Resolving Zero Anaphora in Japanese 
Tadashi Nomoto and Yoshihiko Nitta 
Advanced Research Laboratory, Hitachi Ltd. 
2520, Hatoyama, Saitama 350-03, JAPAN 
E-mail: {nomoto, nitta}@harl.hitachi.co.jp 
Tei.+81-492-96-6111 Fax. +81-492-96-6006 
Abstract 
The paper presents a computational theory 
for resolving Japanese zero anaphora, based 
on the notion of discourse segment. We see 
that the discourse segment reduces the do- 
main of antecedents for zero anaphora and 
thus leads to their efficient resolution. 
Also we make crucial use of functional no- 
tions such as empathy hierarchy and mini- 
mal semantics thesis to resolve reference for 
zero anaphora \[Kuno, 1987\]. Our al)proach 
differs from the Centering analysis \[Walker 
et al., 1990\] in that the resolution works 
by matching one empathy hierarchy against 
another, which makes it possible to deal 
with discourses with no explicit topic and 
those with cataphora \[Halliday and Hassan, 
1990\]. 
The theory is formalized through the 
definite clause grammar(DCG) formalism 
\[Pereira and Warren, 1980\],\[Gazdar and 
Mellish, 1989; Longacre, 1979\]. 
Finally, we show that graphology i.e., quo- 
tation mark, spacing, has an important ef- 
fect on the interpretation of zero anaphora 
in Japanese discourse. 
1 Introduction 
Over the past years, schemes like Focusing and Cen- 
tering have dominated computational approaches 
to resolving anaphora \[Sidner, 1983; Walker et al., 
1990\]. Their success derives from the utility they 
have in identifying salient discourse entities such as 
topic and thereby locating the antecedent for an 
anaphor. But they all suffer from the problem of 
directionality; they process the text (the list of sen- 
tences) from left to right, picking out focus along 
the way and see if a anaphor corefers with a focus 
already encountered. With the one-way processing, 
forward-looking pronouns (cataphora) are not pos- 
sible to resolve. Since Japanese has great tolerance 
with forward reference, a proper theory of zero pro- 
nouns should meet the problem of directionality. 
In what follows, we discuss some points about dis- 
course segment and zero pronoun in Japanese. We 
begin by introducing the idea of discourse segment. 
Consider the pair: 
(1) Taro-go sara<i>-wo dasi, Hanako 
nora plate ace prepare-and 
-go 02<i> ryori -wo morituketa. 
nora food acc arranged 
Taro prepared the plates, Hanako arranged food 
on them. 
(2) Taro -ga sara<~> -wo dasi, Hanako<i> -wa 
top 
01<i> 02<k> ryori-wo morituketa. 
Taro prepared the plates, Hanako arranged food. 
Here, 02 represents a suppressed expression. It acts 
as an indirect object of the verb moritsuketa. 1 1 
and 2 are morphologically identical except that 1 
has ga (nominative marker) where 2 has wa (topic 
marker). But they differ widely in meaning:l im- 
plies that Hanako arranged food on the plates that 
Taro prepared, the reading 2 does not imply; in 2, 
1Here and throughout, we intend the term 01 to rep- 
resent a zero pronoun for the subject, 02 for the indirect 
object, and 03 for the direct object. 
315 
Hanako could have arranged food on plates some- 
body other than Taro prepared. Now locating the 
difference will involve the notion of discourse seg- 
ment. A discourse segment is defined as a set of 
sentences which appear in some region of text and 
which is delimited by a topic-particle wa. Thus 2 
breaks up into two segments, a clause with Taro-ga 
and one with Hanako-wa;1, containing no wa-marked 
element, forms a segment by itself. Section 2.1 pro- 
vides syntactic definitions for the discourse segment. 
Another important feature of discourse segment is 
that of complying with the Minimal Semantics The- 
sis (MST) \[Nomoto, 1992\], a functional property that 
makes a segment cohere. The MST says, 'Assume 
as identical any pair of zero pronouns if it is part 
of some segment and does not occur as arguments 
for the segment's predicate.' Thus any pair of zero 
pronouns that fall into the domain of discourse seg- 
ment are taken to be coreferential, unless they occur 
for the same predicate. 2 Significantly, the MST is 
amenable to syntactic treatment. 
In addition, we make use of ~he empathy hierarchy 
to choose between coreference relationships admitted 
by the MST. We specify a predicate for the empathy 
hierarchy and resolve zero anaphora by unifying one 
predicate's empathy hierarchy with another which 
occurs in the same segment. Since unification is a 
non-directional operation, we are able to treat for- 
ward as well as backward reference. 
2 Theory 
2.1 General 
A discourse segment (DS) is a two-part structure 
consisting of head and body; a head is a nominal 
with a wa marking; a body is a set of sentences, which 
end with a period. Note that an adjunctive clause 
is not a sentence here, since it ends with connectives 
like .node because, .kara because/after, .to and-then, 
etc. Formally, we assume sentence has the following 
analyses, which are given in the DCG formalism. 
(3) S -> C+, N(pp:1~a). 
S -> C*, N(pp:~a) ,C+. 
S -> C+. 
c+ denotes one or more occurrences of clause, C* zero 
or more occurrences of clause, and N (pp : wa) denotes 
a wa-marked nominal;pp:wa specifies that the at- 
tribute pp (for postposition) has wa for the value.3Let 
us define discourse segment by: 
2 \[Hobbs, to appear\] talks about the cognitive economy 
in understanding discourse: it says in effect that coher- 
ence is the result of minimizing the number of entities in 
discourse. 
3We take a wa-marked nominal to be a sentence adver- 
bial. Thus our approach differs from the tiaditional gap 
analysis of topic construction \[Kuroda, 1965; Inoue, 1978; 
Kitagawa, 1982; Gunji, 1987\], which assumes that a wa- 
(4) D -> S+. 
and text by 
(5) T -> D+. 
As discussed in section 1, we choose to restrict D to 
containing at most one ~1 (pp:wa). We implement the 
restriction by way of some additions to the rule set 
3. 
(6) a S(head:X) -> C+, N(morph:X,pp:wa). 
b S(head:X) -> C*, N(morph:X,pp:,a), 
C+. 
Here, the 6 rule takes care of inverted sentence and 
the 6 rule non-inverted sentence. The rule set 6 
enforces unification between the head value and the 
morph value, morph represents the morphology of the 
nominal; thus morph: taro specifies that the associ- 
ated nominal has the morphology "taro". 
Notice that unification fails on a multiply headed 
segment. A head attribute, once instantiated to 
some value, will never unify with another. Unifi- 
cation, therefore, acts to limit each segment in the 
discourse to a single head. Note also that an non- 
headed discourse, that is, discourse with no headed 
segments, has a legitimate DS analysis, for unifica- 
tion is possible between empty heads. The following 
lists the rules for DS Grammar. 
(7) T -> D+(head:_). 
D(head:X) -> S+(head:X). 
S(head:X) -> C+,N(morph:X,pp:wa). 
S(head:X) -> C*,N(morph:X,pp:wa),C+. 
S(head:_) -> C+. 
2.2 Headed vs. Non-Headed Discourse 
The discourse can he perfectly intelligible without 
an explicit topic or wa-nominal, which implies that 
a discourse segment may not be headed at all. It 
appears, however, that a discourse segment always 
comes out headed except when there is no head avail- 
able in the text. In fact, a segment associates with 
a head nominal regardless of where it occurs in that 
segment. 
(8) Taro<i> -wa 01<i> 02<j> seki -we uzutte 
top seat acc give 
-ageta node, 01<i> 02<j> orei -we 
help because thank 
iwareta. Ol<i> chotto terekusa katta. 
say pass slightly embarrased cop 
nominal is dislocated from the sentence and leaves a gap 
behind. In fact the analysis meets some difficulty in ac- 
counting for the wa-nominal having semantic control over 
a set of period-marked sentences. cf. \[Mikami, 1960\]. 
Ours, however, is free from the problem, as we see below. 
316 
Because Taro gave him/her a favor of giving a 
seat, he/she thanked Taro, who was slightly em. 
barrassed. 
(9) 01<i> 02<j> seki-wo uzutte-ageta-node, 
Taro<i> -wa 01<i> 02<j> orei-wo iwareta. 
01<i> chotto terekusak -atta. 
Because Taro gave him/her a favor of giving 
a seat, he/she thanked Taro, who was slightly 
embarrassed. 
(10) 01<i> 02<j> seki-wo uzut-te-ageta-node, 
01<i> 02<j> orei-wo iwareta. Taro<i> -wa 
01<i> chotto terekusak -attn. 
Because Taro gave him/her a favor o/ giving 
a seat, he/she thanked Taro, who was slighau 
embarrassed. 
8, 9 and 10 each constitute a discourse segment 
headed with Taro. 4 A discourse can be acceptable 
without any head at all: 
(11) 01<i> 02<j> seki wo uzutte ageta node, 
seat ace give favor because 
01</> 02<j> orei -wo iwar eta. 01<i> 
thanks ace say pass 
chotto terekusa katta 
slightly embarassed cop 
Because he/she gave him/her a favor of giving a 
seat, he/she thanked him/her, who was slightly 
embarrassed. 
The speaker of 11, or watashi I would be the most 
likely antecedent for the elided subjects here; who- 
ever gave the favor was thanked for the kindness. 
Let us say that a discourse is headed if each of its 
segments is headed, and non-headed, otherwise. Our 
assumption is that a discourse is either headed or 
non-headed, and not both (e.g. figure 1, figure 2). 5 
Formally, this will be expressed through the value 
for the head attribute. 
(12) T -> D(head:empty). 
An empty-headed discourse expands into one seg- 
ment; its head value will be inherited by each of the 
S-trees down below. Note that unification fails on 
4The Centeringalgorithm is not equipped to deal with 
cases like 9 and 10, where the backward-looking center 
Taro refers back to an item in the previous discourse. 
sit is interesting to note that a multiple-head dis- 
course may reduce to a single-head discourse. This hap- 
pens when discourse segments (DS) for a discourse, share 
an identical head, say, Taro and head-unifies with each 
other. In fact, such a reduction is linguistically possible 
and attested everywhere. Our guess is that a repeated 
use of the same wa-phrase may help the reader to keep 
track of a coreferent for zero anaphora. 
T /\ 
D D 
I I 
sl 
Figure 1: Unacceptable DS-tree. "S O" denotes a sen- 
tence with a wa-marked nominal. 
T I 
D /\ 
sl 
Figure 2: Acceptable DS-tree 
the head value if any of the S's should be headed 
and thus specified for the head attribute. 
The following rule takes care of headed construc- 
tions. 
(13) T -> D+(head:.). 
The rule says that each of the segments has a non- 
null specification for the head attribute. 
2.3 Minimal Semantics Thesis 
Minimal Semantics Thesis (MST) concerns the way 
zero pronouns are interpreted in the discourse seg- 
ment; it involves an empirical claim that the seg- 
ment's zeros are coreferential unless considerations 
on the empathy hierarchy (section 2.4) dictate to the 
contrary. 
(14) Kono ryori<i> wa saishoni 01<i> mizu 
this food acc first water 
wo irete kudasai. Tugini 01<i> sio 
acc pour in imperative next salt 
wo hurimasu. 5 hun sitekara, 01<i> 
ace put-in min. after passing 
niku wo iremasu. 
meat ace add 
As for this food, first pour in some water. Then 
put in salt. Add meat after 5 rain. 
We see that 14 constitutes a single discourse segment. 
According to the minimal semantics thesis, all of the 
zeros in the segment are interpreted as coreferential, 
which is consistent with the reading we have for the 
example. Here is a more complex discourse. 
317 
(15) Taro-wa 01<i> machi-niitte, 01<i> huku 
top town to go cloth 
-wokatta. Masako<j> -wa01<k> sono 
acc bought top that 
huku -wo tanjyobi -ni moratte, 01<k> 
cloth acc birthday on got 
totemo yoroko -n'da. 
much rejoice past 
Taro went downtown to buy a clothing. Masako 
got it for her birthday present and she was very 
happy. 
The first two zeros refer to Taro and the last two refer 
to Masako. But this is exactly what the MST pre- 
dicts; 15 breaks up into two discourse segments, one 
that starts with Taro-wa and the other that starts 
with Masako-wa, so zeros for each segment become 
coreferential. 
2.4 Empathy Hierarchy 
It appears to be a fact about Japanese that the 
speaker of an utterance empathizes or identifies more 
with the subject than with the indirect object; and 
more with the indirect object than with the direct 
object \[Kuno, 1987; Kuno and Kaburaki, 1977\]. In 
fact, there are predicates in Japanese which are lexi- 
tally specified to take an empathy-loaded argument; 
yaru give and kureru receive are two such. For yaru, 
the speaker empathizes with the subject, hut with 
the indirect object, in the case of kureru. 
The relevance of the speaker's empathy to the reso- 
lution problem is that an empathized entity becomes 
more salient than other elements in the discourse 
and thus more likely to act as the antecedent for 
an anaphor. 
(16) Taro-ga Masako<j> -ni hon -wo katte 
nom to book acc buy 
-kureta. Imademo 01<i> sono hon -wo 
helped still that book acc 
daijini siteiru. 
care keep 
Taro gave Masako a favor in buying her a book. 
She still keeps it with care. 
In 16, 01, subject of the second sentence, corders 
with the indirect object Masako in the first sen- 
tence, which is assigned empathy by virtue of the 
verb kureta. 
Formally, we define the empathy hierarchy as a 
function with three arguments. 6 
empathy(Z1, Z2, Z3) 
6The definition is based on the observation that 
Japanese predicates take no more than three argument 
roles. 
With the definition at hand, we are able to formulate 
the lexical specification for kureru: 
V(empathy(hrg2, Argl, Arg3), 
subject : hrgl, obj ect2 : Arg2, 
object :Arg3) -> \[kureru\]. 
yaru has the formulation like the following: 
V(empathy(hrgl, Arg2, hrg3), 
subj oct : hrgl, obj ect2: Arg2, 
object :Arg3) -> \[yarun\]. 
Further, let us assume that variables in the em- 
pathy hierarchy represent zero pronouns. If a vari- 
able in the hierarchy is instantiated to some non-zero 
item, we will remove the variable from the hierarchy 
and move the items following by one _position to the 
left; we might call it empathy shifting/ Now consider 
the discourse: 
(17) 01</> 02<i> hon -wo yatta -node, 
book acc favored because 
01<k> 02<a> orei -wo iwareta. 
gratitude ace say cop 
'Because he/she gave a book to him/her, he/she 
was thanked for it.' 
(18) a empathy(01<i>, 02<j>, _) 
b empathy(01<k>, 02<9 >, _) 
18(1) corresponds to the empathy hierarchy for the 
first clause in 17; 18(b) corresponds to the hierarchy 
for the second clause. Unifying the two structures 
gives us the correct result: namely, 01<i> - 01<k>, 
and 02<i> = 02<9 >. Notice that zero items in the 
segment are all unified through the empathy hierar- 
chy, which in effect realizes the Minimal Semantics 
Thesis. As it turns out, the MST reduces the number 
of semantically distinct zero pronouns for a discourse 
segment to at most three (figure 3). We conclude the 
section with a listing of the relevant DCG rules. 
/ 
S (em~Z3) ) S (em~Z3) ) 
Figure 3: 
D(head:X) -> S+(head:X.empathy(Z1.Z2,Z3)). 
S(head:X.empathy(Zl,Z2.Z3)) -> 
C+(empathy(Z1. Z2.Z3)), 
N(morph:X.pp:,a). 
S(head:X,empathy(ZI,Z2.Z3)) -> 
C*(empathy(ZI,Z2,Z3)), 
rThe empathy hierarchy here deals only with pronoun 
variables; we do not want two constant terms unifying via 
the hierarchy - which is doomed to failure. 
318 
N(morph:X,pp:wa), C+(empathy(Zl,Z2,Z3)). 
3 T-structure in Discourse 
3.1 Embedding and Interleaving 
In this section, we will illustrate some of the ways in 
which T-structure figures in Japanese discourse, s 
What we have below is a father talking about the 
health of his children. 
Chichioya<i> -wa 01<i> warat -te, 
father top laugh and 
~Taxo<h>-wa yoku kaze -wo hiku -n'desuyo. 
Taro top often cold acc catch aux-polite 
Kinou -mo 01<t> kaze -wo hi'ire, 01<k> 
yesterday also cold acc catch 
gakko -wo yasu -n'da-n'desuyo. 
school acc take leave past aux-pollte 
Masako<j> -wa 01<./> gen'ldde, Ol<j> kaze 
top healthy cold 
-wo hi'ita koto -ga arimas en. 
acc caught experiende nora occur aux-neg 
01<j> itsumo sotode ason'de -imasuyo." 
often outdoors play aux-polite 
-to Ol<i> itta. 
comp said 
"Taro often catches a cold. He got one 
yesterday again and didn't go to school. 
Masako stays in a good health and has never 
been sick with fin. I often see her playing 
outdoors." Father said with a smile on his 
face. 
Here are the facts:(a) zero anaphora occurring within 
the quotation (internal anaphora) are coreferential 
either with Taro or with Masako; (b) those occurring 
outside (external anaphora), however, all refer to chi- 
chioya; (c) chichioya has an anaphoric link which 
crosses over the entire quotation; (d) syntactically, 
the quoted portion functions as a complement for 
the verb -to itta. It appears, moreover, that an in- 
ternal anaphor associates itself with Taro in case it 
occurs in the segment headed with Taro, and with 
Masako in case it occurs in the segment headed with 
Masako. Then, since the quoted discourse consists of 
a set of discourse segments, it will be assigned to a 
T-structure. But the structure does not extend over 
the part 01 itta, which completes the discourse, for 
the 01 corders with chichioya, and neither with Taro 
or Masako. This would give us an analysis like one 
in figure 4. 
S Here and below we call a tree rooted at T a 'T- 
structure' and one rooted at D a 'D-structure'. 
T 
Figure 4: embedding 
The following discourse shows that the T-structure 
can be discontinuous: 
\[a\] ~Masako<i> -ga kinou sigoto-wo 
nora yesterday work acc 
yasun'da -n'desuyo." \[b\] Hahaoya<k> -wa 
took leave aux-polite mother nora 
01<h> isu -ni suwaru -to 01<t> hanashi 
chair on sit when tell 
hazimeta \[c\] "Kaze-demo 01<i> hi'ita -nolm." 
began, cold acc caught question 
\[d\]-to Chichioya-ga 03<k> tazuneta. 
comp father nom asked 
"Masako took a leave from the work yester- 
day.', Mother began to tell, as she sat on 
the chair. "Did she catch a cold f ", asked 
Father. 
01<i> corders with Masako, so \[c\] forms a T- 
structure with \[a\]. But the two are separated by 
a narrative \[b\]. Similarly, the coreference between 
03<k> and Hahaoya gives rise to a T-structure that 
spans \[d\] and \[b\], but there is an interruption by nar- 
rative \[c\] (figure 5). 
TTT 
Figure 5: interleaving 
3.2 Problem 
There is a curious interaction between a paragraph- 
break and a T/D-structure. \[Fujisawa et al., 
1993\], for instance, observes a strong tendency that 
Japanese zero anaphora are paragraph-bounded. 
The following is from Nihon Keizai Shinbun, a 
Japanese economics newspaper. 
Kawamata Hideo<i>. 01<i> Sagami tetsudo 
Mr. H. Kawamata Sagam/ Railways 
kaichou. \[San-gatsu\] mik-ka gozen juichi-ji 
chairman March 3rd day a.m. 11-hour 
nijusan-pun, kokyuhuzen no-tame 
23-mlnute respiratory insufficiency due-to 
319 
Tokyo Machida de 01<i> sikyo, 01<i> nanajugo 
Tokyo Machida in dies 75 
-Sai. 
yrs. old 
Tanaka Yutaka<k>. 01<k> Moto- Matsushita 
Mr. Y. Tanaka former Matsushita 
tsuushin kogyo senmu. \[San-gatsu\] 
telecom industries exective director March 
mik-ka gozen yo-ji san-pun, sin-huzen 
3rd day a.m. 4-hour 3-mlnute cardiac failure 
no-tame Yokohama Midoriku de 01<k> sikyo, 
due-to Yokohama Midoriku in dies 
Ol<k> rokujuhas-sai. 
68 yrs. old 
Mr. H. Kawamata, 75, chairman of 
Sagami-Railways, died of respiratory insuf- 
ficiency at 11:23 a.m., in Machida, Tokyo, 
March 3. 
Mr. Y. Tanaka, 68, former executive direc- 
tor of Matsushila telecom industries, died 
of cardiac failure at 4:03 a.m., in Midoriku, 
Yokohama, March 3. 
\[Zero-anaphora are made explicit here for expository 
purposes; they are not part of the newspaper. The 
rest appears as it does in the paper.\] From the way 
same-index anaphora are distributed over the dis- 
course, it seems rather obvious that a paragraph 
break has an effect of marking a segment for the 
discourse. 9 The present theory, however, fails to deal 
with the situation like this; it simply assigns a single 
DS structure to the discourse in question, giving a 
wrong interpretation that zero anaphora present are 
all coreferential. As it stands, nothing in the theory 
provides for treating graphological marks such as a 
paragraph break. Yet, it is unclear to us whether a 
paragraph break is a signal for a I"- or D-structure. 
4 Conclusion 
We have developed a computational theory for re- 
solving zero anaphora in Japanese, drawing on the 
results from previous works on Japanese discourse 
\[Kuno, 1987; Kuno and Kaburaki, 1977\], etc). A ma- 
jor departure from the traditional analyses of zero 
anaphora lies in the reduction of the space of an- 
tecedents for zero anaphora. This has been made 
possible by adopting ideas like Discourse Segment, 
Minimal Semantics Thesis and Empathy Hierarchy. 
In particular, we have shown that the Minimal Se- 
mantics Thesis leads to reducing the number of an- 
tecedents for a segment to at most three. Also shown 
in the paper is that the resolution of zero anaphora 
is part of parsing text, so no additional mechanism is 
9 We may note that a recursive embedding of discourse 
of the sort we have discussed above is effected through 
the explicit use of quotation marks; their absence would 
lead to the outright nngrammaticality. 
needed. Furthermore, the present theory compares 
favorably with the previous schemes like Focusing 
and Centering in that it is able to deal with forward- 
and backward-looking anaphora by virtue of the way 
unification operates on the empathy hierarchy. 
Part of our discussion has touched on the effect of 
graphology on the semantics of discourse. To date, 
no significant research has been done on that area 
of academic interests. The literature suggest that in 
the written language, texts, i.e., cohesive discourses, 
are marked through a variety of linguistic and non- 
linguistic means: non-alphanumeric characters (quo- 
tation marks, brackets, parentheses), graphic devices 
t indentation, tabulation, itemization), and so on Nunberg, 1990; Halliday and I-Iassan, 1990\]. Thus 
a discourse segment might qualify for the texthood 
since it has the property that zero pronouns are re- 
solved internally. Its indicator is, of course, the topic 
particle wa. But for the T-structure, it is far from 
clear whether it is anyway cohesive, and if it is, what 
its indicators are. (Quotation mark and paragraph 
break are possible candidates.) 
Some of the technical as well as linguistic details 
are yet to be worked out; we have not talked about 
how the topic comes to be associated with one or 
more zero pronouns in the segment. Considerations 
on empathy may well influence the choice of pro- 
nouns to be matched with. 
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