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Models of primate development. Review of the behavioral development of free-living chimpanzee babies and infants by Frans X. Plooij. Norwood NJ Ablex Publishing Corporation 1985 207 pp $27

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American Journal of Primatology 13:209-211(1987)
Models of Primate Development
Review of The Behavioral Development of Free-living Chimpanzee Babies and infants, by
Frans X. Plooij. Norwood, NJ, Ablex Publishing Corporation, 1985, 207 pp, $27.50.
In his interesting and thought-provoking book, Plooij traces the development of
wild chimpanzees at the Gombe Stream Reserve from the neonatal period through
the 1st year, following the ontogeny of some behavioral aspects up to 20 months of
age. He then compares chimpanzee development with human development and
analyzes the sequence he has described in terms of Powers’ [1973]hypothetical
framework of neurological functioning.
This book is best read along with Powers’ book, Behavior: the Control of Percep
tion. Before reading Powers, I found it difficult to completely understand the model
Plooij is using. For this reason, perhaps a brief discussion of Powers’ system and
how it compares with Piaget’s paradigm-which is probably the best known developmental model-is in order.
Powers proposes that there are nine-plus orders of behavioral regulation organized into a hierarchy. Behavioral regulation is thought to be the result of the
control of perception, as an organism strives to maintain certain set values. These
set values belong to different levels in the hierarchy, controlled by a series of
negative-feedback control systems in which higher-order systems control a number
of lower systems. Powers implies that an organism begins to exert some volition as
the higher orders gain control. He proposes that the highest order, perhaps the
tenth, may be motivational and probably incorporates the “instincts.”
Having worked with Piaget’s developmental framework for a number of years,
I found Plooij’s study especially fascinating. I began wondering how Powers’ framework compares with Piaget’s and how the two paradigms compare in terms of
rationally explaining primate behavioral development.
As I tried to compare the two, it became evident that Powers’ model is based on
the neurological substrates and then moves “ o u ~ ”to include behavior. His model is
heavily based on neurophysiological theory, physics, and computer theory. Piaget’s
model is based on overt behavior and then moves ‘‘in” to focus on the cognitive
level. He hypothesizes on what thinking processes are involved in determining
behavior, based on behavioral observations.
Another notable difference is that Powers has been strongly influenced by
Skinner and the Behaviorist School. The lowest orders in his model are mechanistic.
Only at the highest level or levels is internal motivation or volition incorporated
into the model. In contrast, Piaget’s model incorporates internal motivational determinants from the second stage on.
A third major difference is that Powers’ model is heirarchical but is not necessarily ontogenetic, whereas Piaget’s paradigm is both ontogenetic and heirarchical.
Therefore, Plooij is testing the applicability of Powers’ model to ontogenetic
0 1987 Alan R. Liss, Inc.
210 I Chevalier-Skolnikoff
If the behaviors considered characteristic of Powers’ orders are compared with
those of Piaget’s stages, or if the parameters of these behaviors are compared, certain
similarities can be seen between the two, although they do not correspond precisely.
Likewise, there is some ontogenetic relationship between the two series in terms of
the ages at which primate youngsters show evidence of a particular order or stage.
Powers’ first two orders appear to correspond to Piaget’s first stage, his third
order to Piaget’s second stage, and his fourth order to Piaget’s third stage. However,
Powers’ fifth order corresponds to Piaget’s fourth and fifth stages, his sixth order
corresponds to Piaget’s third, fourth and fifth stages and to levels beyond the
sensorimotor period, and his seventh order corresponds to Piaget’s fifth and sixth
stages and beyond the sensorimotor period.
Powers’ first two orders are especially interesting. The lowest first-order input
function is defined as a set of sensory receptors that always respond at the same
time, only encode intensity, and do not process information identifying the kind of
stimulation. Consequently, all stimuli processed by first-order receptors are qualitatively alike, and there is no distinction between proprioceptive and exteroceptive
stimuli. His second-orderperceptual level functions t o receive first-order stimuli and
to combine them into a single second-orderperception that represents the quality of
a sensation. It is not always possible, however, to classify second-order sensations as
taste, smell, temperature, etc. Since the second-order perceptual level functions
through summation, the sum of several different combinations of stimuli received
may be the same, even if the specific stimuli are very different.
Plooij shows how Powers’ first two orders may explain the functioning of chimpanzee and human neonatal behavior, such as clinging, “mountaineering,” whimpering, and staccato grunting.
As Sackett suggests in his foreword to Plooij’s book, this model also may explain
recent controversial findings on neonates that seem to defy other paradigms such as
Piaget’s. For instance, Sackett points out that human newborns show differential
looking responses directed at three-dimensional visual stimuli based on the correspondence between the shape of the visual stimuli and that of tactile stimulation
from a pacifier. Some researchers have suggested that this behavior occurs through
intermodal matching, a behavioral process that would be inconsistent with firststage functioning according to Piaget’s model, and with other research findings on
the capacities of human neonates. According to Powers’ model, however, different
visual stimuli elicit different stimulus intensities that are matched by intensities
evoked by the tactile stimuli. The behavior is thus determined by intensity equivalence rather than by intermodal matching. Similarly, Powers’ model may explain
Condon and Sander’s [1974] observations that neonatal body movement occurs in
synchrony with adult speech. This model also may explain the common observation
that nursing primate neonates are relatively unresponsive to aversive stimuli.
Plooij describes in some detail the behavioral transitions as chimpanzee infants
presumably reach the third and fourth orders of control between 2 and 6 months.
His comparisons between chimpanzees and humans show striking similarities, for
he finds that many of the developmantal landmarks are almost the same. For
instance, the disappearance of rooting, the emergence of social expressions such as
playfaces and smiles, and the onset of fear of strangers occurred at about the same
time in both species. However, he found that while chimpanzees begin to sit unaided
by 8 weeks and to walk quadrupedally at 5 months, studies on human children from
Western cultures do not report sitting until 5 months or creeping until 7 months.
However, Plooij points out that human motor development is more precocious in
some other cultures, again suggesting strong developmental similarities between
apes and humans.
Models of Primate Development / 211
Plooij then discusses chimpanzee development from 6 months to 1 year. He
describes the development of infants’ excursions away from their mothers, object
play, social gestures, and sociosexual behavior. His discussion relating the development of excursions and of conflict situations and the ethological concept of “displacement activities” to Powers’ model is interesting. However, unlike their treatment of
the first orders, as Powers and Plooij progress through the higher orders-the fifth
order on, they become more general. Powers moves away from the brain, and his
model becomes more theoretical. Plooij’s treatment also becomes more vague and
sometimes confusing. For instance, he ascribes a new level of functioning at 7-9
months to the emergence of Powers’ sixth order, the control of relationships. However, since he notes that control of relationships begins during Piaget’s third and
fourth stages (at about 4 and 6 months in chimpanzees) this discussion is confusing.
One reason for this confusion may be that, as described above, Powers’ model does
not neatly fit the ontogenetic sequence. Some orders emerge gradually over long
developmental periods; order 6 , for instance, emerges throughout all but the first 4
months of development. But Pfooij fails t o point this out.
Even though Powers’ model does not unfold in a neat ontogenetic sequence as
Piaget’s does, this does not necessarily mean that Powers’ model is wrong and that
Piaget’s is correct. It may be that development does not really unfold in a linear
sequence as Piaget has proposed and as his behaviorally based paradigm implies.
On the other hand, Powers admits that behavior is not one of his fields of expertise.
Perhaps he would modify his model if he had a better understanding of the ontogeny
of behavior. I also wondered whether the two models were mutually exclusive or
whether they could be at least partially combined. Logically, a neurological model
should not be incompatible with a behavioral model.
Plooij does not deal with orders 8 and beyond, the orders that Powers proposes
function as motivating forces. Consequently, I have the impression that his view of
chimpanzee and human development is a mechanistic one and does not incorporate
internal motivation.
It is unfortunate that Plooij does not give the ages or ranges of the first
occurrence of specific behaviors. This makes it difficult to compare either his data or
his interpretations with other studies. I noticed too that he discusses, compares, and
integrates the human literature much more thoroughly than the nonhuman primate
literature. The book also is difficult to read because he uses abbreviations for the
various behavioral categories. While this may have been useful for data collection,
with 202 abbreviations to remember readers are unnecessarily forced to search out
their meanings in the glossary.
Though controversial, I think researchers in both the fields of human and
animal development will find this book unusually thought provoking. Perhaps it
will point out some new perspectives and directions for future research.
Suzanne Chevalier-Skolnikoff
Human Interaction Laboratory
Department of Psychiatry
University of California
San Francisco
Condon, W.S.; Sander, L.W. Neonate movement is synchronized with adult speech: Interactional perception and language
acquisition. SCIENCE 183:99-101, 1974.
OF PERCEPTION. Chicago, Aldine Publishing Company, 1973.
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development, living, publishing, corporation, babies, able, frank, infant, model, behavior, primate, free, chimpanzee, plooij, 207, norwood, review, 1985
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