What they developed social impairments that affected

What happens when you make a baby monkey choose between food and comfort? The Harlows answered this question in a series of primate experiments. Love is important, so how will these lonely monkeys function without it?


Though the Beatles confidently tell us that ‘all you need is love,’ behavioral psychologists were skeptical that people and animals need–or are motivated by–anything other than food, water, shelter and sex. Psychologists Harry and Margaret Harlow decided to determine scientifically whether love is something we really need or just a feeling that we have toward things that satisfy our more basic, concrete, physical needs.

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Harlow’s Monkey Experiment

Harry Harlow founded a primate lab and started studying how infant monkeys developed when separated at birth from their mothers. He put these lonely monkeys in cages with two dolls. One was made out of wire with a wooden head and contained a bottle for the monkey’s nourishment.

The other was made of soft foam and covered in cuddly cloth but did not have a bottle. With this setup, Harlow attempted to separate the two things the monkey gets from its mother: nourishment and comfort. The wire mother gave food, while the cloth mother gave warmth and comfort.

Preference For Warmth And Comfort

The prevailing behaviorist theories of the time would predict that the lonely monkey would quickly grow attached to the wire mother, since it dispensed the food. But Harlow was surprised to observe that the monkeys spent an overwhelming amount of time with the cloth mother, moving to the wire mother only when they needed to eat. Their affection for the cloth mother had nothing to do with food and everything to do with warmth and comfort.

Relationship Between Comfort and Security

To further test the monkeys’ attachment to the cloth mother, Harlow decided to scare them to see how they’d react.

He created an ugly, scary paper monster to surprise and frighten them. When the monster appeared, the monkeys screeched and cried and ran immediately to the cloth mother for comfort and security. For all measurable purposes, the baby monkeys seemed to love the cloth mother and interact with it in the same way they’d interact with a real mother. Harlow concluded that the need for love has nothing physically to do with survival–the cloth mother didn’t give the monkey anything it physically needed–but is nevertheless important.

Need for Socialization

Not surprisingly, the cloth mother proved a poor substitute for a real mother. Harlow found that only baby monkeys who were raised with their real mothers and other monkey playmates developed into happy, secure adults.

Babies raised by a real mother and without playmates became more fearful and aggressive, and those initially raised by the cloth and wire mothers had trouble learning to socialize with other monkeys when they returned to communal cages. But the monkeys who were raised by the dolls and had no contact with other monkeys fared the worst; they developed social impairments that affected them the rest of their lives. The cloth mothers provided an outlet for the monkeys’ need for comfort, but not for socialization, which emerged as equally important for normal development.


Harlow’s research had far-reaching consequences. He disrupted the prevailing mindset that love was based purely on physical needs; he showed that love and comfort were non-physical needs that nevertheless were important to normal development. But the cruelty of the experiment also drew attention and led to reforms in regulations about how lab animals were treated.

Subjecting infant monkeys to total isolation impaired them for life; such an experiment would be considered an ethical violation by today’s standards.

Lesson Objectives

Once you complete this lesson you’ll be able to:

  • Explain the findings of Harlow’s monkey experiment
  • Understand why the monkey’s ran to the cloth mother when frightened
  • Comprehend the monkey’s need for both comfort and socialization
  • Consider the relationship between safety and comfort
  • Explain why monkey’s raised without playmates became fearful and aggressive

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