2025.10.26 – When Comfort Was Stronger Than Food: Harry Harlow’s Rhesus Monkeys and the Rise of Attachment Science

Main Ideas

In the middle of the last century, psychology was ruled by behaviorism, a school that claimed affection was nothing more than a conditioned response to feeding. Then came Harry Harlow, who decided to test that belief through an experiment as simple as it was unsettling. His work with rhesus monkeys changed the understanding of early bonding and exposed the moral limits of scientific ambition.

Story

From Behaviorism to a New View of Affection

John B. Watson, the American founder of behaviorism, taught that tenderness weakened discipline. He even advised parents not to kiss or hug their children, arguing that food alone met their needs. In the 1930s and 1940s this view shaped much of Western parenting.
At the University of Wisconsin, Harry Harlow began to doubt it. He suspected that the need for closeness might exist independently of hunger. To explore that idea, he chose the rhesus macaque, a primate that shares most of our genetic code, and built a miniature world to watch them grow.

The Cloth Mother and the Wire Mother

Each newborn monkey found two “mothers.” One was made of wire—cold but equipped with a feeding bottle. The other was soft and warm but provided no food.
If love truly came from nourishment, the infants should have clung to the wire figure. Instead, they spent nearly every moment holding on to the soft one, leaving her only to drink before rushing back. When frightened by moving toys or loud noises, they ran to the cloth mother for comfort.
Harlow concluded that touch, not feeding, was the real foundation of emotional security. “Man cannot live by milk alone,” he liked to say.

The Cost of Growing Up Without Love

The monkeys raised only by these artificial mothers grew into anxious, isolated adults. Males lost interest in mating, and females reacted with fear or aggression when approached. To test whether they could still reproduce, Harlow used a restraining frame that forced contact with males—a grim device he himself called the “rape rack.”
The pregnancies that followed revealed the full cost of emotional deprivation. Many mothers ignored or attacked their newborns; some even killed them. Witnessing these scenes was so disturbing that several of Harlow’s own students abandoned the work altogether. What emerged from those cages was not cruelty for its own sake, but a stark demonstration of what happens when affection is never learned.

Movement, Play, and Partial Recovery

Harlow later introduced motion into his surrogate mothers and allowed isolated infants brief play sessions with socially normal monkeys. The effect was striking. Those exposed to movement and limited companionship grew more curious and less fearful. The lesson was unmistakable: affection is alive—it needs rhythm, touch, and interaction to thrive.

The Nature of Love

In his renowned lecture The Nature of Love to the American Psychological Association, Harlow argued that the bond between infant and caregiver rests on comfort and safety, not on food. His work inspired psychiatrist John Bowlby’s attachment theory and reshaped how hospitals, orphanages, and parents thought about early care.

Ethics and Limits

Viewed today, Harlow’s experiments are painful to imagine. The suffering of his monkeys helped establish modern ethical standards for research. Yet his findings remain powerful evidence that emotional neglect can wound as deeply as hunger. They also remind us that results from animal studies cannot be copied directly onto people, whose bonds are woven through language, imagination, and culture.

A Legacy That Still Breathes

From skin-to-skin care for newborns to modern bonding programs in adoption, traces of Harlow’s discovery remain everywhere. His instruments are long gone, but his insight endures: affection is not an extra—it is the core of life itself.

Sources

• 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.
• Watson, J. B. (1928). Psychological Care of Infant and Child. W. W. Norton & Co.
• American Anti-Vivisection Society. Maternal Deprivation Experiments in Psychology. https://aavs.org/maternal-deprivation-experiments-psychology
• PETA Investigations. NIH Baby Monkey Experiments. https://investigations.peta.org/nih-baby-monkey-experiments/expert-statements

Appendix

Behaviorism

A psychological approach born in the early twentieth century that focused on observable actions and avoided speculation about inner states. Once dominant, it later merged with cognitive science but left a lasting imprint on education and therapy.

Contact Comfort

A concept introduced by Harlow to describe the security derived from gentle physical contact. It became a cornerstone of developmental psychology.

Attachment Theory

Developed by John Bowlby after Harlow’s findings, it explains how early emotional bonds influence trust, resilience, and relationships throughout life. It remains central in child psychology and family policy.