Love in Infant Monkeys
Affection Tn infants was long thought to be generated by the
satisfactions of feeding. Studies of young rhesus monkeys now
indicate that love derives mainly from close bodily contact
T
he first love of the human infant
is for his mother. The tender intimacy
of this attachment is such
that it is sometimes regarded as a sacred
or mystical force, an instinct incapable
of analysis. No doubt such compunctions,
along with the obvious obstacles
in the way of objective study, have
hampered experimental observation of
the bonds between child and mother.
Though the data are thin, the theoretical
literature on the subject is rich. Psychologists,
sociologists and anthropologists
commonly hold that the infant's love
is learned through the association of the
mother's face, body and other physical
characteristics with the alleviation of internal
biological tensions, particularly
hunger and thirst. Traditional psychoanalysts
have tended to emphasize the
role of attaining and sucking at the
breast as the basis for affectional development.
Recently a number of child
psychiatrists have questioned such simple
explanations. Some argue that affectionate
handling in the act of nursing is
a variable of importance, whereas a few
workers suggest that the composite
activities of nursing, contact, clinging
and even seeing and hearing work together
to elicit the infant's love for his
mother.
Now it is difficult, if not impossible, to
use human infants as subjects for the
studies necessary to break through the
present speculative impasse. At birth the
infant is so immature that he has little
or no control over any motor system
other than that involved in sucking.
Furthermore, his physical maturation is
so slow that by the time he can achieve
precise, coordinated, measurable responses
of his head, hands, feet and
body, the nature and sequence of development
have been hopelessly confounded
and obscured. Clearly research into
68
by Harry F. Harlow
the infant-mother relationship has need
of a more suitable laboratory animal.
We believe we have found it in the infant
monkey. For the past several years
our group at the Primate Laboratory of
the University of Wisconsin has been
employing baby rhesus monkeys in a
study that we believe has begun to yield
significant insights into the origin of the
infant's love for his mother.
Baby monkeys are far better coordinated
at birth than human infants. Their
responses can be obsetved and evaluated
with confidence at an age of 10
days or even earlier. Though they mature
much more rapidly than their human
contemporaries, infants of both
species follow much the same general
pattern of development.
Our interest in infant-monkey love
grew out of a research program that
involved the separation of monkeys from
their mothers a few hours after birth.
Employing techniques developed by
Gertrude van Wagenen of Yale University,
we had been rearing infant monkeys
on the bottle with a mortality far less than
that among monkeys nursed by their
mothers. We were particularly careful
to provide the infant monkeys with a
folded gauze diaper on the floor of their
cages, in accord with Dr. van Wagenen's
observation that they would tend to
maintain intimate contact with such soft,
pliant surfaces, especially during nursing.
We were impressed by the deep personal
attachments that the monkeys
formed for these diaper pads, and by the
distress that they exhibited when the
pads were briefly removed once a day
for purposes of sanitation. The behavior
of the infant monkeys was reminiscent
of the human infant's attachment to its
blankets, pillows, rag dolls or cuddly
teddy bears.
These observations suggested the series
of experiments in which we have
sought to compare the importance of
nursing and all associated activities with
that of simple bodily contact in engendering
the infant monkey's attachment
to its mother. For this purpose we contrived
two surrogate mother monkeys.
One is a bare welded-wire cylindrical
form surmounted by a wooden head with
a crude face. In the other the welded
wire is cushioned by a sheathing of terry
cloth. We placed eight newborn monkeys
in individual cages, each with equal
access to a cloth and a wire mother [see
illustration on opposite pagel. Four of
the infants received their milk from one
mother and four from the other, the milk
being furnished in each case by a nursing
bottle, with its nipple protruding
from the mother's "breast."
The two mothers quickly proved to
be physiologically equivalent. The
monkeys in the two groups drank the
same amount of milk and gained weight
at the same rate. But the two mothers.
proved to be by no means psychologically
equivalent. Records made automatically
showed that both groups of infants
spent far more time climbing and clinging
on their cloth-covered mothers than
they did on their wire mothers. During
the infants' first 14 days of life the floors.
of the cages were warmed by an electric
heating pad, but most of the infants left
the pad as soon as they could climb on
the unheated cloth mother. Moreover, as
the monkeys grew older, they tended to
spend an increasing amount of time
clinging and cuddling on her pliant terrycloth
surface. Those that secured their
nourishment from the wire mother
showed no tendency to spend more time
on her than feeding required, contradicting
the idea that affection is a response
that is learned or derived in asso©
1959 SCIENTIFIC AMERICAN, INC
CLOTH AND WIRE MOTHER-SURROGATES were used to test
the preferences of infant monkeys_ The infants spent most of their
time clinging to the soft cloth "mother," (foreground) even when
nursing bottles were attached to the wire mother (background)_
69
© 1959 SCIENTIFIC AMERICAN, INC
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STRONG PREFERENCE FOR CLOTH MOTHER was shown by all infant monkeys.
Infants reared with access to both mothers f!"Om birth (top chart) spent far more time on
the cloth mother (colored curves) than on the wire mother (black curves) _ This was true
regardless of whether they had been fed on the cloth (solid lines) or on the wire mother
(broken lines) _ Infants that had known no mother during their first eight months (bottom
chart) soon came to prefer cloth mother, but spent less time on her than the other infants.
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122
RESULTS OF "FEAR TEST" (see photographs on opposite page) showed that infants
('onfronted by a strange object quickly learned to seek reassurance from the cloth mother
(colored curves) rather than from the wire mother (black curves) _ Again infants fed on
the wire mother (broken lines) behaved much like those fed on cloth mother (solid lines) .
70
ciation with the reduction of hunger or
thirst.
These results attest the importancepossibly
the overwhelming importanceof
bodily contact and the immediate
comfort it supplies in forming the infant's
attachment for its mother. All
our experience, in fact, indicates that
our cloth-covered mother surrogate is an
eminently satisfactory mother. She. is
available 24 hours a day to satisfy her
infant's overwhelming compulsion to
seek bodily contact; she possesses infinite
patience, never scolding her baby
or biting it in anger. In these respects we
regard her as superior to a living monkey
mother, though monkey fathers
would probably not endorse this opinion.
Of course this does not mean that
nursing has no psychological importance.
No act so effectively guarantees
intimate bodily contact between mother
and child. Fmthermore, the mother who
finds nursing a pleasant experience will
probably be temperamentally inclined
to give her infant plenty of handling and
fondling. The real-life attachment of the
infant to its mother is doubtless influenced
by subtle multiple variables, contributed
in part by the mother and in
part by the child. We make no claim to
having unraveled these in only two years
of investigation. But no matter what
evidence the future may disclose, our
first experiments have shown that contact
comfort is a decisive variable in this
relationship.
S
uch generalization is powerfully supt-.
ported by the results of the next
phase of our investigation. The time
that the infant monkeys spent cuddling
on their surrogate mothers was a strong
but perhaps not conclusive index of
emotional attachment. Would they also
seek the inanimate mother for comfort
and security when they were subjected
to emotional stress? With this question
in mind we exposed our monkey infants
to the stress of fear by presenting them
with strange objects, for example a
mechanical teddy bear which moved forward,
beating a drum. Whether the infants
had nursed from the wire or the
cloth mother, they overwhelmingly
sought succor from the cloth one; this'
differential in behavior was enhanced
with the passage of time and the accrual
of experience. Early in this series of experiments
the terrified infant might rush
blindly to the wire mother, but even if it
did so it would soon abandon her for the
cloth mother. The infant would cling to
its cloth mother, rubbing its body against
hers. Then, with its fears assuaged
through intimate contact with the moth©
1959 SCIENTIFIC AMERICAN, INC
FRIGHTENING OBJECTS such as a mechanical teddy bear caused
almost all infant monkeys to flee blindly to the cloth mother, as in
the top photograph. Once reassured by pressing and rubbing
against her. they would then look at the strange object (bottom) .
71
© 1959 SCIENTIFIC AMERICAN, INC
"OPEN FIELD TEST" involved placing a monkey in a room far
larger than its accustomed cage; unfamiliar objects added an addi.
tional disturbing element. If no mother was present, the infant
would typically huddle in a corner (left). The wire mother did
er, it would turn to look at the previously
terrifying bear without the slightest sign
of alarm. Indeed, the infant would sometimes
even leave the protection of the
mother and approach the object that a
few minutes before had reduced it to
abject terror.
The analogy with the behavior of human
infants requires no elaboration. We
found that the analogy extends even to
less obviously stressful situations. When
a child is taken to a strange place, he
usually remains composed and happy so
long as his mpther is nearby. If the mother
gets out of sight, however, the child
is often seized with fear and distress.
We developed the same response in our
infant monkeys when we exposed them
to a room that was far larger than the
cages to which they were accustomed.
In the room we had placed a number of
unfamiliar objects such as a small artificial
tree, a crumpled piece of paper, a
folded gauze diaper, a wooden block
and a doorknob [a similar experiment is
depicted in the illustrations on these two
pages]. If the cloth mother was in the
room, the infant would rush wildly to
her, climb upon her, rub against her and
cling to her tightly. As in the previous
experiment, its fear then sharply diminished
or vanished. The infant would
begin to climb over the mother's body
and to explore and manipulate her face.
Soon it would leave the mother to investigate
the new world, and the unfamiliar
objects would become playthings. In a
typical behavior sequence, the infant
might manipulate the tree, return to the
mother, crumple the wad of paper, bring
it to the mother, explore the block, ex-
72
plore the doorknob, play with the paper
and return to the mother. So long as the
mother provided a psychological "base
of operations" the infants were unafraid
and their behavior remained positive,
exploratory and playful.
If the cloth mother was absent, however,
the infants would rush across the
test room and throw themselves facedown
on the floor, clutching their heads
and bodies and screaming their distress.
Records kept by two independent observers-scoring
for such "fear indices" as
crying, crouching, rocking and thumband
toe-sucking-showed that the emotionality
scores of the infants nearly
tripled. But no quantitative measurement
can convey the contrast between
the positive, outgoing activities in the
presence of the cloth mother and the
stereotyped withdrawn and disturbed
behavior in the motherless situation.
The bare wire mother provided no
more reassurance in this "open field" test
than no mother at all. Control tests on
monkeys that from birth had known only
the wire mother revealed that even these
infants showed no affection for her and
obtained no comfort from her presence.
Indeed, this group of animals exhibited
the highest emotionality scores of all.
Typically they would run to some wall
or corner of the room, clasp their heads
and bodies and rock convulsively back
and forth. Such activities closely resemble
the autistic behavior seen frequently
among neglected children in
and out of institutions.
In a final comparison of the cloth and
wire mothers, we adapted an experiment
originally devised by Robert A. Butler
at the Primate Laboratory. Butler had
found that monkeys enclosed in a dimly
lighted box would press a lever to open
and reopen a window for hours on end
for no reward other than the chance to
look out. The rate of lever-pressing depended
on what the monkeys saw
through the opened window; the sight of
another monkey elicited far more activity
than that of a bowl of fruit or an empty
room [see "Curiosity in Monkeys," by
Robert A. Butler; SCIENTIFIC AMERICAN,
February, 1954]. We now know that this
"curiosity response" is innate. Threeday-old
monkeys, barely able to walk,
will crawl across the floor of the box to
reach a lever which briefly opens the
window; some press the lever hundreds
of times within a few hours.
When we tested our monkey infants
in the "Butler box," we found that those
reared with both cloth and wire mothers
showed as high a response to the cloth
mother as to another monkey, but displayed
no more interest in the wire
mother than in an empty room. In this
test, as in all the others, the monkeys fed
on the wire mother behaved the same
as those fed on the cloth mother. A control
group raised with no mothers at all
found the cloth mother no more interesting
than the wire mother and neither
as interesting as another monkey.
Thus all the objective tests we have
been able to devise agree in showing
that the infant monkey's relationship to
its surrogate mother is a full one. Comparison
with the behavior of infant monkeys
raised by their real mothers confirms
this view. Like our experimental
monkeys, these infants spend many
© 1959 SCIENTIFIC AMERICAN, INC
not alter this pattern of fearful behavior, but the cloth mother
provided quick reassurance. The infant would first cling to her
(center) and then set out to explore the room and play with the
objects (right}, returning [rom time to time for more reassurance.
hours a day clinging to their mothers,
and run to them for comfort or reassurance
when they are frightened. The
deep and abiding bond between mother
and child appears to be essentially the
same, whether the mother is real or a
cloth surrogate.
While bodily contact clearly plays the
prime role in developing infantile affection,
other types of stimulation presumably
supplement its effects. We have
therefore embarked on a search for these
other factors. The activity of a live monkey
mother, for example, provides her
infant with frequent motion stimulation.
In many human cultures mothers bind
their babies to them when they go about
their daily chores; in our own culture
parents know very well that rocking a
baby or walking with him somehow promotes
his psychological and physiological
well-being. Accordingly we compared
the responsiveness of infant monkeys
to two cloth mothers, one stationary
and one rocking. All of them preferred
the rocking mother, though the
degree of preference varied considerably
from day to day and from monkey to
monkev. An experiment with a rocking
crib and a stationary one gave similar
results. Motion does appear to enhance
affection, albeit far less significantly than
simple contact.
The act of clinging, in itself, also
seems to have a role in promoting psychological
and physiological well-being.
Even before we began our studies of affection,
we noticed that a newborn monkey
raised in a bare wire cage survived
with difficulty unless we provided it
with a cone to which it could cling. Recently
we have raised two groups of
monkeys, one with a padded crib instead
of a mother and the other with a cloth
mother as well as a crib. Infants in the
latter group actually spend more time
on the crib than on the mother, probably
because the steep incline of the mother's
cloth surface makes her a less satisfactory
sleeping platform. In the open-field
test, the infants raised with a crib but no
mother clem'lv derived some emotional
support from
'
the presence of the crib.
But those raised with both showed an
unequivocal preference for the mother
they could cling to, and they evidenced
the benefit of the superior emotional
succor they gained from her.
S
till other elements in the relationship
remain to be investigated systematically.
Common sense would suggest that
the warmth of the mother's body plays
its part in strengthening the infant's ties
to her. Our own observations have not
yet confirmed this hypothesis. Heating a
cloth mother does not seem to increase
her attractiveness to the infant monkey,
and infants readily abandon a heating
pad for an unheated mother surrogate.
However, our laboratory is kept comfortably
warm at all times; experiments in a
chilly environment might well yield
quite different results.
Visual stimulation may forge an additional
link. When they are about three
months old, the monkeys begin to observe
and manipulate the head, face and
eyes of their mother surrogates; human
infants show the same sort of delayed
responsiveness to visual stimuli. Such
stimuli are known to have marked effects
on the behavior of many young
animals. The Austrian zoologist Konrad
Lorenz has demonstrated a process
called "imprinting"; he has shown that
the young of some species of birds become
attached to the first moving object
they perceive, normally their mothers
[see "'Imprinting' in Animals," by
Eckhard H. Hess; SCIENTIFIC AMERICAN,
March, 1958]. It is also possible
that particular sounds and even odors
may play some role in the normal development
of responses or attention.
The depth and persistence of attachment
to the mother depend not only on
the kind of stimuli that the young animal
receives but also on when it receives
them. Experiments with ducks
show that imprinting is most effective
during a critical period soon after hatching;
beyond a certain age it cannot take
place at all. Clinical experience with
human beings indicates that people who
have been deprived of affection in infancy
may have difficulty forming affectional
ties in later life. From preliminary
experiments with our monkeys we have
found that their affectional responses develop,
or fail to develop, according to a
similar pattern.
Early in our investigation we had segregated
four infant monkeys as a general
control group, denying them physical
contact either with a mother surrogate
or with other monkeys. After about
eight months we placed them in cages
with access to both cloth and wire mothers.
At first they were afraid of both
surrogates, but within a few days they
began to respond in much the same way
as the other infants. Soon they were
73
© 1959 SCIENTIFIC AMERICAN, INC
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REARED WITH
CLOTH MOTHER
REARED WITH
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REARED WITH
NO MOTHER
SCORES IN OPEN FIELD TEST show that all infant monkeys familiar with the cloth
mother were much less disturbed when she was present (color) than when no mother was
present (white) ; scores under 2 indicate unfrightened behavior. Infants that had known
only the wire mother were greatly disturbed whether she was present (black) or not (white).
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REARED WITH BOTH MOTHERS REARED WITH NO MOTHER
"CURIOSITY TEST" SHOWED THAT monkeys reared with both mothers displayed as
much interest in the cloth mother (solid color) as in another monkey (hatched color) ;
the wire mother I black) was no more interesting than an empty chamber (white). Monkeys
reared with no mother found cloth and wire mother less interesting than another monkey.
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REARED WITH BOTH MOTHERS
FROM AGE 250 DAYS
EARLY "MOTHERING" produced a strong and unchanging preference for the cloth
mother (color) over the wire mother I black). Monkeys deprived of early mothering showed
Jess marked preferences before separation and no significant preference subsequently.
74
spending less than an hour a day with
the wire mother and eight to 10 hours
with the cloth mother. Significantly,
however, they spent little more than half
as much time with the cloth mother as
did infants raised with her from birth.
In the open-field test these "orphan"
monkeys derived far less reassurance
from the cloth mothers than did the other
infants. The deprivation of physical contact
during their first eight months had
plainly affected the capacity of these
infants to develop the full and normal
pattern of affection. We found a further
indication of the psychological damage
wrought by early lack of mothering
when we tested the degree to which infant
monkeys retained their attachments
to their mothers. Infants raised with a
cloth mother from birth and separated
from her at about five and a half months
showed little or no loss of responsiveness
even afteF 18 months of separation. In
some cases it seemed that absence had
made the heart grow fonder. The monkeys
that had known a mother surrogate
only after the age of eight months, however,
rapidly lost whatever responsiveness
they had acquired, The long period
of maternal deprivation had evidently
left them incapable of forming a lasting
affectional tie.
T
he effects of maternal separation and
deprivation in the human infant have
scarcely been investigated, in spite of
their implications concerning child-rearing
practices. The long period of infantmaternal
dependency in the monkey
provides a real opportunity for investigating
persisting disturbances produced
by inconsistent or punishing mother sur­
rogates.
Above and beyond demonstration of
the surprising importance of contact
comfort as a prime requisite in the formation
of an infant's love for its mother
-and the discovery of the unimportant
or nonexistent role of the breast and act
of nursing-our investigations have established
a secure experimental approach
to this realm of dramatic and
subtle emotional relationships. The further
exploitation of the broad field of
research that now opens up depends
merely upon the availability of infant
monkeys. We expect to extend our researches
by undertaking the study of the
mother's (and even the father's!) love
for the infant, using real monkey infants
or infant surrogates. Finally, with such
techniques established, there appears to
be no reason why we cannot at some
future time investigate the fundamental
neurophysiological and biochemical variables
underlying affection and love.
© 1959 SCIENTIFIC AMERICAN, INC
__ Kodak reports on:
blood in depth ... a way to stiffen oil . . • a nice material
to coat with photographic emulsion
The sanguinary details
Among the red cells, three of the principal types of white cells, left to right:
a lymphocyte, representing about 30% of the white cells; a neutrophil,
representing 60%; an eosinophil, representing about 2%.
Look, mal Practically no hydrogenation!
Into a liquid triglyceride such as
cottonseed oil put a small percentage
of monoglyceride made from
any saturated fat.
,
,
3. The oil becomes a stiff solid. The
crystalline matrix holds the oil for
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2. Heat to about 7SoC and cool.
This works ollly with exceptiollally
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out more, write Distillation
Products Industries, Rochester 3,
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Kodak Company).
•
•
I e • •
A·
Megaloblastic cells, characteristic
of the pattern of red cell
maturation in pernicious anemia.
Erythrophogocyte, seen in an uncommon
blood condition, where
white cells swallow red cells.
Here is one that has eaten three.
Flatness
The objective documentation
of hematological observations
through properly controlled
photomicrography is
treated with some depth in the
current issue of our periodical
Medical Radiography and Photography.
It contains some excellent
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including a series which
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than O.OOOO2-inch distant from the plane of the circle.
Though achievement of such flatness is not simple, that is none
of the user's business.
The user's business is to know the path of a man-made
object moving across the sky and to know it with the best'precision
that man can currently master for the task. The work
requires dimensional constancy of the material which bears
the image.
That material is glass, a substance known to the Phoenicians
and since improved. Our kind alters its linear dimensions by
0.00045 % for each degree Fahrenheit of temperature change.
Polyester, the most dimensionally stable of commercially
available plastics, is called "stabilized" when its temperature
coefficient is brought down to twice that. For the hardened
steel from which gage blocks are made, the linear expansion
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George Eastman quit his job as a bank clerk in 1881 when he found
he could coat emulsioll Oil glass and sell if. That's how our whole business
started. DOII't think of the photographic plate as all antique, however.
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© 1959 SCIENTIFIC AMERICAN, INC