In 1958, Harry Harlow stood before the American Psychological Association and gave a talk called "The Nature of Love." His evidence came from baby rhesus monkeys at the University of Wisconsin. Each infant had two fake "mothers": one made of bare wire that held a milk bottle, and one covered in soft cloth that gave no food. The babies went to the wire mother to feed, then went back to cling to the cloth one, especially when frightened.
The misconception Harlow was attacking was that babies love whoever feeds them. His monkeys suggested that comfort, not food, sits at the heart of the bond. Two other researchers, John Bowlby and Mary Ainsworth, built that idea into attachment theory.
Harlow: contact comfort
Psychologists at the time often explained the infant–mother bond with cupboard love theory: food is rewarding, so the baby learns to love the person who brings it. Harlow's contact comfort findings cut against this. His later isolation studies, where monkeys were raised without social contact, produced adults with severely disturbed social behavior. The studies are now a standard example of research that would not pass an ethics review, and monkeys are not people, so they show what's possible rather than proving how human babies work.
Bowlby: an evolved bond
Bowlby, a British psychiatrist, was influenced by animal research such as Konrad Lorenz's imprinting in geese. He argued that crying, clinging and smiling evolved because they keep a caregiver close and a baby safe. In a 1951 report for the World Health Organization, he argued that long early separation from the mother figure could cause lasting harm, the maternal deprivation hypothesis.
His other key ideas:
- Monotropy: one main attachment figure matters most.
- Sensitive period: attachments form most easily early in life; his 1951 report put the limit around two and a half to three years.
- Internal working model: early care becomes a mental template for later relationships.
Several of these were later softened. Michael Rutter argued in 1972 that never forming a bond (privation) is more harmful than losing one (deprivation), and that what surrounds a separation matters. Schaffer and Emerson's 1964 study of Glasgow infants found many formed several attachments, not one.
Ainsworth: measuring attachment
Ainsworth worked with Bowlby in London, then studied mothers and babies in Uganda and in Baltimore. She developed the Strange Situation, a lab procedure of brief separations and reunions, and described three patterns: secure, insecure-avoidant and insecure-resistant (also called ambivalent, or anxious in the AP course). Mary Main and Judith Solomon later added disorganized, for children with no consistent way of responding on reunion.
Ainsworth linked secure attachment to sensitive responsiveness: a caregiver who reads a baby's signals and responds promptly. A 1997 meta-analysis of 66 studies found a moderate link and concluded sensitivity is important but not the only influence.
Where the research is shaky
- Culture. The Strange Situation was designed in the US. Van IJzendoorn and Kroonenberg's 1988 meta-analysis found more variation within countries than between them, but the proportions of each type still differed across samples, and a brief separation means different things in different childcare cultures.
- Temperament. Some researchers argue a baby's inborn temperament explains part of what the Strange Situation measures.
- Long-term prediction. Early attachment predicts later relationships only moderately, and classifications can change as circumstances change.
- Adult "attachment styles." Hazan and Shaver's 1987 questionnaire applied Ainsworth's categories to romantic love. The pop-psychology habit of labeling partners "avoidant" goes far beyond that evidence.
Common mix-ups
- Avoidant vs. resistant. Avoidant children show little distress and ignore the parent on return; resistant children are very upset and both seek and push away the parent.
- Relationship, not personality. A child can be secure with one parent and insecure with another.
- Separation anxiety in a toddler is normal. It is a sign of attachment, not a problem.
- Monotropy is Bowlby's claim, not a settled finding.
Frequently asked questions
What are the four attachment styles in psychology?
Secure, insecure-avoidant and insecure-resistant (from Ainsworth; the AP course says anxious), plus disorganized (added by Main and Solomon). They describe a child's pattern with one caregiver, not a fixed personality type.
What did Harlow's monkey experiment show?
Infant monkeys preferred a soft cloth "mother" to a wire one that gave milk, suggesting comfort matters more than food for the bond. The studies are now widely criticized on ethical grounds.
What is the difference between Bowlby and Ainsworth?
Bowlby proposed the theory of why attachment evolved and why it matters. Ainsworth found a way to measure it, the Strange Situation, and described the different patterns.
Can attachment styles change?
Yes. Research finds moderate stability at best, and classifications can shift when a child's care or circumstances change. Adult relationships are shaped by much more than infancy.
Try it
Next time you see an online quiz that labels you "avoidant" or "anxious," check what it actually measured: a few questions about how you feel, answered today. Then ask yourself whether a toddler in a lab room in Baltimore was measured the same way.