Harlow’s Monkey Love Experiment
The theory of attachment is a well-studied topic in the psychological field. Attachment can be defined
as a deep, emotional bond between two people, in which each seeks closeness and security when in the
presence of one another. This form of attachment can be commonly seen between a child and attachment
figure, typically its primary caregiver. A famous study in psychology that has investigated this theory
is The Monkey Love Experiment, carried out by Henry Harlow in 1958. Harlow conducted an
experiment on newborn Rhesus monkeys. The aim of this experiment was to investigate the attachment
theory by comparing the attachment between infant Rhesus monkeys given a soft, tactile mother, and
those given a wire mother which provides milk.
Significance of this Research
Harlow’s research is significant in psychology as it was one of the first experiments, besides the 44
Thieves study (Bowlby, 1944), that demonstrated the nature of human love and attachment. At
this period of time, there was very minimal research on attachment, so Harlow was intrigued by
Bowlby’s experiment and wanted to further investigate the importance of the relationship between an
infant and mother during the first years of the infant’s life. While Harlow’s work was not entirely
original, the methodology Harlow utalised to conduct his experiment was different from Bowlby’s case
study. This is important as Harlow carried out a controlled lab experiment which, from a scientific
perspective, makes his investigation more valid. This was significant at the time because it provided
much stronger evidence towards the new knowledge gained around attachment, which later stemmed
many other investigations. Harlow’s research has also remained significant in psychology due to the
highly criticized way his experiment was run. He stripped infant monkeys away from their mothers and
forced them into isolation. Additionally, when some of the monkeys were placed back into their normal
environment, they endured lifelong trauma. This is significant because in the present, this would entirely
breach ethical guidelines, however this was not the case in the 1950’s. Consequently, now instead of
his work being celebrated for its findings, it is often scrutinised for its cruelty.
Harlow decided to create an experiment now commonly known as ‘The Monkey Love Experiment’,
which used infant Rhesus monkeys to test whether they are more likely to form a bond with a wire
surrogate mother that provides them with milk, or a comforting cloth mother that does not, if they had
access to both. He hypothesized that the infant monkeys would spend more time with the cloth mother
than with the wire mother which provided food. The results from this investigation proved Harlow’s
hypothesis as he found that only when they were hungry, they would go to the wire mother. In addition
to this, when a fearful stimulus was placed into the cage, the monkey would seek safety by clinging to
the cloth mother. Harlow was able to draw the conclusion that the monkeys benefitted significantly
more from the security that the cloth mother provided as opposed to the food that the wire one provided.
This means that attachment is not based on food, but rather in the sense of comfort and security, which
supports the evolutionary theory.
Harlow then later modified the experiment to further explore the importance of attachment in the critical
period of development in later life. He separated the infant monkeys into two groups, with the first
being given only the terrycloth mother which provided no milk, and the second with the wire mother
which did. The behaviours of the monkeys were observed when they were placed back into their normal
environment after 90 days of being with their surrogate mother. The monkeys who had the wire mother
were timid and did not know how to socialise when interacting with other monkeys. They indulged in
self-mutilation which meant that they were constantly bullied by their peers not knowing how to defend
themselves. Consequently, this led to difficulties when trying to mate, and the few females who
managed to reproduce, showed signs of inadequate mothering. Harlow therefore concluded that the
privation of forming an attachment bond within in the critical period, which he calculated to be the first
6 months of a human’s life, will permanently affect the behaviour of the individual.
One strength of this research is that it was carried out under laboratory conditions which improves the
control of reducing extraneous variables. Harlow controlled the age of the monkeys by separating them
, from their biological mothers immediately after birth and had all of the monkeys placed in identical
cages. This meant that in Harlow’s initial experiment, the monkeys had the same chances of choosing
between the mothers and, in second experiment, the differences between the behaviour of the monkeys
were only a result of which mother they were given. Another situational variable that was controlled
was that the monkeys were individually tested to reduce the disruptive noises of the other monkeys.
This could otherwise have unsettled them or caused them to behave differently which would invalidate
the correlation between the two surrogate mothers and the time the infants spent clung to them. Another
strength of this methodology is that Harlow could observe every behaviour of the infant monkeys,
which improves the accuracy of the results. While this could be a strength, it also could be a limitation
as the experimenters observing the monkeys may cause bias in the results. This is because they are
likely to have their own expectations about the outcome and may subconsciously only see the
observations tending towards that expected outcome. Nonetheless, the internal validity of this
experiment is still relatively high as within the population of the monkeys tested, the observed results
do support the claim that the monkeys benefit more from the cloth mother.
A situational variable that is a weakness to this experiment is that the infant monkeys had just been
moved and placed into the unfamiliar environment of the wire cage without any time to settle. This
could have influenced Harlow’s findings as the monkeys were probably scared and wanted to seek
comfort so, as a behavioural response, go towards the cloth mother. Additionally, the monkeys were
placed in an artificial setting and stripped from their natural environment which may have produced
unnatural behaviours, resulting in low ecological validity. This is because the monkeys are likely to act
scared and confused when placed in an unfamiliar environment, meaning that the results cannot be an
exact representation of what mother the infants are more likely to tend towards in the wild, making it
difficult to generalize the findings to a real-world setting. Therefore, this experiment has poor external
validity because we are unable to depict whether the results are because of the different mothers, or
because the monkeys had been forced into isolation.
The most significant issue with the way in which Harlow carried out his experiment is with the use of
cruel animal testing. He subjected the monkeys to weeks of inhumane treatment, by forcing them into
small cages, stripping them from any social interaction and placing large objects in their cages in
attempt to scare them. Not only this, but when they were released many of the monkeys experienced
permanent emotional damage, with some even so distressed that they died by starving themselves or
eating themselves alive. This is an issue that was overlooked for many years, but today would be
considered highly unacceptable and a breach to ethics. There are also issues regarding the findings that
Harlow discovered. Although Rhesus monkeys are the closest living biological relatives to humans,
the behaviour of the infant monkeys is not an exact representation of the behaviour of infant humans.
Therefore, it is difficult to generalise Harlow’s experiment on Rhesus monkeys to how maternal
deprivation would affect a human infant. Nonetheless, Harlow could not have conducted this research
on humans as this would breach the ethical guidelines at the time, so using Rhesus monkeys are the
closest living thing he could have used.
Influence on Society
Harlow’s experiments had a large influence on society and have caused many aspects to change since.
As Harlow’s experiments on monkeys demonstrated an immense amount of cruelty, a positive change
that has resulted from this issue is the heightened awareness of the treatment of animals used in
laboratories. Additionally, because his work was highly critised for the abuse the monkeys endured,
this contributed greatly to the creation of the ethics regulations we have today. When his studies were
performed, this was before the modern ethic guidelines were in place. However, as a result of his
studies, animal ethics were incorporated to the code of ethics worldwide in the late 20th century to
promote the humane and responsible treatment of the animals used for scientific purposes.
The research is still relevant in society today as it is still taught widely in psychology around the world.
Additionally, today we use Harlow’s findings in many manners. His researched provided significant
evidence that has helped social workers to understand the risk factors and consequences of child neglect
The theory of attachment is a well-studied topic in the psychological field. Attachment can be defined
as a deep, emotional bond between two people, in which each seeks closeness and security when in the
presence of one another. This form of attachment can be commonly seen between a child and attachment
figure, typically its primary caregiver. A famous study in psychology that has investigated this theory
is The Monkey Love Experiment, carried out by Henry Harlow in 1958. Harlow conducted an
experiment on newborn Rhesus monkeys. The aim of this experiment was to investigate the attachment
theory by comparing the attachment between infant Rhesus monkeys given a soft, tactile mother, and
those given a wire mother which provides milk.
Significance of this Research
Harlow’s research is significant in psychology as it was one of the first experiments, besides the 44
Thieves study (Bowlby, 1944), that demonstrated the nature of human love and attachment. At
this period of time, there was very minimal research on attachment, so Harlow was intrigued by
Bowlby’s experiment and wanted to further investigate the importance of the relationship between an
infant and mother during the first years of the infant’s life. While Harlow’s work was not entirely
original, the methodology Harlow utalised to conduct his experiment was different from Bowlby’s case
study. This is important as Harlow carried out a controlled lab experiment which, from a scientific
perspective, makes his investigation more valid. This was significant at the time because it provided
much stronger evidence towards the new knowledge gained around attachment, which later stemmed
many other investigations. Harlow’s research has also remained significant in psychology due to the
highly criticized way his experiment was run. He stripped infant monkeys away from their mothers and
forced them into isolation. Additionally, when some of the monkeys were placed back into their normal
environment, they endured lifelong trauma. This is significant because in the present, this would entirely
breach ethical guidelines, however this was not the case in the 1950’s. Consequently, now instead of
his work being celebrated for its findings, it is often scrutinised for its cruelty.
Harlow decided to create an experiment now commonly known as ‘The Monkey Love Experiment’,
which used infant Rhesus monkeys to test whether they are more likely to form a bond with a wire
surrogate mother that provides them with milk, or a comforting cloth mother that does not, if they had
access to both. He hypothesized that the infant monkeys would spend more time with the cloth mother
than with the wire mother which provided food. The results from this investigation proved Harlow’s
hypothesis as he found that only when they were hungry, they would go to the wire mother. In addition
to this, when a fearful stimulus was placed into the cage, the monkey would seek safety by clinging to
the cloth mother. Harlow was able to draw the conclusion that the monkeys benefitted significantly
more from the security that the cloth mother provided as opposed to the food that the wire one provided.
This means that attachment is not based on food, but rather in the sense of comfort and security, which
supports the evolutionary theory.
Harlow then later modified the experiment to further explore the importance of attachment in the critical
period of development in later life. He separated the infant monkeys into two groups, with the first
being given only the terrycloth mother which provided no milk, and the second with the wire mother
which did. The behaviours of the monkeys were observed when they were placed back into their normal
environment after 90 days of being with their surrogate mother. The monkeys who had the wire mother
were timid and did not know how to socialise when interacting with other monkeys. They indulged in
self-mutilation which meant that they were constantly bullied by their peers not knowing how to defend
themselves. Consequently, this led to difficulties when trying to mate, and the few females who
managed to reproduce, showed signs of inadequate mothering. Harlow therefore concluded that the
privation of forming an attachment bond within in the critical period, which he calculated to be the first
6 months of a human’s life, will permanently affect the behaviour of the individual.
One strength of this research is that it was carried out under laboratory conditions which improves the
control of reducing extraneous variables. Harlow controlled the age of the monkeys by separating them
, from their biological mothers immediately after birth and had all of the monkeys placed in identical
cages. This meant that in Harlow’s initial experiment, the monkeys had the same chances of choosing
between the mothers and, in second experiment, the differences between the behaviour of the monkeys
were only a result of which mother they were given. Another situational variable that was controlled
was that the monkeys were individually tested to reduce the disruptive noises of the other monkeys.
This could otherwise have unsettled them or caused them to behave differently which would invalidate
the correlation between the two surrogate mothers and the time the infants spent clung to them. Another
strength of this methodology is that Harlow could observe every behaviour of the infant monkeys,
which improves the accuracy of the results. While this could be a strength, it also could be a limitation
as the experimenters observing the monkeys may cause bias in the results. This is because they are
likely to have their own expectations about the outcome and may subconsciously only see the
observations tending towards that expected outcome. Nonetheless, the internal validity of this
experiment is still relatively high as within the population of the monkeys tested, the observed results
do support the claim that the monkeys benefit more from the cloth mother.
A situational variable that is a weakness to this experiment is that the infant monkeys had just been
moved and placed into the unfamiliar environment of the wire cage without any time to settle. This
could have influenced Harlow’s findings as the monkeys were probably scared and wanted to seek
comfort so, as a behavioural response, go towards the cloth mother. Additionally, the monkeys were
placed in an artificial setting and stripped from their natural environment which may have produced
unnatural behaviours, resulting in low ecological validity. This is because the monkeys are likely to act
scared and confused when placed in an unfamiliar environment, meaning that the results cannot be an
exact representation of what mother the infants are more likely to tend towards in the wild, making it
difficult to generalize the findings to a real-world setting. Therefore, this experiment has poor external
validity because we are unable to depict whether the results are because of the different mothers, or
because the monkeys had been forced into isolation.
The most significant issue with the way in which Harlow carried out his experiment is with the use of
cruel animal testing. He subjected the monkeys to weeks of inhumane treatment, by forcing them into
small cages, stripping them from any social interaction and placing large objects in their cages in
attempt to scare them. Not only this, but when they were released many of the monkeys experienced
permanent emotional damage, with some even so distressed that they died by starving themselves or
eating themselves alive. This is an issue that was overlooked for many years, but today would be
considered highly unacceptable and a breach to ethics. There are also issues regarding the findings that
Harlow discovered. Although Rhesus monkeys are the closest living biological relatives to humans,
the behaviour of the infant monkeys is not an exact representation of the behaviour of infant humans.
Therefore, it is difficult to generalise Harlow’s experiment on Rhesus monkeys to how maternal
deprivation would affect a human infant. Nonetheless, Harlow could not have conducted this research
on humans as this would breach the ethical guidelines at the time, so using Rhesus monkeys are the
closest living thing he could have used.
Influence on Society
Harlow’s experiments had a large influence on society and have caused many aspects to change since.
As Harlow’s experiments on monkeys demonstrated an immense amount of cruelty, a positive change
that has resulted from this issue is the heightened awareness of the treatment of animals used in
laboratories. Additionally, because his work was highly critised for the abuse the monkeys endured,
this contributed greatly to the creation of the ethics regulations we have today. When his studies were
performed, this was before the modern ethic guidelines were in place. However, as a result of his
studies, animal ethics were incorporated to the code of ethics worldwide in the late 20th century to
promote the humane and responsible treatment of the animals used for scientific purposes.
The research is still relevant in society today as it is still taught widely in psychology around the world.
Additionally, today we use Harlow’s findings in many manners. His researched provided significant
evidence that has helped social workers to understand the risk factors and consequences of child neglect
