REMARKS ON PLURAL ANAPHORA* 
Carola Eschenbach, Christopher Habel, Michael Herweg, Klaus Rehk/imper 
Universit~it Hamburg, Fachbereich Informatik, Projekt GAP 
Bodenstedtstr. 16 
D-2000 Hamburg 50 
e-mail: HABEL at DHHLILOG.BITNET 
ABSTRACT 
The interpretation of plural anaphora 
often requires the construction of complex 
reference objects (RefOs) out of RefOs 
which were formerly introduced not by 
plural terms but by a number of singular 
terms only. Often, several complex RefOs 
can be constructed, but only one of them is 
the preferred referent for the plural anaphor 
in question. As a means of explanation for 
preferred and non-preferred interpretations 
of plural anaphora, the concept of a Com- 
mon Association Basis (CAB) for the 
potential atomic parts of a complex object is 
introduced in the following. CABs pose 
conceptual constraints on the formation of 
complex RefOs in general. We argue that in 
cases where a suitable CAB for the atomic 
RefOs introduced in the text exists, the cor- 
responding complex RefO is constructed as 
early as in the course of processing the ante- 
cedent sentence and put into the focus 
domain of the discourse model. Thus, the 
search for a referent for a plural anaphor is 
constrained to a limited domain of RefOs 
according to the general principles of focus 
theory in NLP. Further principles of inter- 
pretation are suggested which guide the 
resolution of plural anaphora in cases where 
more than one suitable complex RefO is in 
focus. 
* The research on this paper was 
supported in part by the Deutsche 
Forschungsgemeinschaft (DFG) under grant 
Ha 1237/2-1. GAP is the acronym for 
"Gruppierungs- und Abgrenzungsgrozesse 
beim Aufbau sprachlich angeregter mentaler 
Modelle" (Processes of grouping and 
separation in the construction of mental 
models from texts), a research project 
carried out in the DFG-program "Kognitive 
Linguistik". 
1. INTRODUCTION 
Most approaches to processing anaphora 
concern themselves mainly with the case of 
singulars and deal only peripherally with the 
complications of plurals. An analysis of 
plural anaphora should answer the following 
additional questions: 
1) How are the referents of plural terms 
represented by discourse entities (internal 
proxies)? 
2) How is the link between plural anaphora 
and suitable antecedent discourse entities 
established? 
3) How are complex discourse entities con- 
structed from atomic ones? 
4) When are complex discourse entities 
constructed in the process of text com- 
prehension? 
The present paper addresses primarily the 
third and fourth questions. However, we 
will give some sketchy answers to the first 
and second questions as well. 
We consider only two-sentence texts in 
which the second sentence contains an 
anaphoric pronoun that refers to entities 
introduced in the first sentence by various 
constructions: 
(1) a. The children were at the cinema. 
They had a great time. 
b. Michael and Maria were at the cinema. 
They had a great time. 
c. Michael was at the cinema with Maria. 
They had a great time. 
d. Michael met Maria at the cinema. 
They had a great time. 
The question is: To which entities, i.e. 
complex discourse entities, does the plural 
anaphor th_h~ refer? Surely in (1.a) to the one 
corresponding to the children, and in (1.b), 
(1.c) and (1.d) to Michael and Maria. Up to 
now, most analyses of plural anaphora 
- 161- 
investigate cases of the (1.a)- or (1.b)-type, 
i.e. those in which the complex object is in- 
troduced explicitly, either by a simple plural 
NP or by a conjunction of singular or plural 
NPs (which in both cases yields a plural NP 
as well). 
2. A SKETCH ON PLURALITY 
We assume -as is common in most 
recent approaches to anaphora in AI and 
linguistic semantics (e.g. Webber 1979, 
Kamp 1984)- a representation level of 
discourse referents, which are internal 
proxies of objects of the real (or a possible 
or fictional) world. These discourse entities, 
called reference objects (RefOs), are stored 
and processed in a net-like structure, called a 
referential net (RefN), which links RefOs 
and designations. (For a detailed description 
see Habel 1982, 1986a, 1986b and Eschen- 
bach 1988.) The term "RefO" is, when 
strictly used, a technical notion which is 
employed in the framework of our formal- 
ism only. For reasons of simplicity of expo- 
sition, we do not want to restrict the use of 
"RefO" to this formalism in the present 
paper, but rather apply the term to referents 
also, i.e. the objects to which names, 
descriptions and pronouns refer. 
RefOs for complex objects are con- 
structed by means of a sum operation (Link 
1983), so that with respect to (1.b), we have 
the following entries (among others) in the 
RefN. 
rl Michael 
r2 Maria 
r3 = rl • r2 
The sum operation (symbolized by ~) is 
the semantic counterpart of the NP-connec- 
tive and. It defines a semi-lattice (Link 1983, 
Eschenbach 1988). By means of this struc- 
ture, both complex and atomic RefOs can be 
seen as objects of the same logical type and 
are accessible by the same set of referential 
processes. No operations on RefOs other 
than the sum operation will be considered in 
the present context. 
3. CONSTRAINTS ON SUM FOR- 
MATION 
Sentences like (1.a) and (1.b) demon- 
strate that complex discourse referents can 
be created by plural NPs. But there are other 
linguistic indicators for the creation of com- 
plex RefOs.1 The anaphoric pronoun they of 
(1.c) and (1.d) as well as (1.b) refers to a 
corresponding complex RefO. It is obvious 
that besides conjunctions (e.g. and), some 
prepositions and verbs trigger processes of 
sum formation (with-PPs and meet are out- 
standing examples of these types of con- 
structions.) In (1.c), Michael ... with Maria 
triggers the formation of Michael ~ Mafia. 
But consider the following texts: 
(2) a. Michael and Mafia were at the park 
with Peter. 
In the evening they were at a garden 
party. 
b. Michael and Mafia were at the park 
with their frisbee. 
In the evening they were at a garden 
party. 
In (2.a) it is possible that they refers to 
Michael ~ Maria ~ Peter. But in (2.b) they 
is preferably linked to Michael ~ Maria; 
even if Michael and Mafia happened to take 
their frisbee to the garden party, we would 
not want to claim that the plural anaphor they 
in (2.b) refers to a complex discourse entity 
consisting of Michael, Mafia and the frisbee. 
In the preferred reading 6f (2.b), the frisbee 
is excluded from the antecedent of the 
anaphor. 
We have to explain why with-PPs only 
cause sum formation in certain cases. The 
proposed solution to this problem is the 
concept of a Common Association Basis 
(CAB), which is introduced in Herweg 
(1988). The CAB is an extension of the 
Common Integrator (CI), which Lang 
(1984) developed in his general theory of 
coordinate conjunction structures. 
1 The assumption of indicators and 
constraints contrasts to the less restrictive 
assumption of Frey & Kamp's (1986) DRT- 
oriented analysis of plural anaphora, in 
which they claim that "any collection of 
available reference markers, whether 
singular or plural, can be 'joined together' to 
yield the antecedent with which the pronoun 
can be connected" (p. 18). 
- 162 - 
Grouping by with depends on the condi- 
tion that "x with y" leads to "x ~ y" only in 
those cases in which a CAB-relation is ful- 
filled. The most relevant constraint given by 
CAB is the condition that x and y are 
instances of the same ontological type at the 
most fine-grained level. This means two 
humans are good candidates to form a com- 
plex RefO, whereas a frisbee, which does 
not fall under the ontological type of humans 
or animate objects, and the human players 
are not. 
CAB constraints apply not only to cases 
like (1.c) and (2.b), but to sum formation in 
general. Consider this example: 
(3) Michael and his frisbee were at the 
park. 
Here the conjunction explicitly forces the 
sum formation of objects of different onto- 
logical types. This is at least unusual and has 
a strange effect. However, explicit conjunc- 
tion by ~nd presupposes the existence of a 
suitable CAB for the conjoined entities. The 
addressee must assume that the conjunction 
in (3) involves an instruction to derive such 
a CAB (or simply concede that one exists). 
Thus, to make conjunctions like the one in 
(3) acceptable and natural, one normally has 
to assume a CAB which is not explicitly 
specified or immedeatly derivable from the 
information conveyed in the sentence itself 
but which is given by the preceding or extra- 
linguistic context. In (3), the required CAB 
might simply be something like 'the entities 
desperately being looked for by Michael's 
children'. In isolation however, forced sum 
formations like the one in (3) must be con- 
sidered marginally acceptable. 
We now have the following situation: 
Grouping depends on properties of the 
RefOs in question, namely whether a CAB 
exists which constitutes a conceptual relation 
among the RefOs with respect to situational 
parameters given, for example, by predica- 
tive concepts. Furthermore, it is obvious that 
world knowledge and the theme of the 
discourse give evidence for which (complex) 
RefO is most appropriate as the antecedent 
of an anaphoric pronoun. We will propose 
that these factors can be handled by CABs as 
well. 
This leads us to Herweg's (1988) Princi- 
ple of Connectedness: 
All sub-RefOs of a complex RefO must 
be related by a CAB. 
Now consider example (1.d). It shows 
that some lexical concepts possess what we 
call grouping force, i.e. they trigger sum 
formation with respect to atomic RefOs. The 
grouping force of a lexical concept can be 
seen as a special case of a CAB. Without 
going into details of the representation 
formalism we can formulate the relevant sum 
formation processes by this rule: 
If "x meets y", then construct the com- 
plex RefO x ~ y. 
The status of this sum formation rule is 
similar to that of classical inference rules, 
which are used for bridging processes in the 
sense of Clark (1975). Not all verbs possess 
a grouping force as strong as meet; e.g. the 
grouping force of watch is considerably 
lower. Consider: 
(4) a. Michael met Peter and Maria in the 
pub. They had a great time. 
b. Michael watched Peter and Maria in 
the pub. They had a great time. 
In (4.b), the sum of Maria and Peter is 
significantly preferred to the sum including 
Michael as the antecedent of they. In (4.a), 
there presumably is a preference to the 
opposite, i.e. to link they to the sum con- 
sisting of all three persons. In contrast to 
highly associative verbal concepts like meet, 
watch must be classified as a dissociative 
element which does not constitute a CAB for 
its arguments but induces a conceptual sepa- 
ration. Part of the explanation for this prop- 
erty of watch is to be seen in the (normally 
understood) local separation of subject and 
object in the situation described. Again in 
contrast to meet, this local separation usually 
prevents an interaction or some other kind of 
contact which allows one to assume a suit- 
able link (i.e. a CAB) for the persons intro- 
duced based on properties of the situation 
which the sentence describes. 
4. ANAPHORA RESOLUTION AS 
A SEARCH PROCESS? 
Many classical approaches to anaphora 
resolution are based on search processes. 
- 163 - 
Given an anaphor, a set of explicitly intro- 
duced referents is searched for the best 
choice. 2 The crucial point is: "How to deter- 
mine the set of possible antecedents?" 
The most simple solution is the history 
list "of all referents mentioned in the last 
several sentences" (Allen 1987, p. 343). 
Note that most DRT-based anaphora resolu- 
tion processes (Kamp 1984, Frey & Kamp 
1986) by and large follow this line, with a 
few modifications concerning structural 
conditions in terms of an accessibility rela- 
tion. 
But there is also a different perspective 
whose key notion is the well-established 
concept of focus (see e.g. in Computational 
Linguistics Grosz & Sidner 1986) 3. As is 
shown by psychological experiments (an 
detailed overview is given by Guindon 
1985), a very limited number of discourse 
referents are focussed. Referents in the 
focus, which can be described in psycho- 
logical terms as short term memory (see 
Guindon), are quickly accessed; especially 
pronouns are normally used to refer to items 
in the focus and therefore extensive search is 
mostly unnecessary. The most relevant 
question with respect to focus is "Which 
items are currently in the focus? ''4. Answers 
2 Note that the unspecifity of pronouns 
seldom allows the triggering of bridging 
inferences (see Clark 1975) to select 
referents which are only implicitly 
introduced. 
3 Cf. Bosch (1987) and Allen (1987; 
chap. 14). Both give convincing arguments 
against the simplistic view of identifying 
anaphora resolution with searching. Since 
we address matters of pronominal anaphora 
only, we here assume a rather simple 
concept of focus. Further differentiations 
(e.g. Garrod & Sanford's (1982) division of 
focus into an explicit and implicit 
component) which might become necessary 
if non-pronominal anaphora are investigated 
as well are out of the scope of the present 
paper. 
4 A question closely related to this, 
namely at which point of time and in what 
to this question determine which referents 
can be antecedents of pronouns. 
5. PLURALS IN FOCUS 
Following the line of argumentation in 
section 4, the possibility of a reference to a 
complex RefO with a plural pronoun as in 
(1) means that such a complex RefO is in the 
focus after processing the first sentence. 
Thus it is worth taking a closer look at the 
question as to when a complex RefO is 
formed. There are essentially two opportu- 
nities to construct a complex RefO from 
atomic RefOs: it can be constructed and put 
into the focus when the atomic RefOs are 
mentioned, or the construction might be 
suspended until an anaphor triggers the sum 
formation. 5 The second solution has some 
undesirable consequences; the worst is that 
the methods of resolving plural anaphora 
and singular anaphora must be completely 
different. Since the complex RefOs would 
not be in the focus, a direct access to the 
focussed entities could not solve the prob- 
lem. In such cases, the construction process 
would be triggered during anaphora resolu- 
tion. Thus the processing of they with 
respect to Michael (...) with Maria in (1.c) 
and Michael met Maria in (1.d) should be 
more complicated than the cases of the 
children or Michael and Maria, an assump- 
tion for which no evidence exists as yet. 
Therefore, we take the former choice of 
constructing the complex RefO while pro- 
cessing the atomic RefOs. Again, this sug- 
gests two possibilities, namely to construct 
the complex RefO and put only this into the 
focus, or to introduce both the complex and 
the atomic RefOs into the focus. As a 
working hypothesis, we propose the latter 
procedure, since the sentences like (5), 
way the focus is updated, is not relevant as 
long as we confine ourselves to texts 
containing only two sentences. However, it 
becomes important when the analysis is 
expanded to multiple sentence texts. 
5 This distinction corresponds to 
Charniak's (1976; p. 11) well-known 
dichotomy of read-time and question-time 
inferences. 
- 164- 
which contain singular anaphora (cf. (1)), 
are fully coherent: 
(5) a. Michael and Mafia were at the cinema. 
He/She had a great time. 
b. Maria was at the cinema with Michael. 
He/She had a great time. 
c. Michael met Mafia at the cinema. 
He/She had a great time. 
That these findings do not depend on 
linguistic introspection only is established by 
processing-time experiments, which are 
reported in Mtisseler & Rickheit (1989). 6 
The initial results of the experiments suggest 
that the complexities of processing singular 
or plural anaphora (of sentences like (1) vs. 
(5) are not significantly different 7. The 
anaphoric accessibility of the complex RefOs 
which are introduced by the sentences listed 
above is by no means worse than the acces- 
sibilty of the atomic RefOs. 
Let us summarize the discussion so far: 
There are linguistic concepts -such as 
conjunctions, prepositions and lexical con- 
cepts- which trigger the construction of 
complex RefOs. The atomic RefOs as well 
as the complex RefO (which is formed by 
6 Mtisseler's and Rickheit's research at 
the University of Bielefeld is also carded out 
in a project in the DFG-Program "Kognitive 
Linguistik". This project collaborates with 
ours on reference phenomena from 
computational and psycholinguistic points of 
view. 
7 This holds at least for cases where the 
antecedent of the singular anaphor is in 
subject/topic position. Questions concerning 
the accessibility of singular antecedents in 
non-subject/non-topic positions are not 
definitely settled as yet (see Mtisseler & 
Rickheit 1989). Since Mtisseler's and 
Rickheit's experiments are confined to 
German, which has a single form ~ie for 3rd 
pl. pronoun (they) and 3rd sg. fern. pronoun 
(she), not all of their results on the 
processing-time of singular anaphora with 
antecedents in different structural positions 
can be applied to English. 
the sum operation) are introduced into the 
focus. Thus, resolution of anaphora can be 
performed by processes on the focus not 
involving extensive search. 
6. FURTHER PRINCIPLES OF 
ANAPHORA RESOLUTION 
Further interesting problems can be ob- 
served in the interaction of concepts which 
possess grouping capacity. Consider: 
(6) a. Michael and Maria picked up Peter 
and Anne from the station. 
They were happy to see each other 
again. 
b. Michael and Mafia picked up Peter 
and Anne from the station. 
They were late. 
Here the following atomic and complex 
RefOs exist: 
rl - Michael 
r2 - Maria 
r3 Peter 
r4 Anne 
r5 =rl ~r2 
r6 = r3 • r4 
r7 =r5 • r6 = rl • r2 • r3 ~r4 
In the preferred interpretation, they in 
(6.a) refers to r7, in (6.b) either to r5 or r6. It 
follows from this analysis that more than 
one complex RefO can be in focus. Which 
one is the most appropriate to link to the 
pronoun depends on two principles (see 
Herweg 1988): 
Principle of Permanence: 
It is prohibited (unless the text explicitly 
requires it) to link the plural pronoun to a 
proper sub-RefO of a complex RefO in 
focus. Reference to a sub-RefO is only pos- 
sible if it was introduced explicitly into the 
discourse model by a previous inference. 
Principle of Maximality: 
The plural anaphoric pronoun should be 
linked to the maximal sum of appropriate 
RefOs with respect to a suitable CAB, 
unless the text contains explicit evidence to 
the contrary. 
The interaction of the principles of Con- 
nectedness, Permanence and Maximality can 
lead to correct and natural anaphora resolu- 
tion in (6). For (6.a), maximality and per- 
- 165 - 
manence require a maximal sum, which is 
rT; in (6.b), knowledge about the situations 
of picking someone up and being late 
excludes r7 (i.e. no CAB can be established 
which is simultaneously satisfied by all 
atomic parts of r7; therefore, the condition of 
connectedness is not fulfilled) and thus gives 
evidence for a sub-RefO, namely either r5 or 
r6. The principle of Permanence excludes 
other combinations of atomic RefOs, such as 
rl • r3, r2 • r3, etc. Whether r5 or r6 is 
chosen at last can not be decided on the basis 
of the above mentioned principles alone. 
These examples show that a conflict resolu- 
tion strategy is needed, as is not unusual for 
such principles. 
7. IMPLEMENTATION 
The RefN-processes and sum formation 
are currently being implemented in Quintus- 
PROLOG on a MicroVax workstation. The 
present implementation allows one to repre- 
sent and create RefOs and (1) their descrip- 
tions by way of designators (internal proxies 
for names and definite NPs), (2) their de- 
scriptions by way of attributes, which spec- 
ify properties (sorts) of the represented ob- 
jects themselves (not their designations) and 
relations between them. E.g. sums are rep- 
resented by the use of attributes to RefOs. 
The set of RefOs with their descriptions 
can be structured, so that different RefNs, 
whether or not they are independent from 
each other or related by shared RefOs, may 
be represented in parallel. 
The representation of a sample text within 
the formalism is being worked. The transfer 
of segments of the text into simple nets is 
not being done automatically but by hand. 
For each anaphor, a corresponding RefO 
is created but specially marked as an ana- 
phoric RefO. This is intended to trigger the 
automatic resolution of anaphora. 
In the near future, it is planned to 
- determine the potential antecedent-refer- 
ents for an anaphor out of the set of all 
RefOs which are available; 
- define the requirements concerning the 
representation of focus; it is planned to 
test different formats of representation; 
- structure the nets in order to represent 
CABs. 
The function of the last two steps men- 
tioned is to put further restrictions on the set 
of potential antecedent-referents for a given 
anaphor. 
8. SUMMARY 
Compared to the case of singular pro- 
nouns, the resolution of anaphoric plural 
pronouns requires an additional step of pro- 
cessing: the sum formation. It is guided by 
various grammatical and lexical evidence, 
which is accumulated to form a common 
association basis (CAB). The principle of 
connectedness controls the sum formation, 
by which the restriction to a very limited 
number of complex RefOs is possible. The 
role of focus with respect to plural anaphora 
is similar to the singular case, but poses the 
question as to when the sum formation is 
carried out in the process of text compre- 
hension. The resolution processes of the 
singular and plural cases can be made iden- 
tical by assuming that, in cases where a 
suitable CAB is available, the sum formation 
takes place early, i.e. while processing the 
antecedent sentence(s). The principles of 
Permanence and Maximality are two princi- 
ples which are valid especially for plural 
anaphora. 
The use of CABs and the mentioned 
principles of sum formation is a way to 
avoid the inadequacies of prior approaches 
to plural anaphora, which mostly seem to 
follow the motto "Anything goes". 
ACKNOWLEDGEMENTS 
We thank Ewald Lang, Geoff Simmons 
(who also corrected our English) and Andrea 
Schopp for stimulating discussions and three 
anonymous referees from ACL for their 
comments on an earlier version of this 
paper. 
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