Comprehension of Coreferential Expressions 
Peter C. Gordon 
Department of Psychology 
University of North Carolina 
Chapel Hill, NC 27599-3270 
pcg@email.unc.edu 
Abstract 
The ways in which the form of referring 
expressions interacts with the structure of 
language are reviewed. Evidence from a 
number of different methods - quantitatively 
analyzed judgements of acceptable 
coreference, reading time, and corpus 
frequency of different types of coreferenfial 
expressions - converges on a fairly simple 
description of patterns of coreference.- A 
model is presented which integrates 
important aspects of Discourse 
Representation Theory and of Centering 
Theory in order to provide an account of 
how referential expressions are interpreted 
as part of constructing a discourse universe 
from a series of utterances. 
Introduction 
The study of coreference in generative 
linguistics has led to a very strong emphasis on 
how the hierarchical structure of sentences 
interacts with the form of referring expressions 
to constrain coreference (Chomsky, 1986; 
Reinhart, 1976). The resulting principles, 
embodied in the Binding Theory, provide rules 
that are of some use to researchers in natural- 
language processing because they provide 
information about disjoint reference - what an 
expression cannot refer to in certain 
circumstances. However, beyond that use 
theoretical work on the Binding Theory does not 
directly bear on questions central to language 
processing. Questions of how to resolve 
potentially ambiguous expression such as 
pronouns, or how meaning more generally is 
built up incrementally from linguistic 
expressions in context, are beyond its scope. 
Randall Hendrick 
Department of Linguistics 
University of North Carolina 
Chapel Hill, NC 27599-3155 
hendrick@email.unc.edu 
We (Gordon & Hendrick, 1997; 1999) have used 
the basic methods of experimental psychology to 
take a close look at the phenomena of 
coreference and disjoint reference involving full 
expressions (names and descriptions) that have 
been cited in support of Principle C of the 
Binding Theory. I The results show that the 
interaction of form of referring expression and 
language structure is far simpler than it has been 
taken to be in the Binding Theory. Further, 
results on the judged acceptability of different 
configurations of referential expressions are 
consistent with the results of experiments that 
use reading time as ~ano online measure of 
language comprehension (Gordon, Grosz & 
Gilliom, 1993; Gordon, Hendricl~ Ledoux & 
Yang, 1999). Further, those results are consistent 
with the frequency of different types of 
coreferentiai configurations in corpora of 
naturally-occurring language (Ariel, 1994; 
Carden, 1982; van Hock, 1997). The pattern of" 
coreference that is observed with these three 
types of measures (intuitive judgments, reading 
time, and frequency in a corpus) is accounted for 
by a model (Gordon & Hendricl~ 1998) that 
incorporates aspects of Centering Theory 
(Grosz, Joshi, & Weinstein, 1995) into 
Discourse Representation Theory (Kamp & 
Reyle, 1993). 
• '. Principle C of the Binding Theory states that an r- 
expression cannot have a c-commanding antecedent. 
A constituent ct is said to "c-command another 
constituent 13if the first branching node that 
dominates a dominates 13 as well. 
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1 Coreference and the Form of 
Expressions 
Research in psycholinguistics, both ours and that 
of others, supports a fairly simple generalization 
concerning the ease of establishing coreference 
in sequences of different forms of referring 
expressions. Coreference is most easily 
established in •name-pronoun sequences, less 
-easily established in name-name sequences, and 
least easily established in pronoun-name 
sequences. An example of these types of 
sequences taken from Gordon and Hendrick 
(1997) is shown below along with the proportion 
of naive subjects (college students at the 
University of North Carolina) who judged that it 
was grammatically acceptable for the 
expressions in bold-face to refer to the same 
person. 
John's roommates met him at the shop. .94 
John's roommates met John at the shop. .37 
His roommates met John at the shop. .23 
It should be noted that according to the Binding 
Theory coreference ought to be acceptable in all 
thesentences shown above; for both the name- 
name and pronoun-name sequence the second 
referring expression does not have a c- 
commanding antecedent and therefore should be 
free to corefer with the first referring 
expression? This is one example of consistent 
differences that we found between the accepted 
empirical predictions of the Binding Theory and 
the quantitatively analyzed judgments of 
competent native speakers who were naive to 
linguistic theory. 
In Gordon and Hcudrick (1997; 1999) we find 
that this pattern of relative acceptabifity between 
the different types of sequences is shown for 
categorical judgments, for ratings of 
grammaticality, for isolated sentences, for 
sentences in discourse context, and for different 
types of unreduced expressions including names, 
2 Attempts like that of Kayne (1994) to reformulate 
the definition of c-command to account for the 
unacceptability of his roomates met John at the shoo 
do not explain the improved status of John's momates 
met John at the shun. 
definite NPs and quantified NPs. 3 This pattern is 
also supported by on-line measures of reading 
time which show that under certain conditions 
sentences with repeated names are read more 
slowly than matched sentences with pronouns 
(Gordon, et al. 1993; Kennison & Gordon, 
1997); similar patterns of this reading elevation 
are observed within sentences and between 
sentences (Gordon, Hendrick. Ledoux & Yang, 
1999), The similarity of results that we observe 
with judgments of coreference and with on-line 
measures of reading time suggest that judgments 
of acceptable coreference reflect the ease in 
terms of mental processing with which a 
sentence can be understood. The similarity of 
results that we observe in the factors that 
influence the ease of establishing coreference 
within and between sentences suggests that the 
same mechanisms may be used for the two types 
• of coreferential processing. The finding by Ariel 
(1994) that repeated-name coreference is far less 
• common than pronominal coreference in a 
naturally occurring corpus suggests that our 
conjecture about the relative ease in terms of 
cognitive processes of establishing coreference 
for different types of sequences helps to 
understand how people use different forms of 
. coreferential expressions. 
For name-pronoun and name-name sequences, 
the pattern of acceptable coreference is modified 
by the structural pro_minence of the antecedent 
(first) referring expression. Here, we take 
stmcturai prominence to mean the inverse of 
depth of syntactic embeddedness. An example, 
again from Gordon and Hendrick (1997), is 
shown below along with the proportion of 
subjects who judged that it was grammatically 
acceptable for the expressions in bold-face to 
refer to the same person. 
John's roommates met him at the shop. .94 
John met his roommates at the shop. .97 
John's roommates met John at the shop. .37 
John met John's roommates at the shop. .24 
For name-pronoun sequences, coreference is 
more acceptable when the antecedent is .more 
Our findings make English look more similar to the 
survey of crosslinguistic variation in pronoun- 
quantified NP sequences in Bresnan (1998). 
83 
prominent (e.g., a subject) than when it is less 
prominent (a possessive), while for name-name 
sequences the opposite is ti'ue. Gordon and 
Hendrick (I 997) found that this pattern holds for 
a number of manipulations of prominence: 
subject NP versus object NP, subject NP versus 
a component NP of a pair of conjoined NPs, and 
subject NP versus an NP in a relative clause. 
Gordon, et al. (1999) showed similar patterns in 
relative reading times for cases of both 
intersentential and intrasentential coreference. 
Our studies of coreference have mostly focused 
on understanding contrasts in the comprehension 
of full and reduced referring expressions. The 
results that we have obtained in those 
comparisons are consistent with other theoretical 
and methodological approaches. Research on 
psychological heuristics for interpreting 
ambiguous pronouns has provided support for a 
subject-assignment strategy, where a pronoun is 
preferentially interpreted as coreferential with 
the subject of the preceding clause (Crawley, 
Stevenson, & Kleinman, 1990; Frederick.sen, 
1981); in our framework, the subject of a 
sentence has the most structural prominence. 
Research on the success of algorithms for 
pronoun resolution (Lappin & Leass, 1994) 
shows that syntactic factors (such as being a 
subject, being a direct object, and not being 
contained within another noun phrase) 
contribute to the likelihood that an expression is 
the antecedent of a subsequent pronoun. 
2 DPT: An Account of Basic 
Coreferenee 
We have developed a model of coreference 
called Discourse Prominence Theory or DPT 
(Gordon & Hendrick, 1998). It adapts the 
formalism provided by Kamp and Reyle's (1993) 
Discourse Representation Theory (DRT). In this 
approach, construction rules (CRs) map 
linguistic expressions onto universes of 
discourse. In DPT the construction rules and 
representation of DRT are modified so that they 
can account for the basic facts of coreference 
described above. The principle modifications 
are: (1) The construction rules for proper names 
and definite descriptions introduce entities into 
the discourse model as they do in Kamp and 
Reyle (1993), but the construction rule for 
pronouns interprets pronouns as referring 
directly to entities in the discourse model which 
is not how the rule works in Kamp and Reyle 
(1993); (2) Discourse entities in the model are 
ranked in terms of prominence (an idea that 
derives from the set of forward-looking centers 
in Centering Theory, Grosz, et al. 1995) which 
influences the way in which coreference is 
established. Discourse Prominence Theory 
includes three construction rules for the major 
types of reference and coreference, those for 
proper names, pronouns, and equivalence as 
shown below. We explain these rules by 
showing how they account for the differences in 
the ease of establishing coreference in sequences 
of different types of referring expressions. Then 
we discuss how this process is influenced by 
syntactic prominence. 
CR.PN (Construction Rule for Proper Names) 
i 
Triggering Condition: \[__... ~ b,,_\]\]...\] 
Insm~ctions 
• Introduce a new discourse referent u into 
the universe of the DRS, Uk. 
• Introduce a new condition _(u) into the 
condition set of the DRS. 
• Substitute u for \[Jo L,~ -\]\]. 
cR.PRO (Const~ction Rule for Pronouns) 
I I 
Trigguring Condition: 
~ b._J\] 
Inslructions 
• Chose an antecedent vj, after considering 
every vi i<j such that vl and vj exist in the 
ordered set of discourse referents in the 
DRS and are suitable antecedents, and 
substitute vj for \[co ~ \] in the 
triggering condition. 
• ffno suitable antecedent vj is present, 
introduceinto the universe of the DRS a 
new discourse referent u. 
• Substitute u for \[s~ \[too -\] \]. 
"111 I 
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CR.EQ (Construction Rule for Equivalence) 
Triggering Condition: 
\[_...x... y .... \] 
such that (x) and__(y) 
Instructions: 
• Identify the discourse referent x in the 
ordered list of discourse referents va...vo 
by checking vi after v l where i<j. 
* Introduce the new condition x - y 
• Remove ~e condition _(y) 
2.1 Name-Pronoun Coreference 
The example below involves a sentence with a 
name-pronoun sequence. The occurrence of the 
name triggers the construction rule for proper 
names (CR.PN) which posits a new entity (here 
shown as x) in the universe of the discourse and 
which introduces a condition in the universe 
consisting of the name predicated on this new 
entity. The occunence of the pronoun triggers 
the construction rule for pronouns (CR.Pro) 
which searches the discourse universe for a 
*'suitable antecedent" (one that matches on 
grammatically encoded features). For the 
example sentence below this leads fairly directly 
to a discourse universe that successfully 
represents the coreferential interpretation of the 
name and the pronoun? 
E,x: Jane thinks she is sick. 
Construction Rules 
1. CR.PN 
2. CR.Pro 
I X • Jane(x) 
x ,thinks x is sick 
2.2 Name*Name Coreference 
The next example involves a sentence with a 
name-name sequence. The first occurrence of 
4 The nature of the appropriate predicate 
representation is an important question but one that 
we believe can be tackled independently of the 
mechanisms for establishing reference and 
coreference. 
the name triggers the construction rule for 
proper names as does the second occurrence of 
the name. In the example, this leads to a 
situation where there are two entities (x and y), 
both with the name Jane predicated on them, 
where one entity thinks the other is sick. Thus, 
the construction rules triggered by the name- 
name sequence naturally leads to a state of 
disjoint reference as shown by the intermediate 
discourse universe shown below. Coreference is 
only established through the construction rule 
• for equivalence which is triggered by the 
presence in the discourse universe of the same 
name predicated on two distinct entities. This 
rule introduces a condition that equates the two 
entities thereby establishing coreference as 
shown in the second discourse universe below. 
The fact that coreference in name-name 
sequences requires an additional construction 
rule and involves an intermediate representation 
with disjoint reference explains why coreference 
is more difficult to achieve in name-name 
sequences than in name-pronoun sequences. 
Ex: Jane thinks Jane is sick. 
Construction Rules 
I. CILPN 
2, CR.PN 
ill i 
Jane(x) 
Jane (y) 
x thinks y is sick 
3. C'R.EQ 
xy 
Jane(x) 
Jane (3') 
x=y 
, x thinks y is, sick 
2.3 Pronoun-Name Sequences 
The final example involves a sentence with a 
pronoun-name sequence, First. the occurrence of 
the pronoun triggers the construction rule for 
pronouns. Because the discourse universe does 
not contain a suitable antecedent, a new entity is 
introduced. The occurrence of the name 
85 
subsequently triggers the construction rule for 
proper names which introduces a second entity 
upon which the name is predicated. Thus, a 
discourse universe is created with two distinct 
entities but without the identifying information 
that could trigger the construction rule for 
equivalence. Because there is no direct way of 
establishing coreference it is comparatively 
difficult to establish coreference in pronoun- 
name sequences. 
Ex: She thinks Jane is sick. 
Construction Rules 
!. CR.Pro 
2. CR.PN 
xy 
Jane (y) 
x thinks }, is sick 
2.4 Effects of Prominence .. 
As discussed above, the syntactic prominence of 
the antecedent influences the ease of 
establishing coreference: Greater prominence 
facilitates coreference in name-pronoun 
sequences while it inhibits coreference in name- 
name sequences. DPT accounts for this effect by 
adopting the notion, from Centering Theory, that 
entities in the discourse representation are 
ranked in terms of prominence. Based on our 
psycholinguistic results, it is clear that the 
syntactic status of an antecedent expression is a 
major determinant of its "rank. The two 
construction roles that establish coreference use 
this ranking in different ways. The construction 
rule for pronouns searches for a suitable 
antecedent starting with the most prominent 
discourse entity, thus a prominent antecedent 
facilitates coreference with a pronoun. In 
contrast, the construction rule for equivalence 
involves an evaluation that begins with the least 
prominent entity, thus having a pmrm'nent entity 
inhibits coreference with a repeated name. 
3 Backwards Anaphora 
Backwards anaphora involves coreference in 
pronoun-name sequences, something that we 
said that naive subjects do not accept. In fact, in 
a number of experiments (Gordon & Hendrick, 
1997) we found that naive subjects do not accept 
coreference in the kind of pronoun-name 
sequences that played a critical role in 
motivating the construct of c-command. 
However, naive subjects do accept coreference 
in pronoun-name sequences when the pronoun is 
in a fronted adjunct, as shown by the proportion 
of acceptable coreference judgments for the 
sample sentence below: 
Before she began to sing, Susan stood up. .88 
In Gordon and Hendrick (I997) we found that 
coreference was acceptable for pronouns in 
fronted adjuncts that were subordinate clauses or 
prepositional phrases. This restricted range of 
acceptable backwards anaphora is in fact 
consistent with what is observed in corpus 
studies. Carden (1982) found that in over'95% of 
the naturally-occurring instances of backwards 
anaphora that he observed, the pronoun was in a 
fronted adjunct. Carden's finding is bolstered by 
recent work by van Hoek (1997), which 
provides clear support for the idea that in 
backwards anaphora the pronoun is 
overwhelmingly present in a fronted adjunct. 
An additional important fact about coreference 
and fronted adjuncts emerged in our studies of 
reading time in sentences with intersentential 
coreference (Gordon, et al. 1993). Our research 
has shown a consistent effect (the ."repeated- 
name penalty") where a sentence with a repeated 
name is read more slowly than a matched 
sentence with a pronoun. However, this effect is 
not observed when the repetition occurs in a 
fronted adjunct as shown in the example below: 
Susan gave Fred a pet hamster. 
In his/Fred's opinion, she-shouldn't have. 
Giving a pet as a gift is somewhat of an 
imposition. 
The second sentence is read equally fast when it 
Contains a repeated name as when it contains a 
pronoun. 
In Gordon and Hendrick (1998) we account for 
these two important facts about coreference in 
fronted adjuncts - that they enable backwards 
anaphora and that there is no reading time 
penalty for names compared to pronouns - by 
• considering the semantic function of adjuncts. 
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Adjuncts serve to semantically modify the main 
clause to which they arc attached. Accordingly, 
a fronted adjunct causes a departure from the 
normal incremental construction of a discourse 
model where linguistic expressions are added to 
the existing discourse model so as to elaborate 
its semantic content. This is because a fronted 
adjunct must be processed in relation to the 
clause to which it is attached and therefore must 
await processing of that clause when the adjunct 
appears first. In our model, this break in 
inereinental processing is formulated as a 
(possibly temporary) partitioning of the 
discourse universe that is triggeyed bY a fronted 
adjunct. Because a pronoun in a just begun 
discourse segment cannot possibly have a 
referent, it is held in an un-interpreted state and 
therefore can be subsequently equated with a 
following name. The two construction rules 
given below detail the partitioning of the 
discourse segment and the possible 
establishment of coreference for an entity in 
such a segment. The (temporary) semantic 
partitioning of the discourse universe provides a 
unified account of the possibility of backwards 
anaphora in fronted adjuncts and the absence of 
a penalty for names over pronouns in a fronted 
adjunct. 
CR.FRQNTED.Adjunct 
Triggering Condition: " ' 
\[c, \[xP\] cp...\] 
Instructions: " 
• Begin a new DRS Ut,z 
• Introduce a new condition K~ _ \[ } 
into Uk, i. 
• For any \[ps _ \] within XP, introduce 
a new discourse referent u into 
the universe of the DRS Uk÷l. 
• For any other ~_ \] within 
XP, introduce a new discourse 
referent u into K,. 
• Introduce a new condition _(u) 
into Ut.i 
• Substitute u forL _\] XF. ,, 
CR.EQ:Adjunct .... 
Triggering Condition -: 
the condition set _(u) within --Ki where _ is a 
pronoun 
Instructions: 
• Equate u with a discourse referent v that is 
within the universe K~.I that c_ontains K~. 
Conclusion 
The. work reported here argues that allowable 
coreference emerges from how construction 
rules for interpreting different types of noun 
phrases interact dynamically with a structure 
representing the meaning of a discourse. Further, 
it is argued that interpretation of coreferential 
expressions within and between sentences is 
done in a uniform manner, making use of the 
same construction rules and .principles of 
discourse representation. This manner favors the 
use of pronouns compa~d to proper names for 
reference to prominent entities in a discourse. 
The model that is developed - Discourse 
Prominence Theory (DPT) -integrates and 
elaborates on three theoretical sources: 
Discourse Representation Theory (DRT, Kamp 
& Reyle, 1993), Centering Theory (Grosz, et al. 
1995), and the Binding Theory (Chomsky, 
1986). 
DRT is adopted as a formalism because it 
provides explicit mechanisms for mapping 
linguistic input onto semantic representations. 
Further, syntactic forms play an important role 
in the characterization of the linguistic input to 
the DRT construction rules and syntax clearly 
plays an important role in coreference. Finally, 
DRT describes semantic interpretation as an 
incremental process in which the interpretation 
of an utterance involves a dynamic interaction of 
the characteristics of the utterance with the 
discourse universe that represents the meanings 
created from the earlier utterances in the 
discourse. Beyond its usefulness in providing a 
satisfactory semantics, of discourse, this dynamic 
view of language processing makes DRT 
attractive as a framework for characterizing the 
psychological processes of . language 
comprehension which are generally regarded as 
dynamic and incremental. 
87 
Centering Theory provides key theoretical 
notions for understanding how reference and the 
form of referring expressions contribute to 
discourse coherence. DPT directly incorporates 
the idea of a set of forward-looking centers (Cf) 
in order to explain how structural prominence 
affects the interpretation of pronouns and 
repeated full expressions. This incorporation 
extends the role of the Cf so that it plays a role 
in the interpretation of referential expressions 
within an utterance as well as between 
utterances in a discourse segment. The idea of a 
backward-looking center (Cb) is not directly 
incorporated into the representation of discourse 
within DPT. Instead, thc characteristics of the 
Cb emerge from how the model integrates an 
utterance into the current discourse universe and 
thereby linking it to the current discourse 
segment. The preference for realizing the Cb as 
a pronoun occurs because the construction rule 
for pronouns (CR.Pro) interprets a pronoun as 
referring to an entity in the current discourse 
representation and therefore forces integration of 
the utterance into the current discourse 
representation. Absent a clear cue for 
integration, such as a pronoun, there is always 
the possibility that the utterance is the beginning 
of a new discourse segment and should not be 
integrated into the current discourse universe. 
The existence of only a single Cb in an utterance 
occurs because a single pronominal reference is 
sufficient to force the integration of an entire 
utterance into the current discourse universe. 
The Binding Theory contributes to DPT through 
its emphasis on how the syntactic characteristics 
of the antecedent influence patterns of 
coreference. While this is the case, it is obvious 
that the work that we have reviewed is not 
consistent empirically with the generalizations 
typically modeled by the binding theorists. 
Coreference phenomena appear to be simpler 
than. the binding theorists have taken them to be, 
and accordingly the effects of syntactic factors 
on allowable coreference emerge from the 
nature of the mechanisms posited in DPT for 
achieving coreferential interpretations. 
The discrepancies that we observed between the 
empirical characterizations of the Binding 
Theory and those obtained using more 
systematic experimental techniques deserve 
some comment given the current emphasis in 
computational linguistics on appropriate 
empirical methods. We (Gordon & Hendrick, 
1997) found that there were striking differences 
between what was seen as acceptable 
coreference in the linguistics literature and what 
emerged from quantitative analysis of judgments 
that were systematically obtained from subjects 
who were naive to syntactic theory. Further, the 
view of coreferenc¢ that emerged from the 
judgments of naive subjects was consistent with 
online measures of language comprehension and 
with analysis of corpus data. We believe that this 
consistency points to the usefulness of 
considering the kinds of intuitive judgments that 
have traditionally been used in trying to develop 
theories of linguistic competence. However, we 
also believe that it demonstrates the need for 
careful methodology in collecting intuitive 
judgments. Because of substantial variation in 
"i ~-' judgmentsabout coreference, and because of the 
centrality of the theoretical issues at stake, 
claims about the favored or acceptable 
interpretations of linguistic expressions in 
discourse need to be supported by quantitatively 
analyzed data obtained from subjects who are 
naive to the hypotheses being investigated. Data 
on such intuitive judgments can only provide a 
stable, cumulative basis for theoretical 
development if they are collected with such 
methods. 
Acknowledgements 
Preparation of this report was supported by NSF 
grants SBR-98(YT0"28 and IIS-9811129. 

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