provides food or a mother who provides comfort? To answer this question, he designed an elegant experimental setup that would become one of psychology’s most recognizable studies.

The subjects were rhesus macaque monkeys, chosen because of their similarity to humans in terms of social development and emotional expression. Rhesus monkeys have complex social structures, form strong emotional bonds, and show clear distress responses—making them ideal subjects for studying attachment behaviors that might parallel human development.

The experimental setup involved separating infant monkeys from their biological mothers within 6 to 12 hours of birth, well before strong attachment bonds could form. Each infant was then placed in an individual cage containing two artificial “surrogate mothers”—essentially mother substitutes designed to test different aspects of the attachment relationship.

The laboratory environment was carefully controlled to eliminate confounding variables. Each cage was equipped with identical lighting, temperature control, and feeding schedules. The only differences were in the characteristics of the two surrogate mothers, allowing researchers to isolate exactly which features were most important for attachment formation.

Harlow and his team measured several key variables: time spent with each surrogate mother, stress responses when separated from the surrogates, exploratory behavior in the presence of each mother, and comfort-seeking behavior when frightened or distressed.

The Two “Mothers”: Wire vs. Cloth

The genius of Harlow’s experimental design lay in the stark contrast between the two surrogate mothers. Each was designed to test a specific hypothesis about what drives attachment formation.

The wire mother was constructed from welded wire mesh formed into a cylindrical shape roughly approximating a mother’s torso. It was cold, hard, and uncomfortable, providing no tactile comfort whatsoever. However, for half of the experimental subjects, the wire mother was equipped with a nursing bottle, making it the source of food and nutrition.

The cloth mother was built on the same cylindrical wire frame but was covered with soft terry cloth material, making it warm and comfortable to touch. A heating pad placed inside provided warmth similar to a living mother’s body temperature. Importantly, for half the subjects, the cloth mother provided no food—it offered only comfort and warmth.

This experimental design created four different conditions: monkeys with wire mothers who provided food, monkeys with cloth mothers who provided food, and monkeys who had access to both types of mothers with different feeding arrangements. This allowed Harlow to separate the effects of nourishment from the effects of comfort—something that couldn’t be done with real mothers who naturally provided both.

Key Findings and Statistics

The results of Harlow’s experiments were both dramatic and unambiguous, providing clear evidence against the prevailing behaviorist theories of attachment. The data revealed patterns that surprised even Harlow himself and would fundamentally reshape psychological thinking about love and attachment.

Time Allocation: The most striking finding was how infant monkeys allocated their time between the two surrogate mothers. Regardless of which mother provided food, the babies spent dramatically more time with the cloth mother. Monkeys who were fed by the wire mother spent an average of only 1 hour per day with their food source, while spending 17-18 hours daily clinging to the cloth mother who provided no nourishment.

Even more remarkably, monkeys who were fed by the cloth mother spent virtually all their time (22-23 hours daily) with their soft, comfortable mother figure. This data provided overwhelming evidence that attachment was not primarily driven by food provision, as behaviorist theory suggested.

Stress Response: When frightened by loud noises or unfamiliar objects, 100% of the infant monkeys sought comfort from the cloth mother, regardless of which mother had provided their food. This demonstrated what Harlow termed “contact comfort”—the powerful need for physical touch and security during times of distress.

Exploration Behavior: Perhaps most significantly for understanding healthy development, monkeys would only explore new environments and toys when the cloth mother was present. With only the wire mother available, they showed signs of anxiety and refused to engage with novel stimuli. However, when the cloth mother was present, they would venture out to explore while regularly returning to touch or cling to their soft mother figure.

This behavior suggested that secure attachment serves as a “secure base” from which children can safely explore the world—a concept that would become central to modern attachment theory.

Development Outcomes: Monkeys raised with only wire mothers showed severe social and emotional deficits. They displayed abnormal behaviors including rocking, self-clasping, and difficulty forming relationships with other monkeys. In contrast, those raised with cloth mothers, while still showing some developmental challenges due to their artificial rearing, demonstrated more normal social and emotional development.

Experimental Variations

Harlow’s research program expanded to include numerous variations that tested different aspects of attachment formation and maintenance. These studies provided additional evidence for the importance of contact comfort while revealing the complexity of attachment relationships.

Material Testing: Harlow experimented with different materials for the surrogate mothers, including various textures and fabrics. He found that softer, more comforting materials consistently produced stronger attachment responses, while harder or less pleasant textures were less effective at promoting emotional bonding.

Temperature Variations: Studies manipulating the temperature of surrogate mothers confirmed that warmth was an important component of comfort. Heated surrogate mothers were preferred over unheated ones, even when other factors were controlled.

Movement and Responsiveness: Later experiments introduced movement mechanisms, including rocking motions that simulated the natural movements of real mothers. Monkeys showed even stronger preferences for surrogate mothers that moved gently, suggesting that responsiveness and interaction enhance attachment formation.

“Monster Mothers”: In some of the most controversial variations, Harlow created “rejecting” surrogate mothers that would occasionally blow air blasts, shake violently, or otherwise “reject” the infant monkey. Surprisingly, these negative experiences often strengthened rather than weakened the attachment, with infant monkeys clinging even more desperately to their rejecting mothers. This finding provided insights into why children sometimes maintain strong attachments to abusive caregivers—a phenomenon observed in human development as well.

These experimental variations helped establish that attachment involves multiple components—comfort, warmth, responsiveness, and consistency—rather than any single factor. The research demonstrated that while contact comfort was crucial, the most secure attachments formed when multiple positive factors were combined.

The implications of these findings extended far beyond the laboratory. They suggested that human infants, like monkey infants, might have fundamental needs for comfort, security, and emotional connection that went beyond basic physical care—a radical departure from the behaviorist assumption that proper conditioning and need satisfaction were sufficient for healthy development.

Beyond the Surrogate Mothers: Other Harlow Studies

The Pit of Despair Studies

While the surrogate mother experiments brought Harlow fame, his research program expanded into increasingly controversial territory as he sought to understand the depths of attachment and the consequences of its absence. Among the most disturbing of these studies were what Harlow himself grimly called the “pit of despair” experiments.

These studies involved placing young monkeys in vertical isolation chambers—stainless steel boxes with sloped sides that made climbing impossible. The chambers were designed to induce depression and study learned helplessness, psychological conditions that Harlow believed were analogous to human depression and despair.

Monkeys were confined in these chambers for periods ranging from 30 days to 12 months, depending on the experimental condition. The chambers eliminated virtually all environmental stimulation and social contact, creating conditions of profound isolation and sensory deprivation.

The effects were severe and often permanent. Monkeys subjected to these conditions developed behaviors remarkably similar to human depression: they withdrew from social contact, showed little interest in food or play, engaged in repetitive self-soothing behaviors like rocking, and some exhibited self-harm through excessive scratching or biting. Many never fully recovered their normal social functioning, even after being returned to social environments.

Harlow’s motivation for these experiments was to create an animal model for studying human depression and developing potential treatments. However, the severity of the procedures and the permanent psychological damage they caused would be considered unethical by modern standards and contributed to the eventual development of strict animal welfare regulations in research.

Social Isolation Research

Parallel to the pit of despair studies, Harlow conducted extensive research on various forms and durations of social isolation. These experiments examined how different periods of isolation affected social development and whether recovery was possible after isolation ended.

Total Isolation Studies: Young monkeys were completely isolated from all social contact for periods of 3, 6, or 12 months. The effects were proportional to the duration of isolation. Monkeys isolated for three months showed significant social deficits but could partially recover. Those isolated for six months showed more severe problems with limited recovery. Monkeys isolated for a full year showed profound, largely irreversible social dysfunction.

Effects on Social Development: Isolated monkeys typically displayed what Harlow termed “social autism”—they were unable to interact normally with other monkeys, showed inappropriate fear or aggression, and failed to develop normal play behaviors. They often engaged in repetitive, stereotyped behaviors and seemed unable to understand social cues that came naturally to normally raised monkeys.

“Rehabilitation” Attempts: Harlow and his colleagues attempted various interventions to help socially isolated monkeys recover normal functioning. One of the most successful approaches involved pairing isolated monkeys with younger, socially normal “therapist” monkeys who persisted in social contact despite initial rejection. This “monkey therapy” sometimes helped isolated monkeys develop basic social skills, though full recovery was rare.

Maternal Behavior Studies

Perhaps the most ethically troubling aspect of Harlow’s research involved studying how isolation and maternal deprivation affected the parenting abilities of female monkeys who eventually became mothers themselves.

“Motherless Mothers” Experiments: Female monkeys who had been raised in isolation or with only inanimate surrogate mothers were artificially impregnated (through techniques Harlow described in clinical terms that masked their invasive nature) to study their maternal behavior.

The results were profoundly disturbing. These “motherless mothers” showed little to no maternal instinct. Many were indifferent to their offspring, failing to nurse, comfort, or protect them. Some were actively abusive, pushing their babies away, refusing to allow nursing, or even attacking their infants.

Patterns of Inadequate Parenting: Harlow identified several categories of abnormal maternal behavior among these motherless mothers: “indifferent mothers” who simply ignored their babies; “abusive mothers” who actively harmed their offspring; and “inadequate mothers” who showed some maternal behaviors but were inconsistent and unresponsive to their babies’ needs.

Generational Effects: Most significantly, these studies demonstrated how maternal deprivation could have generational effects. Babies raised by inadequate mothers often became inadequate mothers themselves, creating a cycle of poor parenting that could persist across generations. This finding provided early evidence for what would later be understood as the intergenerational transmission of attachment patterns and trauma.

However, Harlow also discovered that this cycle could be broken. Motherless mothers who had multiple babies sometimes improved their parenting with later offspring, suggesting that learning and experience could partially compensate for early deprivation. Additionally, motherless mothers who were provided with social support from other females showed better parenting abilities than those who remained isolated.

Learning and Cognition Research

Throughout his career, Harlow continued his original research into learning and cognition, often connecting these findings to his attachment work. His development of the Wisconsin General Test Apparatus allowed for sophisticated studies of problem-solving and cognitive development in monkeys.

Learning Set Formation: Harlow’s research on “learning to learn” showed that monkeys could develop general problem-solving strategies that transferred to new situations. Importantly, he found that socially isolated monkeys showed deficits in this type of learning, suggesting that normal social development was crucial for optimal cognitive functioning.

Cognitive Development in Isolated vs. Social Monkeys: Comparative studies revealed that while isolated monkeys could learn simple tasks, they struggled with complex problem-solving that required flexibility and adaptation. This finding suggested that social interaction and secure attachment relationships weren’t just important for emotional development—they were also crucial for cognitive growth and intellectual flexibility.

These cognitive studies reinforced Harlow’s broader conclusion that attachment and security were fundamental to all aspects of healthy development, not just emotional well-being. They provided evidence that the effects of early deprivation were not limited to social and emotional domains but extended to learning and intellectual functioning as well.

The breadth of Harlow’s research program, spanning from learning and cognition to attachment and maternal behavior, created a comprehensive picture of how early relationships affect all aspects of development. However, the increasingly invasive and harmful nature of his later experiments would eventually contribute to a significant backlash against his methods and a fundamental reconsideration of the ethics of animal research in psychology.

Revolutionary Findings That Changed Psychology

Contact Comfort Over Cupboard Love

Harlow’s most fundamental contribution to psychology was his definitive demonstration that attachment is not based primarily on food provision, as behaviorist theory suggested, but on what he termed “contact comfort”—the need for physical touch, warmth, and emotional security.

This finding directly contradicted the “cupboard love” theory that had dominated psychological thinking for decades. According to this theory, babies learned to love their mothers through a process of classical conditioning: mothers became associated with the satisfaction of hunger and other basic needs, and this association gradually transformed into what appeared to be love or attachment.

Harlow’s experiments provided unambiguous evidence against this theory. The baby monkeys’ overwhelming preference for the cloth mothers, even when wire mothers provided food, demonstrated that comfort and security were more powerful motivators than nourishment. This wasn’t a minor preference—it was a dramatic, consistent pattern that held across all experimental conditions.

The concept of contact comfort revolutionized thinking about human development as well. It suggested that human babies, like monkey babies, might have fundamental needs for physical affection, emotional warmth, and secure comfort that were distinct from and potentially more important than basic physical care.

This insight had immediate practical implications for child care practices, hospital policies, and parenting approaches. It supported the growing recognition that infants needed more than efficient feeding and diaper changing—they needed responsive, affectionate care that addressed their emotional and psychological needs.

Critical Periods and Attachment Formation

Another crucial finding from Harlow’s research was the identification of critical periods in attachment formation and social development. His studies revealed that there were specific windows of time during which normal attachment relationships could form, and that missing these critical periods could result in permanent developmental damage.

The timing of separation from mothers proved crucial. Monkeys separated immediately after birth and raised with surrogate mothers showed better development than those separated after forming initial bonds with their biological mothers. This suggested that while early attachment relationships were important, the trauma of separation could be worse than never forming the initial bond at all.

More significantly, Harlow’s isolation studies revealed that there were critical periods for social development that, once missed, could not be fully recovered. Monkeys isolated for the first year of life never developed normal social skills, regardless of later intervention efforts. This finding suggested that there might be biological constraints on plasticity—that the brain might have limited windows for learning certain types of social and emotional skills.

These findings about critical periods had profound implications for understanding human development and for policies regarding child welfare, adoption, and institutional care. They suggested that early intervention was not just preferable but essential for healthy development, and that delays in providing appropriate care could have permanent consequences.

Secure Base Behavior

One of Harlow’s most enduring contributions was his documentation of what came to be known as “secure base behavior”—the way that a trusted caregiver serves as a foundation for exploration and learning.

Harlow observed that monkeys would only explore new environments and engage with novel objects when their cloth mother was present. The surrogate mother seemed to provide a psychological “secure base” from which the infant could venture out to explore while maintaining the confidence that safety and comfort were available if needed.

This behavior pattern involved a delicate balance: the infant needed to feel secure enough to explore independently, but also needed to maintain connection with the attachment figure. Monkeys would regularly return to touch or cling to the cloth mother during exploration, as if “refueling” their sense of security before venturing out again.

The concept of secure base behavior became central to understanding healthy child development. It explained how secure attachment relationships don’t create dependency but actually promote independence and exploration by providing the emotional foundation that makes risk-taking and learning feel safe.

Attachment Hierarchy Beyond Feeding

Harlow’s research revealed that attachment relationships involve a complex hierarchy of needs and behaviors that extend far beyond simple feeding relationships. His experiments showed that attachment figures serve multiple functions: they provide comfort during distress, serve as a secure base for exploration, offer emotional regulation, and create a sense of safety and security that enables learning and development.

This multi-faceted understanding of attachment challenged reductionist approaches that tried to explain complex human relationships through simple behavioral principles. Harlow’s work suggested that love and attachment were complex phenomena that required equally complex explanations.

The hierarchy of attachment needs that emerged from his research included: basic comfort and warmth (contact comfort), emotional regulation and soothing during distress, a secure base for exploration and learning, and consistent availability and responsiveness. These findings laid the groundwork for modern attachment theory’s understanding of how healthy relationships function and what children need from their caregivers.

Harlow’s research also revealed that attachment relationships could survive considerable challenges. The “monster mother” experiments showed that even negative experiences with an attachment figure didn’t necessarily destroy the attachment bond—in fact, they sometimes strengthened it. This finding helped explain why children might maintain strong attachments to inconsistent or even abusive caregivers, a phenomenon that had puzzled psychologists and social workers.

These revolutionary findings fundamentally transformed psychology’s understanding of human nature and development. They showed that emotional connection and security were not luxuries or byproducts of proper care, but fundamental human needs as basic as food and shelter. This insight would reshape not only psychological theory but also practical approaches to child care, education, therapy, and social policy for decades to come.

Impact on Modern Psychology and Beyond

Theoretical Revolutions

Harlow’s findings delivered what many consider a death blow to strict behaviorism’s dominance in developmental psychology. The overwhelming evidence that baby monkeys preferred comfort over food directly contradicted behaviorist predictions and forced psychologists to reconsider their basic assumptions about human motivation and development.

The research provided crucial empirical support for John Bowlby’s emerging attachment theory, which was being developed simultaneously in Britain. While Bowlby was developing his theoretical framework based on observations of human children, Harlow was providing experimental evidence from controlled animal studies that supported many of Bowlby’s key insights.

Together, Harlow and Bowlby’s work created a powerful foundation for understanding early development that emphasized the crucial importance of emotional relationships. This collaboration between experimental animal research and human observational studies created a more robust scientific foundation than either approach could have achieved alone.

The research also influenced psychoanalytic theory, particularly object relations theory, which focuses on how early relationships shape internal mental representations of self and others. Harlow’s demonstration that attachment involves internal working models of relationships provided empirical support for psychoanalytic insights about how early experiences create lasting psychological structures.

Furthermore, Harlow’s work laid important groundwork for modern understanding of trauma and post-traumatic stress disorder (PTSD). His documentation of how early separation and isolation created lasting psychological damage helped establish the scientific foundation for understanding how traumatic experiences, particularly those occurring early in life, can have profound and lasting effects on mental health.

Practical Applications in Child Care

The practical impact of Harlow’s research was immediate and far-reaching, transforming policies and practices in hospitals, orphanages, adoption agencies, and child care facilities around the world.

Hospital Policy Changes: Perhaps the most immediate impact was in pediatric hospital care. Before Harlow’s research, many hospitals severely restricted parental visits to minimize infection risk and maintain order. Children were often separated from parents for weeks or months during medical treatment. Harlow’s demonstration of the psychological importance of attachment relationships led to fundamental changes in hospital policies, encouraging parental involvement in care and recognizing that emotional well-being was crucial for physical recovery.

Adoption and Foster Care: The research revolutionized thinking about adoption and foster care practices. Instead of focusing primarily on providing basic physical needs, agencies began emphasizing the importance of emotional attachment and the quality of caregiving relationships. The findings about critical periods influenced policies regarding the timing of adoptions and the importance of early placement stability.

Institutional Care: Harlow’s research contributed to the movement away from large institutional care facilities for children. The evidence that emotional relationships were crucial for healthy development supported arguments for family-based care, smaller group homes, and more individualized attention for children who couldn’t remain with their biological families.

Childcare Standards: The research influenced standards for professional childcare, emphasizing the importance of consistent caregivers, responsive interactions, and emotionally warm relationships. The concept of contact comfort supported practices like allowing children to have transitional objects and encouraging physical affection in appropriate contexts.

Modern Research Connections

Harlow’s foundational work continues to influence contemporary research in numerous fields, with modern neuroscience and genetics providing biological mechanisms for many of his behavioral observations.

Neuroscience of Attachment: Modern brain imaging studies have revealed the neural pathways involved in attachment formation, largely confirming Harlow’s behavioral observations. Research on oxytocin (often called the “bonding hormone”) has shown how physical contact and emotional connection trigger biological systems that promote attachment and emotional regulation. The neuroscience research demonstrates that secure attachment relationships literally shape brain development, affecting areas responsible for emotional regulation, stress response, and social cognition.

Epigenetics and Trauma Transmission: Recent research in epigenetics—the study of how environmental factors influence gene expression—has provided biological mechanisms for Harlow’s observations about the generational transmission of poor parenting. Studies show that traumatic experiences can create changes in gene expression that can be passed to offspring, potentially explaining how the effects of early deprivation might influence multiple generations.

Adult Attachment Styles in Relationships: Harlow’s basic insights about the importance of comfort and security have been extended to understand adult romantic relationships. Research on adult attachment styles shows that the same basic needs for security and comfort that Harlow identified in baby monkeys continue to influence how adults approach intimate relationships throughout their lives.

Treatment of Attachment Disorders: Modern therapeutic approaches for treating attachment disorders, trauma, and relationship difficulties draw heavily on principles derived from Harlow’s research. Therapies that emphasize safety, emotional regulation, and the development of secure therapeutic relationships all trace their theoretical foundations back to insights about the fundamental importance of secure attachment relationships.

The research has also influenced understanding of broader social and cultural phenomena. Studies of social isolation, community mental health, and even workplace relationships often reference Harlow’s basic insights about the human need for connection and security. His work helped establish that emotional relationships are not just personal preferences but fundamental human needs that affect physical health, mental well-being, and social functioning.

Modern applications of Harlow’s insights can be seen in diverse areas including trauma-informed care in schools and healthcare settings, attachment-based parenting approaches, couple’s therapy techniques that focus on emotional connection, and even organizational psychology approaches that recognize the importance of secure relationships in workplace settings.

The enduring influence of Harlow’s work demonstrates how fundamental scientific discoveries can continue to generate new insights and applications decades after the original research. While the specific methods he used would be considered unethical today, the basic principles he discovered about the importance of love, comfort, and security continue to guide our understanding of human development and relationships.

Ethical Controversies: Then and Now

Historical Context of 1950s-1960s Research

To understand the ethical dimensions of Harlow’s research, it’s crucial to examine the historical context in which these experiments were conducted. The scientific landscape of the 1950s and 1960s operated under fundamentally different ethical standards than those that govern research today.

Animal welfare regulations were virtually nonexistent during Harlow’s early career. The Animal Welfare Act wasn’t passed in the United States until 1966, and even then it provided only minimal protections for laboratory animals. Institutional Animal Care and Use Committees (IACUCs), which now review and approve all animal research, didn’t exist. Researchers had essentially complete autonomy to design and conduct experiments as they saw fit, with little external oversight or accountability.

The post-World War II period was characterized by tremendous optimism about scientific progress and its potential to solve human problems. The development of antibiotics, the success of psychological interventions for war trauma, and rapid advances in technology created a cultural context that strongly supported scientific research, even when it involved significant costs or risks.

Additionally, the behaviorist dominance in psychology during this period created a scientific culture that emphasized objective measurement and experimental control while minimizing consideration of subjective experiences like pain, distress, or emotional suffering. The prevailing scientific philosophy suggested that anthropomorphism—attributing human-like emotions to animals—was unscientific and that objective behavioral observations were the only valid form of psychological data.

The absence of institutional review board systems meant that ethical considerations were left entirely to individual researchers’ discretion. While some scientists were certainly concerned about animal welfare, there were no formal mechanisms for evaluating whether research benefits justified potential harms or for ensuring that alternative methods had been considered.

The Experiments Through Modern Ethical Lens

Evaluated by contemporary ethical standards, many of Harlow’s experiments would be considered unacceptable and would never receive approval from modern research oversight committees. Several fundamental ethical principles now considered essential were violated in his research program.

Animal Welfare Violations: The most obvious ethical concerns involve the severe psychological harm inflicted on the research subjects. The “pit of despair” experiments, in particular, involved deliberately inducing depression and psychological distress in young monkeys through prolonged isolation. Many of the experimental subjects suffered permanent psychological damage, developing abnormal behaviors and social dysfunction that persisted throughout their lives.

The maternal deprivation studies separated infant monkeys from their mothers within hours of birth, preventing the formation of natural attachment bonds. While this separation was necessary for the experimental design, it caused immediate distress and likely contributed to long-term developmental problems in the subjects.

Necessity and Alternatives: Modern ethical frameworks require researchers to demonstrate that the potential benefits of research justify the harms caused and that less harmful alternatives have been thoroughly considered. While Harlow’s research did produce valuable scientific insights, critics argue that much of this knowledge could have been gained through less invasive methods, such as observational studies of naturally occurring variations in maternal behavior or non-invasive studies of human attachment patterns.

The requirement to use the minimum number of animals necessary for valid results also raises questions about some of Harlow’s studies. Some experiments were replicated multiple times with different variations, potentially causing unnecessary suffering when earlier results had already established the basic principles.

Long-term Care and Rehabilitation: Modern animal research standards require researchers to provide appropriate care for research subjects throughout their lives and to make efforts to rehabilitate animals when possible. Many of Harlow’s experimental subjects received minimal attention to their psychological welfare after experiments concluded, and little effort was made to help them recover from the experimental manipulations.

Harlow’s Personal Struggles and Reflections

The ethical evaluation of Harlow’s research is complicated by evidence of Harlow’s own psychological struggles and his evolving attitude toward his work. As his research program progressed, Harlow’s descriptions of his experiments became increasingly disturbed and troubling, suggesting that the work was taking a significant psychological toll on the researcher himself.

In his later career, Harlow began using language that seemed to reflect a certain callousness or even sadism in describing his experiments. He referred to his isolation chambers as “pits of despair” and described creating “motherless mothers” in terms that seemed to show little empathy for his research subjects. Some colleagues reported that Harlow appeared to take a kind of grim satisfaction in the psychological damage his experiments produced.

However, this apparent callousness may have been a psychological defense mechanism rather than genuine indifference to animal suffering. Harlow himself struggled with severe depression and alcoholism throughout much of his later career. He was hospitalized multiple times for mental health treatment and underwent electroshock therapy for his depression. Some observers have suggested that his increasingly harsh experimental methods and disturbing language may have reflected his own psychological pain and his difficulty coping with the implications of his research.

In his later years, Harlow did express some ambivalence about his work. While he never explicitly apologized for his methods, he acknowledged the ethical concerns that his research raised and seemed to recognize that his later experiments had crossed ethical lines that his earlier work had not. He also became more supportive of animal welfare regulations, though he continued to defend the scientific value of his research.

The psychological impact of conducting research that deliberately inflicted suffering on sentient beings appears to have contributed to Harlow’s own mental health struggles. This raises important questions about the ethical responsibility of institutions to protect not only research subjects but also researchers themselves from the psychological consequences of conducting ethically questionable research.

Modern Research Standards and Alternatives

Contemporary animal research operates under fundamentally different ethical frameworks than those that existed during Harlow’s era. Modern standards are designed to minimize animal suffering while still allowing for scientifically valuable research that can benefit human and animal welfare.

IACUC Requirements: All animal research in the United States must now be reviewed and approved by Institutional Animal Care and Use Committees, which include veterinarians, scientists, and community members. These committees evaluate whether research benefits justify potential harms, whether alternatives have been adequately considered, and whether experimental procedures minimize pain and distress.

The 3 R’s Principle: Modern animal research is guided by the principle of “Replace, Reduce, Refine”—replacing animal models with alternatives when possible, reducing the number of animals used to the minimum necessary for valid results, and refining procedures to minimize pain and distress. This framework would have prevented many of Harlow’s most extreme experiments.

Current Primate Research Standards: Research with non-human primates is now subject to particularly strict oversight due to their high cognitive and emotional capacities. Studies that would cause severe psychological distress, like Harlow’s isolation experiments, are generally prohibited unless they can be justified by extraordinary potential benefits and no alternatives exist.

Alternative Research Methods: Modern researchers studying attachment and social development have access to numerous alternative methods that didn’t exist in Harlow’s era. These include non-invasive neuroimaging techniques that can study brain development and attachment processes, sophisticated observational methods for studying natural attachment relationships, and computer models that can simulate developmental processes without requiring animal subjects.

Human Research Opportunities: Advances in human research ethics and methodology have also created opportunities to study attachment processes directly in human subjects using methods that are both ethically acceptable and scientifically valid. Mary Ainsworth’s Strange Situation experiment, for example, provided crucial insights into human attachment patterns using observational methods that caused minimal distress to children.

The Ongoing Ethical Debate

The ethical legacy of Harlow’s research continues to generate debate among scientists, ethicists, and animal welfare advocates. This ongoing discussion reflects deeper questions about the relationship between scientific progress and moral responsibility.

Defenders of the Research: Some argue that Harlow’s experiments, while ethically problematic by modern standards, produced knowledge that has benefited millions of children and families. They point to the research’s influence on hospital policies, adoption practices, and therapeutic approaches as evidence that the scientific benefits justified the costs. From this perspective, the experiments were conducted within the ethical framework of their time and produced knowledge that couldn’t have been obtained through other means.

Critics of the Methods: Others argue that the severity of suffering inflicted on Harlow’s research subjects was unjustifiable regardless of the scientific benefits produced. They contend that much of the valuable knowledge could have been obtained through less harmful methods and that the research violated basic principles of compassion and respect for sentient beings. Some critics also argue that the research reflected and reinforced problematic attitudes toward the use of vulnerable subjects in scientific research.

Middle Ground Perspectives: Many contemporary scientists and ethicists take a more nuanced view, acknowledging both the scientific value of Harlow’s findings and the ethical problems with his methods. They argue that while the research produced important insights, it also serves as a cautionary example of how scientific enthusiasm can lead to ethically questionable practices when adequate oversight and ethical reflection are absent.

This perspective suggests that Harlow’s work should be studied and its insights applied, but that it should also serve as a reminder of the importance of robust ethical oversight in research. The goal is to find ways to advance scientific knowledge while maintaining respect for the welfare of research subjects, whether human or animal.

The ongoing debate about Harlow’s research reflects broader tensions in scientific research between the pursuit of knowledge and the protection of research subjects. These tensions continue to evolve as our understanding of animal cognition and welfare advances and as our ethical frameworks become more sophisticated.

Contemporary discussions often focus on how to honor the scientific contributions of ethically problematic research while avoiding similar ethical violations in the future. This includes questions about how to teach about Harlow’s research in ways that convey both its scientific importance and its ethical shortcomings, and how to ensure that the valuable insights from his work continue to benefit human welfare while preventing similar studies from being conducted.

Legacy and Modern Attachment Research

Contemporary Attachment Studies

The foundation that Harlow laid with his monkey experiments continues to influence modern attachment research, though contemporary studies use methods that are both more sophisticated and more ethically sound. Current research builds on Harlow’s basic insights about the importance of comfort and security while employing new technologies and methodologies that provide deeper understanding of attachment processes.

Human Observational Research: Modern attachment research focuses primarily on human subjects using carefully designed observational methods. Researchers study naturally occurring variations in parent-child relationships, examining how different caregiving styles affect children’s development without creating artificial experimental conditions that might cause distress.

Longitudinal Studies: Some of the most valuable contemporary research involves following children and families over many years to understand how early attachment relationships influence development across the lifespan. These studies confirm many of Harlow’s basic insights while providing more detailed understanding of how attachment processes unfold in real-world contexts.

Cross-Cultural Research: Modern researchers have extended Harlow’s insights by studying attachment patterns across different cultures and societies. This research has revealed both universal aspects of attachment (supporting Harlow’s findings about basic human needs for security and comfort) and cultural variations in how attachment relationships are expressed and maintained.

Neuroimaging and Biological Research: Advanced brain imaging technologies now allow researchers to study the neural mechanisms underlying attachment processes. These studies have confirmed many of Harlow’s behavioral observations by showing how secure attachment relationships actually change brain development, affecting areas responsible for emotional regulation, stress response, and social cognition.

Contemporary research has also revealed the biological mechanisms underlying attachment formation, including the role of hormones like oxytocin and vasopressin in bonding processes. This neurobiological research provides a scientific foundation for understanding why the comfort and security that Harlow identified are so crucial for healthy development.

Technology and Modern Bonding: Current researchers are also investigating how modern technology affects parent-child bonding. Studies examine questions like how screen time, social media, and digital communication might influence the formation and maintenance of attachment relationships—extending Harlow’s basic insights about the importance of responsive caregiving into the digital age.

Clinical and Therapeutic Applications

Harlow’s insights about the fundamental importance of secure attachment relationships have been translated into numerous therapeutic approaches and clinical interventions that help children and adults who have experienced attachment difficulties.

Attachment-Based Therapy: Modern therapeutic approaches for treating attachment disorders draw directly on principles derived from Harlow’s research. These therapies emphasize creating safe, secure therapeutic relationships that can help clients develop the capacity for healthy attachment that may have been disrupted by early trauma or deprivation.

Treatment for Reactive Attachment Disorder: Children who have experienced severe early deprivation often develop reactive attachment disorder, a condition that involves difficulty forming appropriate attachments with caregivers. Treatment approaches for this condition apply Harlow’s insights about the importance of consistent, responsive caregiving in structured therapeutic contexts.

Trauma-Informed Care: The recognition that early attachment relationships are crucial for emotional regulation has influenced the development of trauma-informed care approaches in various settings. These approaches recognize that many behavioral and emotional problems stem from attachment disruptions and focus on creating safe, predictable environments that support healing and growth.

Parent-Child Interaction Therapy: This therapeutic approach helps parents develop more secure attachment relationships with their children by teaching responsive caregiving skills and helping families create more positive interaction patterns. The therapy directly applies Harlow’s insights about the importance of comfort, consistency, and emotional attunement.

Adult Attachment Therapy: Therapists working with adults often use attachment theory to help clients understand how their early relationship experiences continue to influence their current relationships. This work helps people develop more secure attachment styles and build healthier relationships throughout their lives.

The clinical applications of Harlow’s research demonstrate how basic scientific discoveries can be translated into practical interventions that help real people overcome difficulties and build healthier relationships. These therapeutic approaches continue to evolve as our understanding of attachment processes becomes more sophisticated, but they all trace their theoretical foundations back to Harlow’s fundamental insights about the importance of love, comfort, and security in human development.

Modern attachment research and its clinical applications represent the positive legacy of Harlow’s work—demonstrating how scientific insights, even those obtained through ethically problematic methods, can ultimately contribute to human welfare when applied thoughtfully and ethically. The goal of contemporary researchers and clinicians is to honor the valuable knowledge that emerged from Harlow’s studies while ensuring that future research and practice maintain the highest ethical standards.

The continuing influence of Harlow’s work more than six decades after his original experiments demonstrates the enduring importance of his basic insights about human nature and the fundamental role of attachment relationships in healthy development. While we can critique his methods and learn from his ethical failures, we can also appreciate how his discoveries continue to help us understand what it means to be human and how we can create better conditions for human flourishing.

Conclusion

Harry Harlow’s monkey experiments fundamentally transformed our understanding of attachment, love, and human development. His research definitively proved that emotional bonds are not formed through feeding relationships, as behaviorist theory suggested, but through contact comfort and security. The baby monkeys’ overwhelming preference for soft, warm cloth mothers over food-providing wire mothers demonstrated that the need for physical comfort and emotional security is more fundamental than basic survival needs.

These findings revolutionized psychology, contributed to the development of modern attachment theory, and influenced countless practical applications in child care, hospital policies, and therapeutic approaches. While Harlow’s methods raise serious ethical concerns by today’s standards, his insights about the crucial importance of love, comfort, and secure relationships continue to guide our understanding of healthy human development and inform evidence-based practices in psychology, education, and child welfare.

Frequently Asked Questions

What was the main finding of Harlow’s monkey experiments?

The primary finding was that baby monkeys preferred surrogate mothers that provided contact comfort (soft, cloth-covered) over those that provided food (wire mothers), spending 17-18 hours daily with cloth mothers regardless of which mother fed them. This proved that attachment is based on comfort and security, not feeding, overturning prevailing behaviorist theories about love and bonding.

Which of the two fake mother monkeys did the baby monkeys prefer and why?

Baby monkeys overwhelmingly preferred the cloth mother because it provided contact comfort—soft terry cloth material and warmth from an internal heating pad. Even when the wire mother provided food, monkeys spent only 1 hour daily with it versus 17-18 hours with the comfort-providing cloth mother. This demonstrated that physical comfort and emotional security were more important than nourishment for attachment formation.

What happened to the monkeys after the Harlow experiments?

Many monkeys suffered long-term psychological damage including social dysfunction, abnormal behaviors like rocking and self-clasping, and difficulty forming relationships with other monkeys. Monkeys from isolation studies showed severe depression-like symptoms and never fully recovered normal social functioning. Female monkeys raised without mothers became inadequate or abusive mothers themselves, demonstrating generational effects of early deprivation.

Is Harlow’s monkey experiment ethical?

By modern standards, Harlow’s experiments would be considered highly unethical due to deliberate infliction of psychological harm, lack of animal welfare oversight, and failure to provide adequate care or rehabilitation. Contemporary research ethics require institutional review, consideration of alternatives, and strict animal welfare protections. While the research produced valuable scientific insights, the methods violated fundamental principles of animal welfare and would not be approved today.

What does “wire mother” mean in psychology?

In Harlow’s experiments, the “wire mother” was a surrogate mother made from welded wire mesh that provided food but no comfort. It represented the behaviorist theory that babies love mothers primarily for food provision. The wire mother was cold, hard, and uncomfortable, serving as a control condition to test whether feeding relationships or comfort relationships were more important for attachment formation.

What was the monkey mother experiment about?

The monkey mother experiment studied whether baby monkeys would prefer a mother who provided food versus one who provided comfort. Harlow created two artificial mothers: a wire mother (sometimes with food) and a cloth mother (soft and warm but no food). The experiment aimed to test competing theories about the basis of attachment—whether love was based on feeding (behaviorist theory) or emotional comfort and security.

How did Harlow’s research influence modern psychology?

Harlow’s research provided crucial evidence for attachment theory, influenced hospital policies to allow parental visits, improved adoption and foster care practices, and laid groundwork for understanding trauma and PTSD. The research contributed to the decline of strict behaviorism in developmental psychology and supported the importance of emotional relationships in child development, influencing modern therapeutic approaches and parenting guidance.

What is contact comfort in attachment theory?

Contact comfort refers to the physical and emotional comfort derived from soft, warm touch and physical closeness with a caregiver. Harlow coined this term to describe why baby monkeys preferred cloth mothers over wire mothers. Contact comfort became a fundamental concept in attachment theory, explaining how physical affection and emotional security form the basis of healthy attachment relationships rather than just meeting basic survival needs.

References

• Ainsworth, M. D. S., Blehar, M. C., Waters, E., & Wall, S. (1978). Patterns of attachment: A psychological study of the strange situation. Lawrence Erlbaum Associates.
• Blum, D. (2002). Love at Goon Park: Harry Harlow and the science of affection. Perseus Publishing.
• Bowlby, J. (1969). Attachment and loss: Vol. 1. Attachment. Basic Books.
• Bowlby, J. (1973). Attachment and loss: Vol. 2. Separation: Anxiety and anger. Basic Books.
• Bowlby, J. (1980). Attachment and loss: Vol. 3. Loss: Sadness and depression. Basic Books.
• Bowlby, J. (1982). Attachment and loss: Retrospect and prospect. American Journal of Orthopsychiatry, 52(4), 664-678.
• Bowlby, J. (1988). A secure base: Parent-child attachment and healthy human development. Basic Books.
• Bretherton, I. (1992). The origins of attachment theory: John Bowlby and Mary Ainsworth. Developmental Psychology, 28(5), 759-775.
• Fraley, R. C. (2002). Attachment stability from infancy to adulthood: Meta-analysis and dynamic modeling of developmental mechanisms. Personality and Social Psychology Review, 6(2), 123-151.
• Harlow, H. F. (1958). The nature of love. American Psychologist, 13(12), 673-685.
• Harlow, H. F., & Zimmermann, R. R. (1959). Affectional responses in the infant monkey. Science, 130(3373), 421-432.
• Hazan, C., & Shaver, P. (1987). Romantic love conceptualized as an attachment process. Journal of Personality and Social Psychology, 52(3), 511-524.
• Johnson, S. (2019). Attachment in psychotherapy. Guilford Press.
• Main, M., & Solomon, J. (1986). Discovery of an insecure-disorganized/disoriented attachment pattern. In T. B. Brazelton & M. W. Yogman (Eds.), Affective development in infancy (pp. 95-124). Ablex Publishing.
• Mikulincer, M., & Shaver, P. R. (2016). Attachment in adulthood: Structure, dynamics, and change. Guilford Press.
• Sroufe, L. A. (2005). Attachment and development: A prospective, longitudinal study from birth to adulthood. Attachment & Human Development, 7(4), 349-367.
• Watson, J. B. (1913). Psychology as the behaviorist views it. Psychological Review, 20(2), 158-177.
• Watson, J. B., & Rayner, R. (1920). Conditioned emotional reactions. Journal of Experimental Psychology, 3(1), 1-14.

Further Reading and Research

Recommended Articles

• Cassidy, J., & Shaver, P. R. (2016). Handbook of attachment: Theory, research, and clinical applications. Guilford Press.
• Shonkoff, J. P., & Phillips, D. A. (Eds.). (2000). From neurons to neighborhoods: The science of early childhood development. National Academy Press.
• Thompson, R. A. (2008). Early attachment and later development: Familiar questions, new answers. In J. Cassidy & P. R. Shaver (Eds.), Handbook of attachment: Theory, research, and clinical applications (pp. 348-365). Guilford Press.

Suggested Books

• Bowlby, J. (1988). A Secure Base: Parent-Child Attachment and Healthy Human Development. Basic Books.
  • Bowlby’s accessible overview of attachment theory’s practical applications for parents, educators, and clinicians, including real-world examples of secure base behavior.
• Levine, A., & Heller, R. (2010). Attached: The New Science of Adult Attachment and How It Can Help You Find—and Keep—Love. TarcherPerigee.
  • A practical guide applying attachment theory to adult relationships, with assessments, relationship strategies, and communication techniques based on attachment styles.
• Siegel, D. J. (2012). The Developing Mind: How Relationships and the Brain Interact to Shape Who We Are. Guilford Press.
  • Comprehensive exploration of how early relationships literally shape brain development, integrating attachment theory with neuroscience research and therapeutic applications.

Recommended Websites

• Center on the Developing Child – Harvard University
  • Provides evidence-based research on early childhood development, attachment, and trauma with downloadable resources for parents and professionals studying the science behind healthy development.
• Circle of Security International
  • Offers training programs, assessment tools, and educational materials for parents and professionals focused on building secure attachment relationships through practical interventions.
• Attachment and Trauma Recovery
  • Comprehensive resource for families and professionals dealing with attachment difficulties, featuring support groups, educational materials, and therapeutic approaches for healing attachment wounds.

Kathy Brodie

Kathy Brodie is an Early Years Professional, Trainer and Author of multiple books on Early Years Education and Child Development. She is the founder of Early Years TV and the Early Years Summit.

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Early Years TV Harry Harlow’s Monkey Experiments: Wire vs Cloth Mothers. Available at: https://www.earlyyears.tv/harry-harlows-monkey-experiments/ (Accessed: 26 November 2025).