Psychological Review - Harvard Graduate School of Education ...

THEORY OF COGNITIVE DEVELOPMENT 515
levels in the same domains. The environmental
diversity of human experience, as
well as the genetic diversity of the human
species, ensures the occurrence of major
individual differences in development.
Skill theory thus makes several general
predictions about the effects of environmental
factors on sequence and synchrony
in development and provides tools for analyzing
some of these effects. In addition,
the structures defined by the theory suggest
a number of general corollaries about structural
relations and how they determine sequence
and synchrony.
Structural Corollaries
I shall not attempt to provide an exhaustive
list of structural corollaries but instead
will present a few illustrations of
potentially useful ones.
Consistent Decalage Within a Task Domain
Unevenness in skills across domains
seems to be a fact of development. But
according to the theory, many phenomena
that are commonly classified as instances of
unevenness are in fact microdevelopmental
sequences: The unevenness follows the
same pattern in virtually all children in a
given social group, and it seems actually to
arise from differences in the complexity of
the skills. Most of the instances of horizontal
decalage (unevenness within a stage or
period) studied by Piaget and his colleagues
show such microdevelopmental sequences.
The skill theory explanation is simplest in
cases where the skills belong to the same
task domain. The skill that develops later
can be derived by the transformation rules
from the skill that develops earlier.
Among the best documented cases of
consistent decalage within a task domain is
the development of conservation of substance
and conservation of weight. Research
has repeatedly shown that school
children develop conservation of substance
1 to 3 years before conservation of weight
(e.g., Hooper et al., 1971; Piaget & Inhelder,
1941/1974, especially in the introduction to
the 2nd edition; Uzgiris, 1964). In conservation
of substance, 14 children understand,
usually by 7 or 8 years of age, that the
amount of clay does not change when a ball
of clay is elongated into a sausage, flattened
into a pancake, or changed into some other
shape. At the same age, however, they still
believe that the weight of the clay ball does
change when the shape changes. Typically,
they will not develop the skill for conservation
of weight until 9 or 10 years of age.
Within Piaget's framework, this consistent
sequence is puzzling because both types of
conservation are said to require exactly the
same kind of concrete operational scheme:
Two factors (height and length) covary in
such a way that changes in one compensate
for changes in the other.
According to skill theory, conservation of
weight develops after conservation of substance
because it requires a compounded
Level 6 skill that subsumes the skill for conservation
of substance. In conservation of
substance, the child must coordinate the
length and width of the original piece of
clay, ?BL,W» with the length and width of the
transformed piece, i# L w (Halford, 1970;
Peill, 1975; Verge & Bogartz, 1978). 15 In
conservation of weight, on the other hand,
the child must go beyond mere amount of
clay and think about weight of clay. That is,
he must relate the changes in the length and
width of the clay to a third factor, such as
the weight readings on a scale or the amount
offeree that he feels when he holds the clay
in his hand, 6 F. To coordinate all three sets
together in a single skill, he must compound
the skill for conservation of the substance
clay with a skill involving the weight of the
clay, such as the skill in which the child
relates the length of pieces of clay (for
instance, sausage-shaped pieces) to their
14 Note that this type of conservation is properly
called conservation of substance. It has often been
erroneously translated as conservation of matter or
conservation of volume. Both matter and volume are
much more abstract and difficult concepts than amount
of a substance such as clay, and they develop at
later ages.
15 A precise measure of the amount of clay, of
course, requires three dimensions (height, length, and
thickness), not just two; but with early Level 6 skills the
child does not yet understand true volume—that is,
three-dimensional volume. His understanding of
amount of clay is based on a relatively crude coordination
of just two dimensions.