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Title: Minimalism Challenge
Submitter: Michael Hegarty
Description: [Richard Sproat and Shalom Lappin (LINGUIST 16.1156)]: ''In fact, we
would be delighted if someone succeeds in meeting our challenge [of
computationally modeling the development of the child's ability to
parse sentences]. Such success would convince us that the P&P
enterprise is, after all, a testable theory with genuine scientific
content.'' [end quote]

Most likely, Sproat and Lappin do not consider amenability to
computational modeling to be a requirement for all scientific theories
(for there are easy counterexamples to that), but for all syntactic
theories which, like Principles & Parameters syntax, aim to account for
how children acquire language. Specifically, they assume that
providing a basis for a computational model of how to attain the ability
to parse sentences is a threshold level of achievement for such a
theory.

But curiously, for all their concern with ''building a computational
model that is able to do what every child is able to do,'' (16.1156)
Sproat and Lappin say essentially nothing about what children
actually do, that is, about the actually observed linguistic behavior of
children through the process of language development. It's one thing
to computationally model processes which achieve a final state of
successfully parsing linguistic input based on what one takes to be the
child's initial state, and quite another thing to provide a computational
model of the development of child language which systematically
accounts for the linguistic forms produced by children and the
characteristic changes in those forms over time. Peter Hallman's post
(16.1251) makes a point along these lines, arguing that, in order to
meet the concerns of P&P theory, a model of child acquisition of
syntax would not just attain a parsing capability equal to that of the
child's final state, but it ''would behave like a language learner in its
acquisition timeline and would fail to acquire languages that violate
language universals.'' Work in computational modeling of language
acquisition has certainly not completely ignored the problem of
accounting for child behavior en route, as attested by connectionist
claims to reproduce child errors. But the full range of developmental
issues which occupy P&P theorists of child language acquisition do
not seem to be dealt with by the work cited by Sproat and Lappin.

A response on behalf of Sproat and Lappin would be that they are
merely demanding something simpler and weaker, as a threshold
condition for a theory of syntax to establish its scientific bona fides.
Marc Hamann's post (16.1472) fits into this response: if P&P theorists
are interested in the acquisitional process in its full glory, then they
should be ready to meet Sproat and Lappin's challenge since, after
all, it pertains to only a part of that whole. But this response assumes
that scientific theories come neatly nested, with all theories in an area
providing coverage of basic phenomena, beyond which theories
become progressively more specific and specialized. That isn't always
how it works. Qualitative physiology gives a high-level account of the
functioning and possible dimensions of impairment of the body. It's
observations have even been systematized in ways which provide a
basis for computational modeling. Cell-level and molecular physiology
deal with lower-level phenomena, providing precise and detailed
accounts of particular low-level processes, which can be
computationally modeled in their own right, but which do not always
lend themselves to ready synthesis into a theory of the whole system.
This is not meant to be an exact analogy to the acquisition of syntax,
and I leave it to others to discuss which of P&P syntax and
computational modeling of child language acquisition is more
analogous to qualitative physiology, and which to cell and molecular
biology. The analogy is merely meant to illustrate the fact that different
scientific approaches to the ''same problem'' might be sufficiently
different in scale and focus that neither of them can account for the
easiest or most basic test cases of the other. As a result, researchers
must make choices regarding their focus.

I want to understand the growth of child language and the nature of
language impairment in the same way in which P&P syntax and,
increasingly, other frameworks help us understand cross-linguistic
variation. In particular, I want to understand things like the progression
through the optional infinitive phase mentioned by Seth Cable
(16.1454), and the phenomena which I understand as reflecting
growth in the child's capacity for representing functional categories,
described below. P&P syntax provides tools for me to make progress
on these problems; to my knowledge, work on computational modeling
of the attainment of the ability to parse sentences does not. Things
change, of course, and mathematically better defined theories of
syntax such as HPSG are increasingly attractive and effective
instruments for the sort of comparative investigation which has long
been the forte of P&P syntax, and computational modeling may
eventually become detailed enough to answer all my questions about
the acquisitional process. In the meantime, I am no more impressed by
the insistence that P&P do what Sproat and Lappin and Hamann want
it to do than I would be by the insistence that cell and molecular
biology demonstrate their bona fides by accounting for whole organ or
whole organism function in line with results of qualitative physiology,
or that qualitative physiology match the explanatory detail of cell and
molecular biology at the microlevel.

A case-study is helpful to break down some stereotypes concerning
P&P syntax, including the supposition that all P&P work is
straightforwardly committed to some identifiable form of the innateness
hypothesis. Forgive the element of self promotion which follows, but I
must choose an example which is ready to hand. In Hegarty
(forthcoming), I report on a study of child acquisition of functional
elements which found that there is a stage at which children have
acquired individual classes of functional elements, including
complementizers, manifestations of inflection, and negation, and
display them one or two at a time within a clause, but, despite more
than adequate phrase structure complexity (and working memory and
processing capacity) they don't display three functional elements
together within a clause. The pattern is general: there are successive
stages at which the child displays a maximum of (successively) 0, 1, 2
and finally 3 functional categories within a clause. I interpret these
results as a consequence of growth in the representational resources
of the child's grammar. The pattern bears some similarity to that
predicted by the bottom-up structure building theory of the acquisition
of functional categories (Vainikka 1993/94, Guilfoyle and Noonan
1992), but is distinct from it.

I discovered this pattern by virtue of working within a feature-based
formulation of the syntax of functional categories, a part of P&P theory
with its roots in Minimalist syntax. The framework served as an engine
of discovery of this empirical generalization, and it provides theoretical
machinery to express it neatly. But other frameworks and approaches
will be able to accommodate the result. (Other hypotheses about
functional category acquisition fail to predict the results I found, but
the failed predictions do not accrue to other frameworks of syntactic
theory.) I look forward to research within any framework or any
approach to the development of clause structure supporting functional
elements which sheds light on what I identify as the growth of
representational resources of the child's grammar, what those
resources are and how they should best be characterized, and what
this means for the innateness hypothesis and strong continuity
approaches to functional category development (which I see my
results as challenging, at least in some measure). I also look forward
to developing proposals regarding the neurological instantiation of this
growth in representational capacity along the lines of proposals by
Wakefield and Wilcox (1995) and Wilkins and Wakefield (1995) --
which addressed the neurological instantiation of Radford's (1990)
theory of the maturation of functional categories -- but in light of more
current neurolinguistic ideas of the sort surveyed in the issue of
Cognition edited by Hickok and Poeppel (2004). The body of known
generalizations about child language development has grown with the
line of work of which mine is a part, our understanding of the process
has grown, new directions for research have been uncovered, and
our capacity to decide between competing fundamental hypotheses
about acquisition, innateness versus maturation, for example, or to
refine those hypotheses in response to facts about child language
acquisition, has grown. I don't see any reason why this work should
fail to count as science because it hasn't yielded improvements in
computational modeling of the child's ability to parse sentences.

Now we can return to the heart of Sproat and Lappin's argument for
the legitimacy of their challenge:

[Sproat and Lappin (16.1439)] ''Regarding B, we remind readers that
humans do assign structure to sentences, that assigning structure to
sentences is surely a part of what syntax is about, that humans
acquire this knowledge as part of language acquisition, and that P&P
claims to provide an explanation of how this is achieved. So we are at
a loss to understand why inducing a large-scale working parser from
sample data is not a valid test of P&P.'' [end quote]

I hope to have provided enough background to make clear the central
problem with this apparently simple and straightforward line of
reasoning. No one person speaks for P&P theory. If there is someone
who claims to have an explanation of how humans acquire the
knowledge to assign structure to sentences, then Sproat and Lappin's
challenge might be rightfully directed at that person. But I and the
researchers whose work I have cited make no such claim. We claim to
have accounts of some central aspects of the development of the
functional category system. More generally, we have accounts of
aspects of child syntax, and aspects of the development of child
syntax which, as far as I know, other theories have not even noticed.
But we don't claim to have an explanation of how humans acquire the
ability to parse sentences. For my own part, I don't even know if this
development is accurately characterizable as an increase in
knowledge, as Sproat and Lappin understand it, which seems to
assume that the child's relevant cognitive structures and capacities
are fully in place, simply waiting to iterate a learning procedure to
convergence. For this reason, it's not even clear to me that existing
computational models of how humans acquire syntax actually model
the development of knowledge of language. One of the things I hope
to get out of work on child language acquisition is a better
understanding of what syntactic knowledge is, along with clarification
regarding its genesis (innateness versus alternatives).

''What would count as a valid test of such a theory?'' is a common
refrain in this discussion. Well, making predictions based on such
theories, checking the data, and seeing which theory (or theories)
best accords with the data sounds like a valid test. This is what I did
with existing theories of functional category acquisition with regard to
the development of the functional category system, given the child's
inventory of expressed functional elements and level of phrase
structure complexity. Why isn't this a valid test of a theory in progress?

The amenability to computational modeling of the sort Sproat and
Lappin demand is by no means a worthless quality for a theory to
have, as Emily Bender's contribution to the discussion (16.1454)
makes particularly clear. If everything else is equal, it is worth striving
for. But it doesn't seem reasonable to regard it as the sole or primary
measure of the scientific value of a syntactic theory.

Michael Hegarty
Department of English and Program in Linguistics
Louisiana State University

References:

Guilfoyle, Eithne, and Maire Noonan. 1992. Functional categories and
language acquisition. Canadian Journal of Linguistics 37(2): 241-272.

Hegarty, Michael. Forthcoming. A Feature-Based Syntax of Functional
Categories: The Structure, Acquisition and Specific Impairment of
Functional Systems. Mouton de Gruyter.

Hickok, G. and D. Poeppel, eds. 2004. Cognition 92.

Radford, Andrew. 1990. Syntactic Theory and the Acquisition of
English Syntax: the Nature of Early Child Grammars of English.
Oxford: Oxford University Press.

Vainikka, Anne. 1993/94. Case in the development of English syntax.
Language Acquisition 3(3): 257-325.

Wakefield, Jennie, and M. Jeanne Wilcox. 1995. Brain maturation and
language acquisition: A theoretical model and preliminary
investigation. In Dawn MacLaughlin and Susan McEwen, eds.,
Proceedings of the 19th Annual Boston University Conference on
Language Development, vol. 2, 643-654. Cascadilla Press.

Wilkins, Wendy K., and Jennie Wakefield. 1995. Brain evolution and
neurolinguistic preconditions. Behavioral and Brain Sciences 18: 161-
182.
Date Posted: 11-May-2005
Linguistic Field(s): Computational Linguistics
Linguistic Theories
Language Acquisition
Discipline of Linguistics
LL Issue: 16.1506
Posted: 11-May-2005

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