an epigenetic perspective on the development of self
TRANSCRIPT
An Epigenetic Perspective on the Development of Self-Produced Locomotion and ItsConsequencesAuthor(s): Bennett I. Bertenthal, Joseph J. Campos, Rosanne KermoianSource: Current Directions in Psychological Science, Vol. 3, No. 5 (Oct., 1994), pp. 140-145Published by: Blackwell Publishing on behalf of Association for Psychological ScienceStable URL: http://www.jstor.org/stable/20182292Accessed: 24/11/2009 12:12
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140 VOLUME 3, NUMBER 5, OCTOBER 1994
events. Pilot data collected in our
laboratory suggest that young infants
expect a moving object to stop when
it encounters a tall, thin box but not
a short, wide box, even when the
latter is considerably larger in vol
ume than the former. We suspect that infants are led by the dominant
vertical axis of the tall box to per ceive it as a wall-like, immovable
object, and hence categorize the
event as an instance of a barrier phe nomenon; in contrast, infants tend
to view the wide box as a movable
object, and hence categorize the
event as an instance of a collision
phenomenon, resulting in incorrect
predictions. The foregoing discussion high
lighted several types of developmen tal sequences that would be antici
pated in an innate-mechanisms view
but not (without considerable elab
oration) in an innate-principles view. To gain further insight into the
nature and origins of these develop mental sequences, we have adopted a dual research strategy. First, we
are examining the development of
infants' understanding of additional
physical phenomena (e.g., gap,
containment, and occlusion phe nomena) to determine how easily these developments can be captured
in terms of the patterns described in
the model and to compare more
closely the acquisition time lines of
phenomena that are superficially distinct but deeply related. Second, as was alluded to earlier, we are at
tempting to teach infants initial con
cepts and variables to uncover what
kinds of observations, and how
many observations, are required for
learning. We hope that the pursuit of
these two strategies will eventually allow us to specify the nature of the
learning mechanisms that infants
bring to the task of learning about
the physical world.
Acknowledgments?This research was
supported by grants from the Guggenheim Foundation, the University of Illinois Cen
ter for Advanced Study, and the National
Institute of Child Health and Human De
velopment (HD-21104). I would like to
thank Jerry Dejong, for his support and
insight, and Susan Carey, Noam Chom
sky, Judy DeLoache, Cindy Fischer, John
Flavell, Laura Kotovsky, Brian Ross, and
Bob Wyer, for many helpful comments
and suggestions.
Notes
1. J. Piaget, The Construction of Reality in the Child (Basic Books, New York, 1954).
2. E.S. Spelke, Preferential looking methods as tools for the study of cognition in infancy, in Mea surement of Audition and Vision in the First Year of
Postnatal Life, G. Gottlieb and N. Krasnegor, Eds.
(Ablex, Norwood, Nj, 1985). 3. R. Baillargeon, The object concept revisited:
New directions in the investigation of infants' phys ical knowledge, in Visual Perception and Cognition in Infancy, CE. Granrud, Ed. (Erlbaum, Hillsdale,
NJ, 1993). 4. E.S. Spelke, K. Breinlinger, J. Macomber, and
K. Jacobson, Origins of knowledge, Psychological Review, 99, 605-632 (1992).
5. R. Baillargeon, L. Kotovsky, and A. Need
ham, The acquisition of physical knowledge in in
fancy, in Causal Understandings in Cognition and
Culture, G. Lewis, D. Premack, and D. Sperber, Eds. (Oxford University Press, Oxford, in press).
6. R. Baillargeon, A model of physical reasoning in infancy, in Advances in Infancy Research, Vol. 9,
C. Rovee-Collier and L. Lipsitt, Eds. (Ablex, Nor
wood, NJ, in press). 7. R. Baillargeon, Physical reasoning in infants,
in The Cognitive Neurosciences, M.S. Gazzaniga, Ed. (MIT Press, Cambridge, MA, in press).
8. E.S. Spelke, Physical knowledge in infancy: Reflections on Piaget's theory, in The Epig?nesis of
Mind: Essays on Biology and Cognition, S. Carey and R. Gelman, Eds. (Erlbaum, Hillsdale, NJ, 1991).
9. A.M. Leslie, ToMM, ToBy, and Agency: Core architecture and domain specificity, in Causal Un
derstandings in Cognition and Culture, G. Lewis, D.
Premack, and D. Sperber, Eds. (Oxford University Press, Oxford, in press).
10. P. Rochat and A. Bullinger, Posture and functional action in infancy, in Francophone Per
spectives on Structure and Process in Mental Devel
opment, A. Vyt, H. Bloch, and M. Bornstein, Eds.
(Erlbaum, Hillsdale, NJ, in press). 11. K.D. Forbus, Qualitative process theory, Ar
tificial Intelligence, 24, 85-168 (1984). 12. This example focused exclusively on the
size of the cylinder, but what of the distance trav eled by the bug in each event? It seems likely that infants encode this information not in quantitative terms (e.g., "the bug traveled x as opposed to y distance"), but rather in qualitative terms, using as their point of reference the track itself (e.g., "the bug rolled to the middle of the track"), their own spatial position (e.g., "the bug stopped in front of me"), or the brightly decorated back wall of the apparatus (e.g., "the bug stopped in front of such-and-such section of the back wall").
An Epigenetic Perspective on the
Development of Self-Produced Locomotion and Its Consequences Bennett I. Bertenthal, Joseph J. Campos, and Rosanne Kermoian
One of the most striking charac
teristics of early human develop ment is its consistency and stability.
Infants show considerable unifor
mity in the nature and timing of new
behaviors. A principal contributor to
this early uniformity is the develop
ment of species-typical behaviors, such as vocalization, locomotion, and reaching. These behaviors ensure a common set of experiences
with far-reaching consequences for
development. In this article, we
review a specific example of this
epigenetic process involving the
development of self-produced locomotion.
EXTERNAL VERSUS SELF-PRODUCED FORMS
OF EXPERIENCE
The importance of self-produced experiences is often overlooked by researchers, and, indeed, most stud ies investigating early development focus on the effects of stimulation
from the environment. A paradig matic example is the study of infants'
Published by Cambridge University Press
CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 141
responses to the social stimulation of
their caretakers. The infant is viewed
as a passive recipient of environ
mental stimulation, and responses are typically assumed to be contin
gent on the actions of the caretaker.
The alternative view is that infants
are active participants in learning about self and environment, and
they provide through their own ac
tions at least some of the experi ences necessary for further growth and development. In contrast to ex
ternally produced forms of stimula
tion, these new experiences are
available to all infants regardless of
their rearing environments.
Our own research focuses on
how the onset of crawling, the emer
gence of independent mobility, in
fluences subsequent development. In most infants, crawling emerges
between 6 and 9 months of age, and
coincides with numerous changes in
sensorimotor intelligence, including new ways of coding spatial rela
tions, new concepts about objects, new forms of social communication, a burgeoning of fear, and the further
differentiation of other emotions.1 Is
it possible that experiences provided
by the emergence of crawling are
functionally related to some of these
other major developmental changes
occurring at the same time? During the past decade, research conducted
in our respective labs has shed some
light on the answer to this provoca tive question.
Bennett I. Bertenthal is Professor of Psychology at the University of
Virginia. Joseph J, Campos is Pro fessor of Psychology and Director of the Institute of Human Develop ment at the University of California at Berkeley. Rosanne Kermoian is Assistant Research Psychologist at
the University of California at
Berkeley. Address correspondence to Bennett I. Bertenthal, Depart
ment of Psychology, Gilmer Hall, University of Virginia, Charlottes
ville, VA 22903-2477.
Fig. 1. Photograph of infant crawling on the visual cliff.
FEAR OF HEIGHTS
During the third quarter of the 1 st
year, infants show a dramatic in
crease in the intensity and probabil
ity with which they express fear.
These changes are so abrupt and so
adaptive for survival that it is often
assumed that neuromaturational fac
tors are the principal cause of this
developmental shift. An especially
compelling case is made for fear of
heights because it is such a biologi
cally significant behavior. Although we do not dispute a contribution by neuromaturation, our research sug
gests that the development of fear of
heights is considerably more com
plex, and is based on the interplay between neuromaturation and other
factors, especially self-produced lo
comotor experience.
Wariness of heights is often stud
ied using a "visual cliff." Figure 1
shows a picture of this apparatus, which consists of a large sheet of
glass (8x4 ft) suspended almost 4 ft
above the floor. A narrow board is
placed across the middle, dividing the sheet into two sides. On one side
(referred to as the shallow side), a
textured checkerboard pattern is
placed directly under the glass, so
that it appears as a rigid and support able surface. On the other side (re ferred to as the deep side), the tex
tured checkerboard is placed 4 ft
below the glass, so that this side ap
pears as a cliff, or as an apparent
drop-off. In most studies, infants are
placed on the centerboard and en
couraged to cross to the mother, who stands alternately across the
deep or shallow sides of the cliff.
Our early observations testing in
fants on the visual cliff revealed that,
contrary to the predictions of other
researchers, they did not avoid the
deep side (i.e., the apparent drop off) immediately following the onset
of crawling. Instead, it was generally 6 to 8 weeks following the onset of
crawling that avoidance was first ob served. This observation was subse
quently replicated and extended in a
series of experiments.2 In one study, crawling and pre
crawling infants were tested on the
visual cliff at the same age (7.3
months). Infants were held 3 ft
above the glass surface of the cliff by an experimenter, and slowly low
ered onto the surface. During this
descent, their heart rate was mea
sured and compared with their heart
Copyright ? 1994 American Psychological Society
142 VOLUME 3, NUMBER 5, OCTOBER 1994
rate during a baseline period. In gen- I
eral, heart rate decelerates in states
of orienting and attentiveness, and
accelerates in states of defensiveness or fearfulness.3 Precrawling infants
showed no significant cardiac
changes as they were lowered onto
either side of the visual cliff (see Fig. 2). By contrast, crawling infants
showed significant cardiac accelera
tion when lowered onto the deep
side, and no cardiac change when
lowered onto the shallow side.
Although these results suggested that crawling experience affected
fear of heights, they were by no
means definitive. Logically, crawl
ing experience could covary with
any number of other developmental variables that might contribute to the
development of fear of heights. The
challenge of subsequent research
was to show that the relation be
tween crawling experience and vi
sual-cliff performance was causal
and not merely correlative. To do
so entailed a series of converging
experiments. The most convincing experiment
was one in which locomotor experi ence was manipulated in a quasi
experimental manner. A group of
precrawling infants were given 40 hr
of locomotor experience in their
homes by their caregivers, who
placed them in infant walkers for
some period of time each day. These
walkers enable infants to locomote
by supporting them in a small seat
that is attached to a frame on
wheels. Infants with walker experi ence were tested with the descent
paradigm on the visual cliff. An age matched control group of precrawl
ing infants without walker experi ence was also tested on the visual
cliff. The results provided firm sup
port for the conclusion that experi ences with self-produced locomo
tion contribute to the development of fear of heights. Specifically, the
group of precrawling infants with
walker experience showed heart rate
acceleration when lowered onto the
deep side of the visual cliff; the age matched control group showed only a slight deceleratory shift.
Another study confirmed that the
relation between locomotor experi ence and fear of heights was not
task-specific. In this experiment, wariness of heights was tested by en
couraging infants to locomote across
the deep and shallow sides of the
cliff. To assess independently the ef
fects of age of onset of crawling and
crawling experience, infants who
began to crawl at 6, 7, or 8 months
of age were tested after 11 or 41
days of locomotor experience. The
probability of an infant crossing the
cliff, as well as the latency (or
elapsed time) to begin crossing, were significantly related to crawl
ing experience, but not to the age of
onset of crawling (see Fig. 3). Taken together, these results offer
compelling evidence that crawling
experience contributes significantly to the development of fear of
heights. The explanation for this re
lation is still not completely under
stood, but we do know that falling
experiences alone are not sufficient
to account for this developmental shift on the visual cliff. A more plau sible interpretation for this shift is
that active control of locomotion, unlike passive locomotion, demands
continuous updating of one's orien
tation relative to the spatial layout. This information is provided through multimodal sources, such as visual and vestibular coding of angular ac
celeration. With locomotor experi ence, changes in angular accelera tion detected by the visual system are mapped onto analogous changes detected by the vestibular system. Fear or avoidance ensues when the
expected mapping between visual
and vestibular information is vio
lated. This violation occurs when in
fants are placed on the deep side of
the visual cliff, because angular ac
celeration on the retina is scaled to
the distance of the nearest visible
texture, whereas no such scaling oc
curs for the vestibular system.4 As a
consequence, visual and vestibular
specification of self-motion become
discrepant, and produce an aver
sive, vertiginous response by infants,
CL SI LU (D
z <
o
< ce I rr
< LU
X
3 H
O? loe deep
#? loe shallow
-B? preloc deep - ?
preloc shallow
-i i i i i r
0.5 1.5 2 2.5
SECONDS OF DESCENT
3.5
Fig. 2. Mean heart rate changes in 7.3-month-old infants as a function of locomotor
experience. Data are plotted to show second-by-second heart rate changes during
descent onto deep and shallow sides of the visual cliff. Heart rate change is calculated
using beats per minute (BPM). Loc = crawling infants; preloc
= precrawling infants.
Published by Cambridge University Press
CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 143
I LU
O z LU ?C LU
5
I Z < LU
2
LU ?_
100
80 i
? 60 1
40 i
20
-#? 41 days loe. exp.
-0? 11 days loe. exp.
6.5 7 7.5 8 8.5 9
AGE AT TESTING (MONTHS)
9.5 10
100
LU
i ?o ?. LU LU ? Z Q O
60 H
40
20
- 41 days loe. exp.
-0? 11 days loe. exp.
6.5 7.5 8.5
AGE AT TESTING (MONTHS)
9.5
Fig. 3. Results from a visual-cliff experiment testing infants who began to crawl at 6, 7, or 8 months of age. The top panel shows the mean latency differences between moving onto the deep and shallow sides of the visual cliff as a function of both locomotor
experience (loc. exp.) and age at testing. The bottom panel shows the percentage of
infants not crossing the deep side of the cliff as a function of locomotor experience and
age at testing.
not unlike what happens to adults
when looking down from a very high
skyscraper.5
SPATIAL SEARCH
The search for hidden objects is
another skill that is linked to crawl
ing experience. It is fairly well estab
lished that young infants are rarely successful in searching for a hidden
object following a displacement of
themselves or the object. Perhaps the best known example of this def
icit is the A-not-B error shown by infants on Piaget's object-perma
nence test. In this test, infants con
tinue to search in a previously
successful location even after they observe the toy hidden somewhere
else. This situation changes around
8 to 9 months of age, when infants
begin to show correct search for a
displaced object. The temporal cor
respondence between this develop mental shift in search behavior and
the onset of crawling led a number
of investigators to suggest a causal
connection between crawling expe rience and the development of new
search skills.
Two recent experiments support this predicted relation. The first
study tested hands-and-knees
crawling infants, precrawling in
fants, and precrawling infants with
walker experience (mean age = 8.5
months) on a series of hiding tasks
corresponding to Piaget's object permanence scale.6 Hands-and
knees-crawling and walker-assisted
precrawling infants passed more
items on this scale than did pre
crawling infants. Infants with the
most crawling experience (9 weeks)
passed tasks that involved searching for objects hidden in a new, spatially discriminable location, whereas pre
crawling infants were not able to
pass even a task involving a single
hiding location if a second (unused)
hiding location was present. The second study involved a dif
ferent task, but the results were sim
ilar.7 Hands-and-knees-crawling,
belly-crawling, and precrawling in
fants (mean age = 7.5 months) were
tested on trials in which a toy was
hidden in one of two differently col
ored containers placed in front of the
infant. After the hiding was com
pleted, either the infant or the table was rotated 180? so that the correct
left-versus-right location of the hid
den toy was reversed. The results re
vealed that crawling experience was
systematically related to search per formance when the infant was
moved, but not when the table was
rotated (see Table 1). When the in
fant was rotated, hands-and-knees
crawling infants searched the correct
container more often than predicted
Copyright ? 1994 American Psychological Society
144 VOLUME 3, NUMBER 5, OCTOBER 1994
Table 1. Number of infants showing correct and incorrect search for hidden
toy on first trial
Search
Infant group Correct Incorrect
Infant-displacement condition
Precrawling 5 15
Belly crawling 3 7 Hands-and-knees crawling 13 5
Table-displacement condition
Precrawling 11 9
Belly crawling 3 7 Hands-and-knees crawling 10 8
by chance, precrawling infants
searched the incorrect container
more often than predicted by
chance, and belly-crawling infants
showed a mixed response. How does crawling experience
contribute to infants' searches for
hidden objects? Prior to the onset of
crawling, infants remain in one lo
cation for extended periods of time.
They are thus fairly successful in
coding the location of an object with a body-centered frame of reference.
Following the onset of crawling, in
fants are moving much more often, and it becomes inefficient for them
to code the location of an object us
ing a body-centered frame of refer
ence. During the initial stages of this
transition, infants are likely to show
much greater variability in perfor mance, which is exactly what we
observed with belly-crawling in
fants. From our perspective, it is the
greater variability in performance that occurs during this time that
drives the system to a new level of
organization. The newly emergent search strategy no longer relies on a
body-centered frame of reference, but instead uses landmarks or some
other strategy, such as visual track
ing, for continually updating the lo
cation of an object. It is noteworthy that search per
formance was related to locomotor
experience only in the infant
rotation condition. This condition,
compared with the table-rotation
condition, more closely approxi
mates the experience of infants fol
lowing the onset of crawling. Infants
learn from this experience that they must update their spatial code for
the location of an object following a
self-displacement. Apparently, the new search response that emerges
with crawling experience does not
generalize immediately to other
conditions in which this response would also improve performance.
THE ORGANIZATION OF CRAWLING BEHAVIOR
A complementary theme high
lighted by the preceding research is
that all forms of locomotor experi ence are not equivalent, and that the
transition from belly crawling to
hands-and-knees crawling is espe
cially important. This developmen tal transition shares much in com
mon with the other shifts reviewed.
Indeed, the development of hands
and-knees crawling offers one of the
clearest examples of how the selec
tion of new behaviors by infants rep resents an emergent process based
on the confluence of multiple organ ismic and environmental factors.
In a recently completed study, we
assessed longitudinally the develop ment of crawling in six infants.8 Ki
nematic analyses of interlimb coor
dination revealed that the most
stable pattern of movement corre
sponds to a diagonal coupling of the
limbs, in which diagonally opposite limbs, such as the right arm and left
leg, move simultaneously, and 180? out of phase with the other pair of
limbs. The development of this diag onal pattern is an emergent process fueled by the initial experience of
moving on hands and knees. Limb movements corresponding to a diag onal pattern are rarely observed
prior to the time when the infant de
velops sufficient strength to support the torso off the ground. By contrast, a diagonal pattern is observed a little more than 50% of the time just 1 to
2 weeks after the torso is supported off the ground (see Fig. 4).
Our interpretation for this finding is that hands-and-knees crawling re
quires that the torso remain sup
ported and balanced during forward
progression; these requirements are
irrelevant during the preceding stage of belly crawling and creeping. Once infants develop sufficient
strength in their arms and legs to
support the torso, they begin to ex
plore the various interlimb patterns available for movement, such as
moving only one limb at a time or
moving both limbs on the same side
of the body at the same time. Fol
lowing a relatively brief opportunity to explore the various interlimb pat terns available to them, infants con
verge on the same diagonal pattern because it provides greater effi
Fig. 4. Mean percentage of time that limbs are diagonally coupled during
crawling. Data are plotted from 2 weeks
prior (-2) to the onset of hands-and
knees crawling through 6 weeks follow
ing ( + 6) the onset of hands-and-knees
crawling.
Published by Cambridge University Press
CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 145
ciency and stability than any of the
alternatives.
Two points should be empha sized about this developmental shift.
First, the transition to hands-and
knees crawling begins with a period
during which success in producing forward prone progression is quite variable. This experience informs
the infant that the previous interlimb
organization is no longer adequate for the new task. Gradually, the in
fant converges on a new form of in
terlimb patterning representing the
most dynamically stable organiza tion given the task at hand. Second, these developmental changes are
neither prescriptive nor obligatory, but rather represent the natural out
come of a system governed by trying to achieve the most dynamically ef
ficient solution. It is noteworthy that
this sequence of increased variabil
ity followed by greater stability and
greater generalizability is a common
theme that cuts across many differ
ent developmental theories.9
GENERALIZATIONS ABOUT DEVELOPMENT
At a specific level, the findings from this research program provide
compelling evidence that locomotor
experience is functionally related to
the development of a number of new
behavioral forms. At a more general level, these findings lend support to
three important generalizations about the developmental process.
First and foremost, our research
program shows that some of the most significant early experiences are those produced by the infant's own actions. This finding represents a radical departure from the tradi
tional perspective of viewing these
actions as merely products and not
processes of development. More
over, it confirms our contention that
some of the relevant factors contrib
uting to the uniformity of early be
havioral development are indeed ex
periential. An important goal for
future research is to assess the gen
eralizability of our finding to deter
mine whether other forms of self
generated experiences, such as
those produced by sitting or reach
ing, also contribute to the develop mental process.
Second, our findings underscore
the limitations invoked by assuming a linear model of development. Such a model makes the simplistic
assumption that performance con
tinues to improve at a constant rate
as experience increases. Logically, this prediction is problematic be cause most behaviors reach an as
ymptotic level of performance and
then show no further improvement. Moreover, the results from many studies are not consistent with the
expectation that change occurs at a
constant rate. For instance, the de
velopment of a diagonal gait pattern
following the onset of hands-and
knees crawling corresponds to an
abrupt nonlinear change rather than a monotonically increasing func
tion. Our experience is that the ap
plication of linear models, such as
linear regression, to the study of the
effects of experience sometimes
produces misleading or incorrect
results.10
Finally, this research underscores
the importance of conceptualizing
development from a systems per
spective. Developmental change is
rarely attributable to a single cause.
For example, self-produced locomo tor experience contributes to search
for hidden objects, but so do im
proved trunk control, ability to se
quence actions, and increased vi
suai attentiveness.6 It is essential that
investigators conceptualize early de
velopment as responsive to multiple factors that interrelate and subsume
organism-environment coactions.
From this perspective, behavioral
development is multidetermined, re
lational, and emergent.
Acknowledgments?The research de
scribed in this review was supported by National Institutes of Health Grants
HD16195 and HD23144, and also by a grant from the John D. and Catherine T.
MacArthur Foundation. We thank Jean nine Pinto for her helpful comments.
Notes
1. A comprehensive review of these develop mental changes is presented in B.I. Bertenthal, J.J. Campos, and K.C. Barrett, Self-produced locomo tion: An organizer of emotional, cognitive, and so cial development in infancy, in Continuities and Discontinuities in Development, R.N. Emde and R.J. Harmon, Eds. (Plenum, New York, 1984).
2. J.J. Campos, B.I. Bertenthal, and R. Ker
moian, Early experience and emotional develop ment: The emergence of wariness of heights, Psy chological Science, 3, 61-64 (1992).
3. J.J. Campos, Heart rate: A sensitive tool for the study of emotional development, in Develop
mental Psychobiology: The Significance of Infancy, L.P. Lipsitt, Ed. (Erlbaum, Hillsdale, NJ, 1976); L.A. Sroufe and E. Waters, Heart rate as a convergent measure in clinical and developmental research, Merrill-Palmer Quarterly, 23, 3-27 (1977).
4. T. Brandt, W. Bles, F. Arnold, and T.S.
Kapteyn, Height vertigo and human posture, Ad vances in Oto-Rhino-Otolaryngology, 89, 88-92 (1979).
5. For more details, see B.I. Bertenthal and J.J. Campos, A systems approach to the organizing ef fects of self-produced locomotion during infancy, in
Advances in Infancy Research, Vol. 6, C. Rovee Collier and L. Lipsitt, Eds. (Ablex, Norwood, NJ, 1990).
6. R. Kermoian and J.J. Campos, Locomotor ex
perience: A facilitator of spatial cognitive develop ment, Child Development, 59, 908-917 (1988).
7. D.L. Bai and B.I. Bertenthal, Locomotor sta tus and the development of spatial search skills,
Child Development, 63, 215-226 (1992). 8. R.L. Freedland and B.I. Bertenthal, Develop
mental changes in interlimb coordination: Transi tion to hands-and-knees crawling, Psychological Science, 5, 26-32 (1994).
9. G. Edelman, Neural Darwinism (Basic Books, New York, 1987); E. Thelen, Evolving and
dissolving synergies in the development of leg co
ordination, in Perspectives on the Coordination of Movement, S. Wallace, Ed. (Elsevier, Amsterdam,
1989). 10. B.I. Bertenthal and J.J. Campos, A reexam
inaron of fear and its determinants on the visual
cliff, Psychophysiology,21, 413-417 (1984).
Copyright ? 1994 American Psychological Society