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Key Study: Attachment in
Infant Monkeys

BACKGROUND & AIMS: When Harry Harlow, who had studied intelligence (among other things) under Lewis Terman at Stanford University, took a job at the University of Wisconsin-Madison in 1930, he planned to study rats, but he wound up with rhesus monkeys, a small agile breed. Reflecting his studies under Terman, he began by devising a test of monkey intelligence, a sort of simian IQ profile. Among Harlow’s early (and largely uncredited!) assistants was Abraham Maslow, later developer of the Hierarchy of Needs.

However, the behaviour of the monkeys soon drew his research into a different direction. When he separated the infants from their mothers and peers, the infants,became extremely attached to the terry cloth towels covering the cage floors. They would lie on them, grip them in their tiny fists and throw tantrums if they were taken away. The monkeys seemed to ‘love’ these towels - just like a human infant with a ratty blanket or a stuffed bear.

To investigate this, Harlow separated newborn rhesus monkeys from their mothers within 6-12 hours of birth and raised them in cages on their own, each cage containing a ‘baby blanket’. The babies became extremely distressed whenever the blanket was removed. It seemed to Harlow the distress at having the blanket removed was similar to the distress infant monkeys normally show when separated from their mothers. Harlow began to argue that the monkeys ‘loved’ the comfort their mothers and became attached to the blankets for similar reasons. Therefore, attachment wasn’t just based on food as Behaviourists like John Dollard & Neal Miller (1950) proposed. (See: Theories of Attachment.)

Behaviourism, in the United States especially, strongly influenced child rearing from the 1930s through to the 1950s - eg: John B Watson (1928, p82) advised:  "Do not overindulge them. Do not kiss them goodnight. Rather, give a brief bow and shake their hand before turning off the light."

Harlow, however, was not afraid to talk about ‘love’. At one conference, he was repeatedly rebuked for use of this term by a colleague who kept saying: "You must mean proximity, don't you?" Finally, Harlow told his colleague: "It may be that proximity is all you know of love…I thank God I have not been so deprived." (Lauren Slater, 2004)

Harlow was influenced in his approach by the work of John Bowlby (1951) and the concept of Maternal Deprivation. The classic study discussed here was just one in a range of studies Harlow conducted both before and after 1959 into the nature of attachment using rhesus monkeys.

PROCEDURE (METHOD): In the initial study 8 infant rhesus monkeys were taken from their mothers shortly after birth and kept separately in a cage with two surrogate mothers - a ‘cloth mother’ covered with a soft blanket and a skeletal ‘wire mother’. Both ‘mothers’  were of the same size and shape as an adult monkey. For 4 of the 8 monkeys the wire mother incorporated their feeding bottle; for the other 4 the cloth mother had it. The monkeys were kept in these conditions for a period of time and then released into a cage with a group of normally reared monkeys.

To see how the monkeys would react when frightened, Harlow put a teddy bear drummer toy into the cage. (In later replications a crab-like ‘monster’ machine with snapping jaws was used to frighten the monkeys - see top left video below.)

A further condition - the ‘Open Field Test’ - was created by introducing the monkeys for 3 minutes into the strange environment of a room measuring 6 feet by 6 feet by 6 feet and containing toys and other stimuli known to elicit curiosity-manipulatory responses in baby monkeys. The monkeys were placed in this situation twice a week for 8 weeks with no mother surrogate present during alternate sessions and the cloth mother present during the others. A terry cloth towel was always available as one of the stimuli throughout all sessions.

RESULTS (FINDINGS): The infant monkeys preferred to spend time with the cloth mother even when they got their food from the wire mother. When the cloth mother had the bottle, they didn't go to the wire mother at all. The graphs below left show the mean amount of time spent on the mother in relation to which mother had the feeding bottle.

Regardless of which mother supplied them with food, the monkeys all ran to the cloth mother when they were frightened by the teddy bear drummer. They rubbed against the cloth mother and eventually calmed down. Harlow theorised that they used their mothers as a ‘psychological base of operations’, allowing them to remain playful and inquisitive after the initial fright had subsided. (Some of the monkeys even challenged the frightening stimulus!)

In the Open Field Test condition, after one or two adaptation sessions, the infants always rushed to the cloth mother when she was present and clutched her, rubbed their bodies against her and frequently manipulated her body and face. After a few additional sessions, the infants began to use this surrogate mother as a source of security, a base of operations. They would explore and manipulate a stimulus and then return to the mother before adventuring again into the strange new world. The behaviour of these infants was quite different when the mother was absent from the room. Frequently they would freeze in a crouched position and cry, crouch down or suck their thumbs. Some of the monkeys would even run from object to object, apparently searching for the cloth mother as they cried and screamed. In a variation of this condition, monkeys placed in this situation with their wire mothers exhibited the same behaviours that the monkeys with no mother accompanying them did

When returned to the company of other monkeys, Harlow’s monkeys showed signs of inappropriate social behaviour and delinquency. They were aggressive or indifferent towards other monkeys, unable to form normal relationships. Some sat clutching themselves, rocking constantly back and forth; a stereotypical behaviour pattern for excessive and misdirected aggression. Males were unable to mate successfully - they actually didn’t seem to know what to do! - while females attacked any male that tried to mate with them. If they did have offspring, these "motherless mothers" (as Harlow later termed them) were either negligent or abusive. Negligent mothers did not nurse, comfort, or protect their young, nor did they harm them. (The first one to have a baby ignored it and pushed it away when it tried to make contact.) The abusive mothers violently bit or otherwise injured their babies, to the point that many of them died. Deprivation of emotional bonds to live mother monkeys (as infant monkeys) these (now adult) monkeys were unable to create a secure attachment with their own offspring.

CONCLUSIONS: It seems the privated monkeys suffered serious emotional deprivation - arguably, privation - resulting in delinquent and anti-social behaviour. Harlow concluded that the infant monkeys had an innate need for contact comfort.

He also concluded that the mother serves as a ‘safe base’, both from which to explore the world and to retreat to when threatened.

The study does seem to contradict theories that the infants attach for food. Gavin Bremner (1994) describes these findings as inconsistent with Secondary Drive Theory. Harlow  drew the conclusion that comfort and security formed the basis for attachment, rather than food.


Harlow carried out a number of variations and extensions on the 1959 study - among the most important of which were:-

Harry Harlow (1959)

Updated: 26 June 2013

eat solid foods, they were separated from their cloth mothers for 3 days. When they were reunited with their mothers, they clung to them and did not venture off to explore as they had in previous situations. Harlow claimed from this that the need for contact comfort was stronger than the need to explore. In these findings, Harlow appears to be pre-empting Ainsworth & Bell’s (1970) identification of ‘anxious-resistant’ attachment.

2 general criticisms of Harlow’s work are:-

  1. Care must be taken in generalising Harlow’s rhesus monkey studies to human beings since even the higher mammals appear unable to learn by reflection in the way humans can.
  2. Many of his experiments would be considered unethical today. With tens of monkeys either dead, disturbed or socially unadaptive he was held accountable for unethical work. Their nature of the experiments and Harlow's descriptions of them heightened awareness of the treatment of laboratory animals and thus contributed to modern ethics regulations. Gene Sackett, one of Harlow's doctoral students, told Deborah Blum (2002) that he believes the animal liberation movement in the U.S. was born as a result of Harlow's experiments.
    Part of the negative reaction to Harlow’s studies was due to the dark, dramatic language he insisted on using - such as the ‘pit of despair’ and ‘rape rack’ (for the structure to which unwilling female monkeys were tied).
    Willam Mason, another of Harlow's students who continued deprivation experiments after leaving Wisconsin, told Blum (1994, p96)  that Harlow "kept this going to the point where it was clear to many people that the work was really violating ordinary sensibilities, that anybody with respect for life or people would find this offensive. It's as if he sat down and said, 'I'm only going to be around another ten years. What I'd like to do, then, is leave a great big mess behind.' If that was his aim, he did a perfect job."
     However, Harlow argued that the benefits in terms of knowledge far outweighed the ethical problems.

After Harlow's second wife, Peggy, died from breast cancer in 1970, he remarried first wife Clara Mears in 1971. However, he also suffered severe Depression - having had periodic bouts of the illness previously. He was given a programme of Electro-Convulsive Therapy a the May Clinic in Minnesota.

Unfortunately he continued to suffer Depression - which may explain his morbid interest in creating a ‘primate model of mental illness’. He compounded his condition by slipping into alcoholism and becoming estranged from his own children.

During his later years he suffered badly from Parkinson’s Disease and couldn’t stop shaking. He died from a brain tumour late in 1981.

An excerpt from a 1960s documentary on Harlow’s work, including the crab-like monster and the ‘Strange Situation’-type room experiment

An excerpt from a later documentary hinting at the devastating emotional effects on the experimental monkeys [Photos and video footage copyright © University of Wisconsin]

The basic condition…

…plus teddy bear drummer!

The ‘with’ and ‘without’ conditions of the Open Field

They concluded that the impact of early maternal deprivation could be reversed in monkeys only if it had lasted less than 90 days, and estimated that the equivalent for humans was 6 months. After these critical periods, no amount of exposure to mothers or peers could alter the monkeys’ abnormal behaviours and make up for the emotional damage that had already occurred. When emotional bonds were first established was the key to whether they could be established at all