Harry Harlow - Developmental Psychology - The Canon

Psychology: Essential Thinkers, Classic Theories, and How They Inform Your World - Andrea Bonior 2016

Harry Harlow
Developmental Psychology
The Canon


BORN 1905, Fairfield, Iowa


DIED 1981, Tucson, Arizona


Educated at Reed College and Stanford University



Many people are familiar with Harry Harlow’s classic study of rhesus monkeys raised without their mothers from infancy. In this study, he pitted surrogate mothers made of wire and wood against surrogate mothers made of terry cloth to see which surrogates the monkeys preferred (only the wire-and-wood surrogates provided milk). Less known, however, is the scope of Harlow’s research, which included a number of other monkey studies that went beyond comparing terry-cloth surrogates to wood-and-wire surrogates. Indeed, Harlow’s extensive work was instrumental in propelling forward big ideas about attachment and peer relationships.

In Harlow’s most noteworthy study, monkeys craved the contact comfort of terry cloth more than they valued the ability to be nourished by milk. Of course, it makes sense that it was more comfortable for them to sit on terry cloth than on wire and wood. But the level to which the monkeys gravitated toward the terry-cloth surrogates, especially in times of fear, spoke to something larger about attachment. We see this in human children, who are comforted by stuffed animals or other cuddly toys. Harlow’s discovery went squarely against the psychoanalytic cupboard model of previous theories on mother-child attachment, which had said that the mother provides nourishment and satisfies basic biological needs, and that therein lies the origin of the bond with her child. Harlow’s findings also went against the behaviorist model, which said that children are conditioned to be attached to their mothers because of the positive reinforcement and conditioning of having their biological needs met. In addition, his findings helped dislodge the prevailing mindset of the time, which held that too much physical cuddling and comforting can spoil a baby. Furthermore, the presence of the surrogates allowed the monkeys to be more adventurous. They took more healthy risks in venturing out when their surrogates were near, showing how a strong attachment that provides comfort can help lay the foundation of security that provides the confidence to be more autonomous in the world.

Harlow’s findings also suggest how strongly we attach to the faces of those we love. The terry-cloth surrogates were given slightly different faces, and each monkey not only came to know and recognize its own surrogate’s face but also clearly preferred its surrogate over others that were similar.

What gets less attention is Harlow’s exploration of the role of peers, in terms of attachment and development. In one aspect of his experimentation, Harlow compared monkeys who had contact only with their mothers to monkeys who had contact only with their peers. Neither group fared particularly well. The monkeys who were in the mother-only contact group failed to develop healthy social behaviors; when finally exposed to their peers, they would avoid them or even become highly aggressive. The monkeys in the peer-only contract group would cling to the peers they had grown to know but were aggressive toward monkeys outside the peer group when finally exposed to them. In general, they were more irritable and agitated than the monkeys in the mother-only contact group.

Harlow’s most heartening research involved the addition of carefully screened monkey “therapists.” The monkeys serving in the therapeutic role would hang around with the previously isolated monkeys, clinging to them. In time, the previously isolated monkeys clung back, and their behavior began to look more normal and adaptive, in terms of their interactions with others, though it’s worth noting that they always remained more acutely susceptible to stress.

Harlow felt strongly that he was revealing deeper truths that apply to the human experience. Strikingly, he said that the focus of his studies was love, not attachment, even though many theorists overwhelmingly preferred the latter focus. Harlow himself, however, was unsentimental, and he openly acknowledged that he didn’t really like animals and had no love for the monkeys he had spent years experimenting on. Some people consider his experiments to have been unethical by their very nature.

KEY EXPERIMENT Harry Harlow used rhesus monkeys for his experiments, and his most classic experiment looked at monkeys who had been raised from infancy without their mothers. He had two different cages for each monkey, and each cage contained a primitive mother figure, or surrogate. One of the surrogates was made of wire and wood, and the other was covered in terry cloth. Even though only the wire-and-wood surrogate had a nipple that yielded milk, the monkeys overwhelmingly preferred to spend time with their terry-cloth surrogates. When the monkeys were frightened, they tended to cling to their terry-cloth surrogates and never once went to the milk-yielding surrogates when they were afraid.

Harlow also observed monkeys that were raised in isolation for anywhere from three months to one year. Three months of isolation had minimal effects, but longer periods of isolation were striking for the damage they seemed to cause. When finally faced with other monkeys, the previously isolated monkeys tended to sit in a corner rocking, sometimes biting themselves; they showed no interest or ability in playing and were unable to fight back if provoked. Later, when some of these monkeys became parents themselves, they tended to ignore or even attack their offspring. This behavior in the adult monkeys was most pronounced toward their firstborn offspring, but the experience of parenthood seemed to teach the previously isolated monkeys better ways to behave, and they behaved in less problematic ways with subsequent offspring.


Harlow’s work on monkey attachment directly influenced later researchers on human attachment, including his students John Bowlby and Mary Ainsworth, later famed for their exploration of different attachment styles. For example, Ainsworth developed the “strange situation” task, which has been used for decades to study the strength of young children’s attachments to their parents or caregivers and to observe their responses in somewhat stressful situations. Children between the age of 12 months and 24 months are the typical participants. In this experimental setup, a stranger is introduced to each child in the presence of his or her caregiver, and then the caregiver leaves the child alone with the stranger. Later the caregiver comes back and the stranger leaves, and then the caregiver leaves again and the stranger returns. Finally, the children are reunited with their caregivers. The various steps of this process are observed by researchers through two-way mirrors, with specific attention given to the children’s emotional responses, their ability to soothe themselves and play, and their ability to be soothed by the strangers and/or by their own caregivers. Ainsworth categorized the patterns she observed into those that reflected secure attachment and those that reflected insecure attachment, with the latter set further categorized as reflecting an insecure-avoidant pattern of attachment (the children showed few emotional reactions and little connection to their caregivers) and an insecure-resistant pattern of attachment (the children were so distressed by the departure and even the return of their caregivers that it was difficult for them to be consoled). Later, a colleague of Ainsworth’s, Mary Main, added the category of a disorganized/disoriented pattern of attachment, which included mixed distress signals and disorientation upon a caregiver’s return. With this work, Ainsworth helped establish attachment theory, which posits that the strength of young children’s attachments to primary caregivers helps set the stage for many aspects of later development. The specific attachment styles she identified have been shown to have implications for later personality traits, emotional health, and interpersonal relationships.



Whether you’ve spent one afternoon or 10 straight years with babies, you’ve probably picked up on the fact that they like to be held. Close contact, warmth, and proximity soothe them in an immediately noticeable way. In fact, holding a baby can often be the only thing that will stop his or her crying, much to the frustration of a parent who hasn’t slept well in weeks.

New parents are often bombarded with messages about the right way to do things, and many birth mothers feel shamed and anxious if they are unable to breast-feed their babies, believing that the mother-infant bond may be in jeopardy. But it’s the cuddling and comfort of feeding, not the exact mechanism of the milk’s delivery, that clearly makes for good bonding. Thankfully, this provides a chance for others besides a biological mother to bond beautifully as well, whether feeding the baby bottled breast milk or formula.

Harlow’s studies on monkeys raised without any parenting at all also force us to think in stark ways about how the quality of our early attachments can influence our later attachments. Harlow’s findings suggest that if we are not shown love ourselves, it will be difficult for us to know how to show it to others. Perhaps your parents were the opposite of affectionate, and their coldness and sternness translated into rejection. Or perhaps there was even abuse. Your experiences of trying to find love in young adulthood might have left you feeling completely out of sorts, not knowing what it means to express affection, how to go about it, how far you should go to do it, or what it should feel like. Maybe you overcompensated and jumped into a whirlwind relationship with the first person who ever complimented you. Or maybe someone treated you well, and you cared for that person a great deal, but his or her nurturing was so foreign to you that it made you uncomfortable. It’s not uncommon for people who have never been shown much open affection to have no idea how to act when it’s finally shown to them, and they may become profoundly doubtful and ill at ease.

The same feelings might apply if you did not have parents who were particularly loving but eventually go on to have your own children. Perhaps you feel you have no “script” for how to show affection to your kids: “Is my praise too much?” “I feel self-conscious cooing back at my baby.” “Do people actually say I love you to their kids at bedtime?” “Do I hug my kids at preschool pickup?” You love your children a great deal, and you wish that expressing it came naturally, but it just doesn’t seem to. Harlow’s studies show that this is less likely to be a problem with your genes than a challenge you must overcome because of the deficits in your early attachments.

But the good news is that the challenge can be overcome. Just like Harry Harlow’s monkey “therapists,” other people in your life can make a difference with their encouraging, patient, empathetic support. These people may be the surrogate families you developed among your high school friends’ parents, a first real confidant, neighbors or colleagues, a romantic partner, or a psychotherapist. Past deficits can be undone. Lack of warmth and comfort in childhood may have played a part in the person that you’ve become, but that doesn’t have to be the whole story.