Comments on Marchman's note

[Steve Pinker]

One thing that is not in dispute in the past tense debate: we could
use more data on children's development and on the behavior of network
models designed to acquire morphological regularities.  It is good to
see Virginia Marchman contribute useful results on these problems.  In
a complex area, however, it especially important to be clear about the
factual and theoretical claims under contention.

In OLC, we praised R-M for their breadth of coverage of developmental
data (primarily a diverse set of findings from Bybee & Slobin's
experiments), and reviewed all of these data plus additional
experimental, diary, and transcript studies.  The thrust of Marchman's
note is that "the data on the acquisition of the past tense in real
children may be very different from the patterns assumed by either
side in this debate". More specifically, she cites Jay McClelland's
recent prediction that future research will show that we have
"substantially overstated the empirical situation in several
respects". We are certainly prepared to learn that future research
will modify our current summary of the data or fail to conform to
predictions. But Marchman's experiment, as valuable as it is, largely
replicates results that have been in the literature for some time and
that have been discussed at length, most recently, by R-M and
ourselves. Furthermore, the data she presents are completely
consistent with the picture presented in OLC, and she does not
actually document a single case where we "underestimated the
complexity and degree of individual variation inherent in the process
of acquiring the English past tense".

1. Marchman reports that 'a child can be in the "stage" of
overgeneralizing the "add -ed" rule anywhere between 3 and 7 years
old.' The fact that overregularizations occur over a span of several
years is well-known in the literature, documented most thoroughly in
the important work of Kuczaj in the late 1970's. It figures
prominently in the summary of children's development in OLC (e.g. p.
137).

2. She calls into question the characterization of children's
development as following a 'U'-shaped curve. The 'U'- sequence that
R-M and we were referring to is simply that (i) very young children do
not overregularize from the day they begin to talk, but can use some
correct past tense forms (e.g. 'came') for a while before (ii)
overregularizations (e.g. 'comed') appear in their speech, which (iii)
diminish by adulthood. Thus if you plot percentage of
overregularizations against time, the curve is nonmonotonic in a way
that can be described as an inverted-U.  This is all that we (or
everyone else) means by 'stages' or 'U-shaped development', no more,
no less. No one claims that the transitions are discrete, or that the
behavior within the stages is simple or homogeneous (this should be
clear to anyone reading R-M or OLC).  Marchman does not present any
data that contradict this familiar characterization.  Nor could she;
her study is completely confined to children within stage (ii).

3. She reports that "errors took several forms beyond the one
emphasized by P&P, i.e.  overgeneralization of the "-ed" rule to
irregular forms. Instead, errors seem to result from the
misapplication of *several* (at least two) past tense formation
processes" (identity mapping, vowel changes, and addition of 'ed').
But the fact that children can say 'bringed', 'brang', and 'brung' is
hardly news.  (We noted that these errors exist (e.g.  'bite/bote', p.
161, p. 180) and that they are rarer than '-ed' overregularizations
(p. 160).)  As for its role in the past tense debate, in OLC much
attention is devoted to the acquisition of multiple regularization
mechanisms in general (pp. 130-136) and identity-mapping (pp. 145-151)
and vowel-shift subregularities (pp. 152-157) in particular.

(Marchman does call attention to the fact that vowel-change
subregularization errors can occur for *regular* verbs, as in
'pick/puck'. We find cases like 'trick/truck' in our naturalistic data
as well. Interestingly, the R-M model never did this. All of its
suprathreshold vowel-shift errors with regular verbs blended the
vowel-change with a past tense ending (e.g.'sip/sepped').  Indeed even
among the irregulars it came up with a bare vowel-change response in
only 1 out of its 16 outputs. This is symptomatic of one of the major
design problems of the model: its distributed representations makes it
prone to blending regularities rather than entertaining them as
competitors.)

4. Contrary to the claim that we neglect individual variation in
children, we explicitly discuss it in a number of places (see, e.g. p.
144).

5. Marchman writes, "In several interesting ways, children can be
shown to treat irregular and regular verbs similarly during
acquisition."  This is identical to the claim in OLC (pp. 130-131,
135-136), though of course the interpretation of this fact is open to
debate. We emphasized that the regularity of the English '-ed' rule
and the irregularity of the (e.g.) 'ow/ew' alternation are not innate,
but are things the child has to figure out from an input sample.  This
learning cannot be instantaneous and thus "the child who *has not yet
figured out* the distinction between regular, subregular, and
idiosyncratic cases will display behavior that is similar to a system
that is *incapable of making* the distinction" (p. 136).

6. According to Marchman, we suggest that regulars and irregulars are
tagged as such in the input. To our knowledge, no one has made this
very implausible claim, certainly not us.  On the contrary, we are
explicitly concerned with the learning problems resulting from the
fact that the distinction is *not* marked in the input (pp.128-136).

7. Finally, Marchman previews a report of a set of runs from a new
network simulation of past tense acquisition.  We look forward to a
full report, at which point detailed comparisons will become possible.
At this point, in comparing her work to the OLC description of the
past tense, she appears to have misinterpreted what we mean by the
'default' status of the regular rule. She writes as if it means that
the regular rule is productively overgeneralized. However, the point
of our discussion of the difference between the irregular and regular
subsystems (pp. 114-123) is that there are about 6 criteria
distinguishing regular from irregular alternations that go beyond the
mere fact of generalizability itself.  These criteria are the basis of
the claim (reiterated in the Cog Sci Soc talk) that the regular rule
acts as a 'default', in contrast to what happens in the R-M model (pp.
123-125). Marchman does not deal with these issues.

In sum, Marchman's data are completely consistent with the empirical
picture presented in OLC.

Steven Pinker
Alan Prince