Harlow’s Monkey Experiment – The Bond between Infants and Mothers

This is the 4th post in our interesting psychological studies series. In the previous posts, we talked about the following psychological studies:

Harry Harlow was an American psychologist whose studies were focused on the effects of maternal separation (the separation of an infant from its mother), dependency, as well as social isolation on monkeys in an attempt to understand the significance of companionship and care giving when it comes to both mental and social development. He conducted a series of experiments on rhesus monkeys, observing how isolation and separation can affect the subjects in the latter years of their lives.

Objective of the Harlow’s Monkey Experiment

The idea came to Harlow when he was developing the Wisconsin General Testing Apparatus or the WGTA to study the mental processes of primates, which include memory, cognition and learning. As he developed his tests, he realized that the monkeys he worked with were slowly learning how to develop strategies around his tests.

To understand how this happens, Harlow wanted to study developing primates, taking them to a nursery setting away from their biological mothers. The succeeding behaviours further inspired Harlow to look into the introduction of surrogate mothers, and how they can be a ‘replacement’ for the affection that should have been given by the biological mothers.

Harlow had the idea that infant monkeys who are separated from their mothers at a very early age (within 90 days) can easily cope with a surrogate, because the bond with the biological mother has not yet been established. Furthermore, he also wanted to learn about whether the bond is established because of pure nourishment of needs (milk), or if it involves other factors.

How did the Harlow’s Monkey Experiment work?

Harlow’s experiment tackled both hypotheses – (1) if surrogates can take the place of the biological mother, and (2) if the bond between mother and child is purely based on physiological need. To do this, Harlow separated infant monkeys from their biological mothers within 6 to 12 hours after being born. He then placed these infants in a nursery with inanimate ‘surrogate’ mothers – one who is made of heavy wire mesh and the other made of wood that was covered in terry cloth. Both surrogates were of the same size; however the wire mesh mother did not have any soft surface, while the terry cloth mother was soft to the touch and appeared to be cuddly.


In the first experiment, both the surrogates were placed with the infant monkeys, so the infants would have a ‘choice’ where to go. Both surrogates are able to provide nourishment to the infants.

In the second experiment, the infant monkeys were divided into two groups (wire mesh or terry cloth), and the infants had no choice which one they would go to.

Results of the Harlow Monkey Experiment

After observing the infant monkeys over time, it was found that even though the infants received nourishment from the wire mesh mother, they still spent more time cuddling and being affectionate with the terry cloth mother. This shows that the bond between mother and infant was not solely based on whether the former is able to give the latter’s physiological needs.

Furthermore, the results of the second experiment showed that while the infants from both groups consumed the same amount of milk from their mother, the infants who grew up with the terry cloth mother exhibited emotional attachment and what is considered as normal behaviour when presented with stressful variables. Whenever they felt threatened, they would come close to the terry cloth mother and cuddle with it until the monkeys were calm.

The results for the wire mesh mother were the opposite. They reacted rather differently with the same stimulus – throwing themselves on the floor, rocking back and forth, and evidently did not go to the wire mesh mother for comfort.

Significance of the Harlow’s Monkey Experiment

Harlow’s Monkey experiment reinforced the importance of mother-and-child bonding. He suggested that the same results apply for human infants – that the timing is critical when it comes to separating a child from his or her mother. He said that it is at 90 days for monkeys, and about 6 months for humans. Moreover, it was found that the establishment of bond between the infant and his or her mother is not purely dependent on the satisfaction of one’s physiological needs (warmth, safety, food) , but also emotional (acceptance, love, affection).

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