58cv网址导航58cv网址导航Learnability, Hyperlearning, and the Poverty of the Stimulus
Learnability, Hyperlearning, and the Poverty of the Stimulus*
Geoffrey K. Pullum
University of California, Santa Cruz
Presented at the Parasession on Learnability,
22nd Annual Meeting of the Berkeley Linguistics
Society, Berkeley, California, February 18, 1996.
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| Copyright (c) 1996 by the Berkeley Linguistics Society.
All rights
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This is a preliminary draft of a paper that will be
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Please do not cite or quote this draft in any published |
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Internet address: "pullum@ling.ucsc.edu"; postal |
| address: Stevenson College, UCSC, Santa Cruz, California 95064, USA. |
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* ACKNOWLEDGEMENT NOTE:
I am grateful to Geoffrey Sampson for
sending me a copy of Sampson (1989), which started me thinking
about this topic, to Hinrich Schuetze for sending me a copy
of his dissertation, to Dave Chalmers, Gerald Gazdar, Lotus
Goldberg, Tom Wasow, and the students in my research seminar
for useful comments and references, and particularly to Barbara
C. Scholz, who subjected my first thoughts on this topic to
much-needed penetrating interrogation.
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Stimulus poverty and hyperlearning.
Hornstein and Lightfoot (1981) make a bold and unambiguous claim
concerning first language acquisition:
"People attain knowledge of the structure of their language for
which NO evidence is available in the data to which they are
exposed as children."
(Hornstein and Lightfoot 1981:9)
I propose to question whether hyperlearning is in fact ever attested
in the domain of human language acquisition.
This issue is of importance primarily because of its central
relevance to the Argument from Poverty of the Stimulus (APS).
The claim that rationalism' is confirmed over
empiricism' by
findings about language acquisition is the most celebrated of the
claimed impacts of generative linguistic research on philosophy and
psychology, and the APS is generally agreed to be the most important
argument in this connection.
I believe the argument is not sound.
Let me therefore begin by attempting to do that.
We will need to
distinguish between two ways in which an infant might in principle
learn a language.
(The reader will doubtless see in them the
outlines of
empiricism' and
rationalism' as characterized in
traditional epistemology, but in most of this paper I will avoid
using those terms because of the baggage they carry.)
The first involves nothing but methods of rational belief
fixation (inductive inference) applied to a corpus of observed
utterances in natural contexts.
Crucially, the learning is assumed
not to be informed by preconceptions of any sort about what languages
I will call this "data-driven learning".
It is important
that if an infant could acquire a language L by means of data-driven
learning after being exposed to nothing more than a corpus C of
observed utterance tokens, then any rational being could in principle
do the same.
Notice, therefore, that if this is the way first
language acquisition is accomplished, we will never see an instance of
hyperlearning in the first language acquisition domain.
The second way in which languages might be learned involves
the learner being primed ab initio with special information,
or endowed with special internal mechanisms that make available
specific information about the domain.
Call learning of this sort
"innately-primed learning".
By definition, innately-primed learning
proceeds in a way that does not limit the learner's resources to the
corpus of relevant observations.
It is compatible with the occurrence
of hyperlearning, because although the corpus might not suffice for
learning what is learned, the sum of the contributions of the corpus
and the innate priming might suffice.
Some may take it to be trivial that one or the other of these
must be correct: either there is innate priming or there is not.
Others claim that additional distinct positions lying between the two
can be made out (see in particular Stich 1979).
Garfield ()
notes that some regard "empiricism" and "rationalism" as merely two
regions in a continuum, so that there are indefinitely many
alternative positions.
Ignoring these disputes, I propose simply to
grant for the sake of argument what advocates of the APS assume: that
data-driven learning and innately-primed learning are distinct,
mutually exclusive, and jointly exhaustive.
That yields a disjunction
that constitutes the initial premise of the APS, which I can now set
out as in (2).
The Argument from Poverty of the Stimulus (APS)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Human infants learn their first languages either by
data-driven learning or by innately-primed learning.
[D by assumption.]
If human infants learn their first languages by means of
data-driven learning, then hyperlearning will never be
observed in this domain.
[Immediate, from the
characterization of data-driven learning methods.]
Hyperlearning does in fact occur in the domain of first language
acquisition by infants.
[Empirical premise asserted in (1).]
Human infants do not learn their first languages by means of
data-driven learning.
[From (b) and (c) by modus tollens.]
Human infants learn their first languages by means of
innately-primed learning.
[From (a) and (d) by disjunctive syllogism.]
This is a valid argument.
My concern is with its soundness, and in
particular with whether premise (2c) is true.
My strategy will be to
examine the strongest and best-known pillar of support for (2c) that
advocates of the APS have put forward, to show that it will not bear
the load assigned to it.
The claim I have in mind concerns auxiliary
fronting in polar interrogatives (e.g. "Are you happy?", the polar
interrogative corresponding to declarative "You are happy").
crucial point about it is the one stated in (3):
The rule for initial auxiliary verb position in polar
interrogatives (and a variety of other construction types)
in such languages as English or Spanish is based on structural
relations rather than just linear sequence: it is the main
clause auxiliary verb that is assigned initial position,
not, e.g., whatever is the leftmost auxiliary in the
corresponding declarative clause.
The first reference to the structure-dependence of auxiliary
fronting -- the fact that it needs dominance as well as precedence to
state it -- is in Chomsky ().
Chomsky () repeats
the point, adding that the "language-learner knows" only to use
structure-dependent operations.
Chomsky () begins to
expound an explicit case for hyperlearning, and this is echoed by
similar passages in Chomsky (; 153 154), Piattelli-Palmarini
(), and (adapting the data to Spanish), Chomsky ().
The claims have since been repeated by many others (e.g. Marcus 1993:
80, Pinker ; 233 4).
Quite apart from its celebrity, there are reasons for taking this
case to be potentially the strongest kind of case for the APS, because
it is purely syntactic.
A question like how the child learns that
Mary appealed to the men to like each other is well-formed while *Mary
appeared to the men to like each other is not (Chomsky 3)
cannot sensibly be considered in isolation from issues of how the
child learns about the uses of reciprocals and the control patterns of
and these issues seem intimately bound up with semantics.
in the auxiliary fronting case we are dealing with a purely syntactic
device for signalling a difference in sentence type.
It is purely
syntactic evidence that is needed to identify which device is being
Chomsky never offered evidence for the empirical claim that
children always do correctly identify that syntactic device.
took it to be intuitively obvious.
But Crain and Nakayama (1987) have
examined the matter experimentally, and their results did support the
claim, by showing that when children are manipulated into a situation
where asking a question like (4a) is appropriate, they never utter
strings like (4b) instead.
Is the boy who is in the corner smiling?
*Is the boy who in the corner is smiling?
This is not sufficient as a basis for the APS.
No argument for
innate priming can be developed merely from that fact that the
generalization is strucuture-dependent and the fact that children
are uniformly successful at discovering it.
What is critical is
an additional claim, the one that involves stimulus poverty.
Stimulus poverty and auxiliary fronting.
The claim about auxiliary fronting that permits it to be used in an
instance of the APS is the very specific one in (5).
The Stimulus Poverty claim about auxiliary fronting:
The corpus of evidence presented to children during their
language learning is insufficient to permit data-driven
inference to the selection of the correct auxiliary fronting
generalization and the elimination of all the alternatives.
Chomsky asserts this claim in quite extreme terms.
He claims not just
that the crucial kinds of example to distinguish structure-dependent
from structure-independent formulations of auxiliary fronting have a
lower frequency than examples with simple subject NPs (which is
certainly true); in the paper and discussions published in Piattelli-
Palmarini (1980), he makes the much stronger claims quoted in (6).
"A person might go through much or all of his life
without ever having been exposed to relevant evidence,
but he will nevertheless unerringly employ [the
structure-dependent generalization], on the first relevant
occasion" (Chomsky, in Piattelli-Palmarini 1980:40)
"the more complex cases that distinguish the
hyp you can easily live your whole
life without ever producing a relevant example to show
that you are using one hypothesis rather than the other
one." (Chomsky, in Piattelli-Palmarini )
"The examples cited are the only kind for which the
hypotheses differ, and you can go over a vast amount of
data of experience without ever finding such a case.
in many cases the Martian scientist could not know by
passive observation whether the subject is using the first
hypothesis or the second one."
Note in passing that (6b), if true, undercuts (6a) and (6c).
If people so rarely produce utterances that exhibit their grasp of
the structure-dependent character of the auxiliary fronting
generalization, then there could well be many speakers who have
acquired an "incorrect" structure-independent generalization instead
but who are never detected because of the rarity of the crucial
situations in which they would give themselves away.
Here I prescind
away from this suspicious epistemological aspect of the claim, and
turn to the issue of its truth.
One scholar has suggested in print that the claims in (6) are
empirically false.
Sampson (1989) notes that when he turned to the
list of "wonder questions" in a children's encyclopedia, he found
crucial examples of the relevant sort within the
and he points out that William Blake's poem
Tiger', which no one
seems to go through grade school without encountering, contains the
line "Did He who made the lamb make thee?", also crucial positive
evidence for the structure-dependent rule.
These observations are
anecdotal, certainly, but they certainly need some sort of response
from defenders of the APS.
A few minutes of critical reflection suffice to raise some
Could you really expect to live your whole live as an
English speaker, or even reach kindergarten, without running into
any sentences of the sort illustrated in (7)?
(In these examples,
the position in each string where the main clause auxiliary would
be if it were not fronted is marked with underlining.)
Would anyone who is interested __ see me later?
Could the man who has lost his ticket __ come to the desk?
Can the people who are leaving early __ please sit near the door?
Will those who are coming __ raise their hands?
Can a helicopter that has lost its tail rotor __ still fly?
Will the owner of the car that is blocking the driveway __ please
Is the boy who was bothering you __ still here?
Could a tyrannosaur that was sick __ beat a triceratops in a fight?
These examples have an auxiliary verb within the subject NP, and thus
the auxiliary that appears initially would not be the first auxiliary
in the declarative.
But of course the extra auxiliary does not need to
be in the subject NP in order for there to be a contrast between
fronting the main clause auxiliary and fronting the first auxiliary.
All that is needed, as Sampson recognizes, is for any auxiliary to
precede the main clause auxiliary.
And that condition would be met in
examples like the ones in (8) as well.
If you don't need this, can I have it?
Since we're here, can we get some coffee?
When you're done, could I borrow your pencil?
Given that I'm not needed, can I go home?
While you're getting cigarettes could you get some more milk?
Though you won't like me asking, did you brush your teeth?
The notion of fronting the wrong auxiliary might seem perverse
here, because the preposed clauses precede the whole domain of the
correct auxiliary fronting.
But of course, the very idea of a
structure-independent rule demands that we imagine ourselves with no
access to such notions as clause boundaries.
The question is whether
there is naturally-occurring evidence against the hypothesis that a
string like (9a) can be made into a polar interrogative by fronting
the first auxiliary (word 3, rather than word 7).
The answer is that
corresponding to (9a) we find (8a) = (9b), rather than (9c), or for
that matter (9d), the result of fronting the right auxiliary but
fronting it too far in the string.
if1 you2 don't3 need4 this5 I6 can7 have8 it9
if1 you2 don't3 need4 this5 can7 I6 have8 it9
*don't3 if1 you2 need4 this5 I6 can7 have8 it9
*can7 if1 you2 don't3 need4 this5 I6 have8 it9
This crucially confirms the structure-dependent generalization over the
structure-independent one.
The range of relevant examples is yet wider once we notice that
wh-movement questions in which the wh-phrase is a nonsubject always
incorporate an auxiliary fronting construction.
(We find, for
example, strings of the form WX where W is a wh-word and X is a string
of the sort that instantiates auxiliary fronting.)
Thus any evidence
that we find examples like (10a) rather than (10b) is crucial evidence
in favor of the structure-dependent auxiliary fronting hypothesis:
How could anyone who was awake not hear that?
b. *How was anyone who awake could not hear that?
These examples all look plausible enough, but they are invented.
What we need to know is whether such examples actually turn up in
natural language use.
This calls for a corpus search.
Ideally, what
we would want is a large machine-readable corpus containing a
transcription of most of the utterances used in the presence of some
specific infant over a period of years.
By large I mean tens of
millions of words.
Less desirable but still of some use would be a
large corpus of representative utterances used in natural contexts in
the presence of a number of infants in their critical period for
language acquisition.
As far as I know, there are no such corpora in
existence.
Lacking a corpus that was anywhere near ideal, I searched what
I had: the text corpus on the CD-ROM made available by the ACL
(Linguistic Data Consortium 1993).
This contains about forty million
words of newspaper articles from the "Wall Street Journal" between
1987 and 1989.
Now, let me be the first to concede that even bankers'
children do not spend their early years being read to from the
But the exercise is not as misguided as it might
superficially seem.
First, some data is better than no data at all.
Second, the WSJ
material is more suitable
it contains a lot of
structurally simple colloquial speech in verbatim quotes from ordinary
people interviewed in news stories, as well as ordinary English of
every journalistic genre from news features to theater reviews to
humorous essays.
And third, note that many statistically defined
syntactic properties of running text vary little from genre to genre
(recall the surprising result of Hudson 1994 that about 37% of the
word tokens in running text are nouns regardless of genre, style,
modality, source, or even language).
How fundamental are the changes these events portend?
(LDC-93-T1:WSJ\:3963)
*How fundamental do the changes these events portend are?
The crucial evidence is supplied by the fact that (11a) occurs rather
than (11b), with fronting of the supportive do that would be the
auxiliary of the relative clause.
is the entirely unproblematic example (12a), actually involving two
instances of the copula, exactly like Chomsky's hypothetical
The crucial fact is that we do not find (12b) instead.
Is what I'm doing in the shareholders' best interest?
(LDC-93-T1:WSJ\:2991)
*Am what I doing is in the shareholders' best interest?
It would be possible to engage in a certain amount of debate about
what exactly one should count as a relevant example.
(I will not
illustrate this in detail here for reasons of space.
The facts are
readily accessible to anyone else who wants to take a look.)
it to say that at least five crucial examples occur in the first 500
for example, the 387th interrogative in the corpus is
(13), and the 456th is (14).
Is a young professional who lives in a bachelor condo as much
a part of the middle class as a family in the suburbs?
(LDC-93-T1:WSJ\:2813)
Why did "The Cosby Show's" Lisa Bonet, who has a very strong
screen presence, think that participating in a graphic sex scene
would enhance her career as a legitimate actress?
(LDC-93-T1:WSJ\:16426)
Finding five crucial examples in the first 500 interrogatives suggests
that there may be as many as one crucial auxiliary fronting case for
every hundred cases of interrogative mood.
And focussing more closely
on polar interrogatives seems to push the number
my examination of the full corpus of polar interrogatives in the
corpus suggested that about 12 percent of the examples crucially
confirmed the structure-dependent regularity over the
structure-independent one.
Chomsky's assertion that "you can go over a vast amount of data of
experience without ever finding such a case" is unfounded
hyperbole.
And this is apparently the best candidate for a case
of attested hyperlearning ever put forward -- the candidate
Chomsky has been using to support the APS for over twenty years.
I conclude that the defenders of the hypothesis that there is a
specialized language-acquisition brain module pre-programed with
universals of language are sorely in need of a new well-confirmed
case of hyperlearning.
Implications.
I am not aiming to present here an argument for adopting an
empiricist' view of language acquisition.
However, casting doubt on
the Stimulus Poverty claim about auxiliary fronting does remove a key
reason for thinking that the success rate of children at learning the
structure-dependency of auxiliary fronting cannot be explained in
terms of data-driven learning.
The utterance tokens that could
provide the crucial data apparently make up between 1% and 10% of
interrogatives.
A child obviously hears hundreds of thousands of
sentences while engaged in language acquisition, and thus will hear
thousands of examples that crucially confirm the structure-dependence
of auxiliary fronting.
This does not show that there is no innate
priming, but it greatly weakens the support for it.
Children could
be learning in a data-driven way
that auxiliary fronting is
structure-dependent.
The APS, then, loses its vital premise (2c), and
cannot at present be shown to be sound.
Searching for new cases of linguistic hyperlearning will involve
generative linguists in intimate involvement with two things toward
which they have typically shown considerable antipathy: research
results on formal learning theory, and the methods of corpus
linguistics.
The relevance of formal learning theory (see Osherson et
al. 1986 for an introduction) is that it is the mathematical study of
the limits on data-driven learning, and without clear results on that,
hyperlearning cannot even be characterized.
And corpus study is
relevant because a claim that hyperlearning occurs will incorporate a
specific claim about what occurs in typical corpora of material
available to infants during their critical period for language
acquisition.
Thus generative linguists, if they are going to develop
serious conclusions about language acquisition using the APS, are
going to have to become more broad-minded in these two respects.
Serious work on establishing the soundness of the APS is
almost certainly going to be partially self-undercutting.
it will involve close study of the capabilities of data-driven
learning procedures, it is highly likely that improvements in the
success rate of such procedures will be attained in the course of
Such successes will eliminate many apparent cases of
hyperlearning.
I have no space here to expand on this point, but
already Brent (1993) has demonstrated that an "unsupervised"
algorithm taking as input just raw, untagged text plus extremely
rudimentary knowledge about grammatical cues (e.g., "if X occurs
and Xing occurs, then X is probably a verb"; "if a determiner
occurs before X then X is probably not a verb"; "the word THE is
a determiner") can identify the verbs of a language and their
subcategorization frames.
Even more remarkably, Schuetze (1995) develops methods for
deducing syntactic category, semantic class, word sense for ambiguous
forms, and subcategorization information from raw text alone, on the
basis of no syntactic information except the distributional
information in the corpus.
In short, more is learnable from an
unanalyzed corpus than most linguists think.
A serious defense of the APS will have to be based on a study
of work of this sort.
The strategy will involve establishing known
limits on what data-driven learning algorithms can do, and then
searching for instances of children acquiring knowledge about a
language that such algorithms provably could not induce (within a
reasonable time) from a corpus that could plausibly have been
presented to a child under normal child-rearing conditions.
like this has yet been attempted.
I have shown that the case of the
structure-dependence of auxiliary fronting is probably not a good
place to look.
It may indeed be the case that something construable as a system
of Cartesian innate ideas is in play when human infants learn natural
languages.
But as yet, there is no support for that view to be
garnered from the Argument from Poverty of the Stimulus.
REFERENCES
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From grammar to lexicon: unsupervised learning
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Computational Linguistics 19, 243-262.
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Aspects of the Theory of Syntax.
Cambridge,
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Chomsky, Noam.
Language and Mind.
New York: Harcourt,
Brace and World.
Chomsky, Noam.
Problems of Knowledge and Freedom.
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Chomsky, Noam.
Reflections on Language.
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Chomsky, Noam.
Rules and Representations.
Oxford: Basil Blackwell.
Chomsky, Noam.
Language and Problems of Knowledge.
Cambridge, Massachusetts: MIT Press.
Crain, Stephen, and Mineharu Nakayama.
Structure dependence
in grammar formation.
Language 63, 522-543.
Demopoulos, William.
On applying learnability theory to the
rationalism-empiricism controversy.
Matthews and Demopoulos,
eds., 77-88.
Garfield, Jay L.
Innateness.
Samuel Guttenplan, ed.,
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Basil Blackwell.
Gold, E. Mark.
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Explanation in
Linguistics, 9-31.
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About 37% of word-tokens are nouns.
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I am not suggesting that this is an adequate exact
definition.
For one thing, I am being deliberately vague about
the character of K we can take it to be propositional
knowledge, so that we can talk about such things as inferring K,
but that is not ultimately essential, and one might discuss such
things as skill acquisition in similar terms.
And for another,
there is really a different definition of hyperlearning for each
fully precise definition of data-driven learning procedures.
However, I think the rough definition offered in the text
suffices to gloss the shorthand term that is all I need for
present purposes.
Chomsky () gives some general discussion
of how learning is "better understood as the growth of cognitive
structures along an internally directed course under the
triggering and partially shaping effect of the environment"
(p.33), and then states that he is giving "a variant of a
classical argument in the theory of knowledge, what we might
call `the argument from poverty of the stimulus.'"
and Lightfoot (1981) take up the phrase but do not spell out an
Lightfoot () insists on the importance of the
"poverty of stimulus problems, i.e. where there are no data
available to the child which will suffice to establish some rule
or principle," but also states
(1989) critiques the argument but offers only inexplicit
quotations from C whole articles such as
Demopoulos (1989), Matthews (1989), and Wexler (1991) are
devoted to discussing the argument without clearly stating it.
(Let me quote Wexler () introducing the topic, for
example: "Chomsky... asked the question of explanation: How
does the child construct her grammar?
In other words, why
the adult output grammar the one that it is?
Chomsky's answer
notes that the attained grammar goes orders of magnitude beyond
the information provided by the input data and concludes that
much linguistic knowledge must therefore be innate."
And having
thus confounded four different questions (how grammars develop,
why grammars are the way they are, whether hyperlearning takes
place, and what is innate), Wexler simply announces that "As
Chomsky pointed out, this is an application of the classic
rationalist argument from the poverty of the stimulus."
argument is given or referenced.)
Stich () and Garfield (1994) come a bit closer
to offering a statement of the argument, but miss the central
point about learning things for which there is no evidence in
the corpus, and confuse it with other points (underdetermination
occurrence of
etc.) from which Sampson (1989) is careful to distinguish it.
Space precludes a detailed discussion here.
I am aware of one study that looked at the issue
in connection with second language acquisition.
() summarizes work by Y. Otsu and K. Naoi in which an
attempt was made to assess by forced question construction
whether the structure-dependent generalization would be employed
by Japanese subjects of 14 to 15 years old who had learned
simple polar interrogatives in English but had not yet learned
relative clauses, and whose native language did not use a
structure-dependent constituent-order variation to signal
interrogative sentence type (Japanese merely adds a question
particle to the declarative).
In these experiments, one of the
eleven subjects did produce errors of the "impossible"
structure-independent type (and another three remained neutral
by finding ways to avoid constructing strings of the desired
This is not particularly strong support of the claim
that innate priming defines the possibilities.
The example is in w7_001, the first file in the
1987 directory.
This UNIX command will pick it out:
fgrep "?" w7_001 | head -15 | tail -1
Sentences quoted from the Wall Street Journal corpus are given
with an identifier of the form X:Y:Z where X is the Linguistic
Data Consortium catalog number for the CD-ROM, Y is the MS-DOS
path name of the file on the disk, and Z is the line number in
The reader can verify this with the following UNIX
command on the files in the 1987 directory of the WSJ corpus:
fgrep "?" * | head -180 | cat -n | tail
Sampson also has an epistemological argument
against the APS: he argues that anyone defending the stimulus
poverty claim by exhibiting a fact F about a language L that
could not be induced from the evidence of ordinary use of L
must face the question of how they know that F is a fact.
If the warrant they offer for F comes from evidence of use
of L, they have contradicted themselves by conceding that
the e if the warrant is held to be the
result of knowledge gained via innate priming they have
committed the fallacy
and there are no
other cases.
This may be a valid additional objection to
the APS, but I have not appealed to it here.