Memory & Cognition 1984, 12 (5), 489-497 Establishing and accessing referents in discourse

GREGORY L. MURPHY Brown University, Providence, Rhode Island

Four experiments explored the processes by which people understand references. Subjects read naturalistic stories one sentence at a time, and their reading-comprehension times were record­ ed. The first two experiments investigated differences between definite and indefinite reference. Their results suggest that establishing a new discourse referent (as indicated by an indefinite article or pronoun) produces longer comprehension times than referring to an already existing referent. The last two experiments investigated reference to one versus two discourse referents. They showed that accessing more referents led to longer reading times even when the referring sentences were identical. The results are explained in terms of a discourse model approach to comprehension. According to this view, readers construct a mental model of the discourse do­ main, and the ease of comprehension is related (in part) to the number of elements in the model that are accessed and the operations performed on them.

Reference is one of the primary functions of commu­ and successfully refer to the desired person, even when nication. From the prelinguistic pointing of young chil­ the man is not wearing a cheap suit (perhaps it just looks dren to the use of elaborate descriptions in fluent adults, cheap, or perhaps it is not really a suit). Furthermore, people find it necessary to pick out individual entities or the reference may be successful even if the speaker and groups of entities. It is no surprise, then, that linguists, the hearer both know that the description is false (Clark philosophers, and psychologists have devoted much ef­ & Marshall, 1981). These cases can be handled by the fort to devising an account of reference. This task is a simple fact that people's discourse models can include ob­ difficult one because it requires (eventually) an explana­ jects and information that do not correspond to objects tion of how language corresponds to the world, and that and facts of the real world. Ifthe speaker and listener have correspondence is anything but straightforward. For psy­ similar models, then an incorrect description may still pick chologists, the problem ofunderstanding reference raises out one element sufficiently for comprehension (Clark & issues of memory representations, linguistic processes, Carlson, 1981; Clark & Marshall, 1981; Johnson-Laird perception, and reasoning. & Garnham, 1980). One recent approach to explaining reference has been Another useful result of considering understanding to the use of mental "discourse models." As a listener un­ involve such discourse models is that this approach may derstands some discourse, he or she builds a mental world help explain how people understand potentially ambigu­ that represents the information in the discourse, and refer­ ous, vague, or difficult texts (see Collins, Brown, & Lar­ ence is made by picking out objects in this model of the kin, 1980, and Johnson-Laird, 1981). Because the world. A reference to something that is not in the model discourse model is a sort of mental world, people can use will not be understood, or else will require work on the their real-world knowledge to organize and supplement part ofthe listener to alter the model. Johnson-Laird and it. The use of mental models in problem solving and Garnham (1980) demonstrated that this approach can ac­ reasoning was reviewed by Gentner and Stevens (1983). count not only for "normal" reference, but also for refer­ Much of the support for this approach to language com­ ence to nonexistent objects or to objects that are prehension comes from studies of ambiguous or misdescribed (cases that have posed serious problems for anomalous reference. The experiments described in this some past theories). article focus on simpler cases of "normal" reference. For example, someone might say at a crowded party, In particular, this approach suggests that an important factor in understanding is whether the referent is al­ (l) The man in the cheap suit is a gangster. ready included in the discourse model. In most cases, a definite reference will in fact pick out something already in the model-that is, something that has been mentioned I would like to gratefully acknowledge the advice of Herbert Clark in the discourse or that is present in the context (Clark on these experiments and the helpful comments of Richard Gerrig, Ray­ & Marshall, 1981). Usually, indefinite references in­ mond Gibbs, and Ed Wisniewski on an earlier draft of this paper. This troduce a new entity into the conversation. When the research was supported by a BioMedical Research Grant from Brown listener hears an indefinite article (or no article for plural University and by Grant MH-20021 to Herbert Clark. Requests for reprints may be addressed to Gregory L. Murphy, Hunter Laboratory nouns), he or she can guess that a new entity is being men­ of Psychology, Brown University, Providence, Rl 02912. tioned and that a new element in the discourse model must

489 Copyright 1984 Psychonomic Society, Inc. 490 MURPHY be created. All other things being equal, it should there­ easier and faster to do. In order to test this prediction, fore take longer for listeners to understand indefinite refer­ it is necessary to construct passages that either invoke a ences than to understand definite references. Experiments new element or refer to an old element, with the refer­ 1 and 2 investigated this claim. ring expressions carrying the same amount of informa­ Another implication of this approach is that reference tion. That is, it is important not to confound the to more than one object in the discourse model should complexity ofthe expression or its meaning with the fac­ take longer than reference to a single object. Since the tor of interest. Experiment 1 therefore compared noun entities in the discourse model are in fact separate "ob­ phrases in identical contexts that differed only in the use jects," picking out two or more of them should be more of the definite or indefinite article. time consuming than picking out one, even when the mul­ Experiment 1 used the reading-time paradigm, in which tiple objects are described by a single noun. This hypothe­ subjects read stories, a sentence at a time, and pressed sis was elaborated and investigated in Experiments 3 a button as they understood each sentence. One story con­ and 4. . tained the following sequence in which the article was Although the experiments described here were inspired varied for different subjects: by the discourse-model approach to comprehension, their primary goal was not to evaluate that approach as a whole, (2) Though driving 55, Steve was passed by a truck. but to investigate specific hypotheses about reference. Later, George was passed by a/the truck too. There is no single theory of the use of discourse models in understanding (which is why I have been calling it an Notice that the same actions and properties are predicated "approach," rather than a "theory"), and different in­ in both versions. Furthermore, both versions repeat the stantiations of the approach could be devised to make noun in the second sentence (i.e., although the indefinite different predictions than those I give. These experiments version introduces a new object, it does not introduce a can be thought of as providing data on which more detailed totally new concept). The only difference between the two accounts of understanding can be based in any theoreti­ is whether the actions and properties are predicated of a cal framework, although they are particularly relevant to new object or of one already mentioned, as signaled by the discourse-model approach. the article. There are a number ofalternative theories of reference An alternative hypothesis is that the definite article that may suggest other psychological explanations of how prompts the reader to search memory for the object be­ reference is understood. However, like the discourse­ ing referred to, whereas the indefinite article indicates that model approach, these theories usually do not provide the a new object is being mentioned and thus no such search necessary detail to make clear predictions for on-line is necessary (Irwin, Bock, & Stanovich, 1982). There­ measures of comprehension. Rather than describing al­ fore, indefinite reference should be easier to comprehend. ternative approaches here, I consider more specific al­ This hypothesis has some plausibility, and it may be true ternatives for the individual reference processes of the when the definite reference is only obscurely related to separate experiments. its referent. However, the main hypothesis assumes that fmding the antecedent of a simple definite reference should be easier than establishing a whole new referent in the EXPERIMENT 1 discourse model. Other investigators have looked at various effects of As Clark and Marshall (1981) showed in detail, under­ definite and indefinite articles, but apparently never in standing definite reference can often involve complex order to investigate instantiation of referents. A number reasoning about possible referents based on the hearer's of experiments have suggested that memory for text is knowledge ofthe domain of discourse and of the speaker organized around discourse referents, rather than (see also Clark & Murphy, 1982). Definite reference can­ individual sentence meanings (Bransford, Barclay, & not be interpreted through syntactic rules, although there Franks, 1972; de Villiers, 1974; Ehrlich & Johnson­ are some syntactic constraints on reference (see, e.g., Las­ Laird, 1982; Garnham, 1981), but they do not address nik, 1976). Furthermore, it cannot be interpreted solely the questions of understanding processes raised here. through semantic analysis of the referring expression. As More recently, Irwin et al. (1982) showed that the ar­ mentioned above, the noun phrases used to refer are often ticle preceding a noun influenced the time it took to per­ vague, ambiguous, or even incorrect from an "objective" form a lexical decision on the noun, in particular, that viewpoint [as in (1)]. It is only by virtue of similar dis­ the lexical decision was facilitated by a preceding definite course models (and similar processes to act on them) that article. Although these results are consistent with the speaker and hearer can communicate. predictions made here, their experiment did not actually Ifthe discourse model is a mental representation con­ measure sentence comprehension, but, rather, identifica­ taining various elements with associated schemata, then tion of a single test word. It is conceivable that their results introducing a new element will require the model to be derived from strategies operating in the lexical decision updated. Simply referring to an already established ele­ task that would not be as important in a more naturalistic ment will not require such a change, and thus should be reading situation. Experiment I reproduced the normal REFERENTS IN DISCOURSE 491 reading situation more closely, in an attempt to replicate Results and Discussion their results. The reading times did show an effect ofthe definite and Finally, an experiment by Haviland and Clark (1974) indefinite article: Sentences with the definite article took may be the closest conceptually to Experiment 1. In 1,556 msec to understand, and those with the indefinite their Experiment 2, they compared (3) and (4). article took 1,764 msec. This 208-msec difference was reliable in the subject analysis [F(1, 17) = 7.01, P < .02, (3) Ed was given an alligator for his birthday. MSe = 6,159] and in the item analysis [F(I,35) = 5.62, The alligator was his favorite present. P < .025, MSe = 6,726]. (4) Ed wanted an alligator for his birthday. Thus, as Haviland and Clark (1974) found, people took The alligator was his favorite present. longer to read and comprehend sentences that mentioned a new object than sentences that referred to an old one. They found that the second sentence of (4) took longer This experiment rules out the problem of misuse of the to understand than that of(3), presumably because an ac­ definite article that may have been present in Haviland tual alligator had already been established in (3). and Clark's experiment. Moreover, it found an effect of However, the definite noun phrase is somewhat anomalous article definiteness similar to that of Irwin et al. (1982), in (4), since it is being used to introduce an element rather who used a lexical decision task. The effect here than to refer to an old one. So, this difference in compre­ (208 msec) was much larger than any found in their ex­ hension times may reflect either the time to establish a periments (their largest analogous effect was 46.5 msec, new discourse referent or the time to override the anomaly in Experiment 4). Possible reasons for this difference are (or both). Experiment 1 eliminated this potential problem. discussed after Experiment 2. The results also argue against the alternative hypoth­ Method esis that definite reference should be more difficult than Subjects. Thesubjects were 18BrownUniversity undergraduates who indefinite reference, since it requires the reader to find were paid for their participation. an antecedent. Apparently, the time needed to establish Materials. Eighteen storieswereused, each II lineslong, witha true­ falsequestionat theend. Eachstory contained two test sentences, which a new element overrides any such effect. It should be were always the 5th and 10th lines. Two versions of each story were noted, however, that the references here were all straight­ created; they differed in whether an indefinite or definite article was forward ones requiring no difficult inferences. Direct usedin the test sentences (andalsoin the following sentence of the story, references involving extensive or tenuous inferencing may to be consistentwiththis difference). See Appendix A for an example. Becausedefiniteand indefinitereferenceare ubiquitousin all texts, the well take longer than indirect references, which usually presence of test sentences wasopaqueto the subjects, whoneverreported do not require such computations. guessing the purpose of the experiment in the debriefing session. (In The results ofExperiment 1 do not definitely establish fact, this is true of all of the experiments.) Each line of the story con­ the discourse-model explanation of how the article affected tained a complete sentence or clause. the understanding process. Another possibility is that the The true-false question following each story was written to ensure that the reader had paid close attentionto the story. The questionsdid definite article facilitates the perception and interpreta­ not, however, alwaysfocuson the test sentences,since that mighthave tion of the following noun: When readers encounter the, givenawaythefocusof thestudy. For this reason,thesequestionscould they may activate memory representations of words or not be used to monitor the readers' interpretations of the referring ex­ concepts that have already been mentioned and that match pression. (Sincethere were two test sentences per story, this wouldalso have required more than one question for each story.) the repeated noun. Since the indefinite article usually in­ Procedure. The subjectsread the instructions at the beginning of the dicates the introduction of a new referent, it would not experiment. The instructions explained the taskand emphasized thatthe engender this activation. Experiment 2 attempted to tease subjectsshouldfully understandeach sentencebefore pressingthe but­ apart these two possibilities, eliminating the articles but ton to beginthe nextpresentation. The subjects then repeated the instruc­ retaining the contrast between definite and indefinite refer­ tionsto the experimenter, whocorrectedany misconceptions and added any missinginformation. The subjects then read four practice stories. ence, by using pronouns. after whichthe instructions were reemphasized. These instructions were designed to encourage thesubjects to complete any processes of assigning referents so that they would be included in the comprehension times. The stories were presented on a standard video monitor controlled EXPERIMENT 2 by an Apple II+ microcomputer, in upper- and lowercaseprint. Each story presentation began with the message "PUSH BUTTON FOR The understanding of pronouns must be similar in many NEXTSTORY.,. After the subjectsdepressed the responsekey, there ways to the interpretation of full noun phrases. For both, was a 2-secdelay, followed by a plus sign as a fixationpoint. For each line of the story, the fixation point appeared for 750 rnsec, followed the listener must try to pick out a referent based on the by a 250-msec blank screen and then the sentence. The sentencedis­ context and the information in the referring expression. For appeared after the subjects' responses, and, 500 msec later, the fixa­ pronouns, that information is restricted primarily to num­ tion plus reappeared. The computer recorded the readingtime of each ber, gender, and animacy; noun phrases can contain as sentence. At the end of the story, the true-falsequestionappeared, and little information as a pronoun, or considerably more. the subjectsrespondedby pressinga "true" or a "false" buttonon the responsepanel. The time of this responsewas not recorded. The order Like noun phrases, pronouns differ in their definiteness: of the stories was determined randomly for each subject, and half of Personal pronouns are all definite; indefinite pronouns in­ the subjects were assigned at random to each story version. clude one(s) and some. Compare: 492 MURPHY

(5) a. Sandy rode a bicycle and I rode it too. periment 1 was replicated: Sentences with indefinite b. Sandy rode a bicycle and I rode one too. pronouns took 287 msec longer to read than those with definitepronouns [F(1,19) = 17.79, P < .001, MSe = Thedefinite pronoun it refersto thepreviously mentioned .0926, inthesubject analysis; F(l,39) = 13.58, p < .001, bicycle; theindefinite pronoun onerefers to a new bicycle. MSe = .2454, in the item analysis]. This result suggests Experiment 2 useddefinite and indefinite pronouns to thattheeffectof definiteness cannot be completely caused extend the results of Experiment I. For example, Experi­ by the presence of the definite article, since in Experi­ ment 2 compared the two versions of (6) embedded in a ment2 definiteness wasvariedby thepronoun ratherthan story. by an article. (Of course, this does not mean that the definite article does not have a facilitative effect, but it (6) Karen gave money to an old woman. arguesthattheaddition of a discourse element influences Dan gave her/one some money too. understanding beyond that.) This effect also rules out anotheralternative explana­ In general, the indefinite pronoun onewascontrasted with tion for Experiment l 's results(andthoseoflrwin et al., the grammatically appropriate definite pronoun, she, him, 1982). Perhapssubjects were disturbed by the repetition it, etc. Because definite and indefinite articles were not of a noun that was not coreferential with its first occur­ varied here, and because there was no repeated noun rence. Thatis, if people readthe wordbicycle twice,they phrase, anyperceptual facilitation or guessing strategy that mayexpectboth occurrences to refer to the sameobject. may haveoccurredin Experiment 1could not operatein In the indefinite condition, however, the words refer to Experiment 2. differentobjects,and it mighthave beenthis violated ex­ A secondfactor in Experiment 2 wasthe lengthof the pectation that slowed reading, rather than the instantia­ antecedent. Because thepronoun mayin someway"rein­ tion of a new discourse element. Contrary to this state" its antecedent (Frederiksen, 1981), the length of hypothesis, Experiment 2 found a similareffect, withno the antecedent might be expected to influence its com­ ·lexical repetition. prehension. Furthermore, thiseffect might be particularly Therewasonlya smallandnonsignificant effectof an­ pronounced for indefinite pronouns, since they require tecedent length [F(l,19)=2.02, p > .10, MSe=.0358, the instantiation of a new element thatcorresponds insome over subjects; F(1,39)= .28, p > .50, MSe= .2652, wayto theantecedent. Thefullreasoning behind thispre­ over items]. Because the 6O-msec effect was in the ex­ dictionis described in Murphy (1982, in press); because pected direction, it may be premature to affirm the null of theequivocal results, it willnotbe fully discussed here. hypothesis here. More convincing was the finding of no interaction of definiteness and antecedent length (both Method Fs < 1.0; MSe = .0443 and .0723 for subject and Subjects. Twenty Stanford University undergraduates performed the experiment to fulfill a course requirement. itemanalyses, respectively). Onepossible result discussed Materials and Procedure. Twenty stories similar to those used in above was that antecedent length would affect any new Experiment 1were constructed. Each story contained two test sentences, discourse elements being instantiated andthuswould have one containing a definite and the other containing an indefinite pronoun a greater effect for indefinite than for the definite (varied between subjects). The indefinite pronoun was always one;the pronouns. However, the interaction was nonsignificant, definite pronoun was the grammatically appropriate counterpart. These sentences were preceded by a sentence containing an antecedent that and the means actually go in the opposite direction (see was either long or short. The two versions of the antecedent noun phrase Table 1). had the same head nouns, but the long version contained additional ad­ The advantage of definite reference in Experiments 1 jectives and adjective phrases or a relative clause modifying the noun. and 2 was an order of magnitude larger than that found Appendix B contains an example. Four different lists of the stories were by Irwinet al. (1982). This may notbe surprising, given created so that each test sentence appeared in all conditions. Equal num­ bers of subjects received each version. that Irwin et al. used a task quite different from the The procedure was identical to that of the first experiment. reading-time procedureusedhere. Their lexicaldecision task was probably sensitive to perceptual and semantic Results and Discussion facilitation caused bythedefinite article, among otherfac­ The meanreadingtimesare presented in Table 1. The tors. However, because subjects were instructed tojudge mainresult is that the effectof definiteness found in Ex- as quickly as possiblewhetheror not the test string was a word, the lexicaldecision timesmay nothavereflected all thediscourseprocesses that were operating. Even so, Table 1 the finding of an advantage of definitereferencein both Mean Reaction Times (in Seconds) for Experiment 2 as a Function of Pronoun Type and Length of Antecedent paradigms is goodconverging evidence. The resultfrom Experiment 2, thatthisadvantage doesnotdepend on the Antecedent Pronoun presence of articles, adds furtherevidence thatinstantiat­ Type Long Short ing new discourse elements has a measurable effect. Definite 1.594 1.509 As a whole, the resultsgive more evidence for the im­ Indefinite 1.856 1.820 portance of instantiating newdiscourseelements, but lit- REFERENTS IN DISCOURSE 493 tle evidence for any reinstatement of the linguistic respectively. Notice, however, that not all plural nouns antecedent in understanding pronouns, due to the null ef­ refer to two (or more) elements in the discourse model fect of antecedent length. (However, the latter finding is even if they refer to many objects in the world. For ex­ ambiguous enough to require confirmation in future ample, consider the delegates in the following. research.) Assuming that the null effect of antecedent length is "real," it may pose a problem: Ifindefinite refer­ (7) Each of the 50 states elected a delegate to the convention. ence involves creating new objects in the model, one might Unanimously, the delegates voted against the exclusion rule. expect the antecedent to influence how much new infor­ mation needs to be added. The complete absence of an It seems unlikely that readers establish a separate entity effect of length on indefinite pronouns is a little mystify­ for each delegate referred to-not only would this exceed ing. One possible answer is to make a distinction between memory capacity, but it is also unnecessary, since the descriptions and objects in the mental model similar to delegates are not differentiated in any way. Rather than the type-token distinction. (Garrod & Sanford, 1977, 50 elements, the reader probably establishes one element p. 88, also made this distinction.) Indefinite anaphoric for the whole set of delegates (Johnson-Laird, 1983, reference introduces a new object but uses the same p. 443, discusses a number of such possibilities). description as some other object (see Webber, 1979). The stories used here were designed to avoid the "set" Thus, in Dan gave one some money too, one refers to a interpretation of plurals as much as possible. First, the new object, but the object is identified by a description plurals referred to only two entities, rather than to a large (old woman, or whatever) already mentioned. To inter­ number. It should be relatively easy to establish two dis­ pret one, the subject may have discovered the antecedent course entities, whereas it may beimpossible to differen­ and added a new element with a pointer to the descrip­ tiate larger numbers of similar objects or people in a tion already present, which would produce no effect of story. Second, the two objects were given differentiating antecedent length, but which would find an influence of descriptions. In a story describing two children, one was definiteness. This process would be more efficient than described as Japanese and pretty, the other as noisy; for reinstating all the information associated with the antece­ two paintings, one was a landscape and one was brightly dent, since it would not require the reader to represent colored. Providing different information about each ele­ information redundantly. Experiments 3 and 4 further in­ ment should cause subjects to represent them separately vestigated the question of economy in discourse represen­ to avoid confusing them. tation. In the singular condition, the information predicated of the two objects was simply predicated of one object. Thus, EXPERIMENT 3 the differentiating propositions were different, but not con­ tradictory. In this way, the total amount of information Experiments 1 and 2 found differences between refer­ presented was roughly equal in the two versions. Follow­ ence to an element and instantiation of a new element in ing is one of the stories in both of its versions. the discourse model. But even within the realm of refer­ ence to an already established element, there may be differences in what references are easy to understand. (8) Linda was looking for a new house. Other research has found effects of referential continuity There were a lot of houses for sale in her town. (Ehrlich & Johnson-Laird, 1982), the typicality of the con­ She' visited a house with a fireplace in every room. cept being referred to (Garrod & Sanford, 1977), the plau­ It was only a block away from the train station. The house was far too expensive, however. sibility of the reference (Marslen-Wilson & Tyler, 1980), She decided to stay in her old place after all. the focus of antecedents (Shwartz, 1981; Yekovich, Walk­ (9) Linda was looking for a new house. er' & Blackman, 1979), the distance ofantecedents (Clark There were a lot of houses for sale in her town. & Sengul, 1979), and so on. Experiments 3 and 4 further She visited a house with a fireplace in every room. explored the notion of objects in a discourse model by Another was only a block away from the train station. comparing reference to one versus two discourse ele­ The houses were far too expensive, however. ments. The discourse-model approach suggests that refer­ She decided to stay in her old place after all. ring to one element will require less processing than referring to two or more: If discourse understanding is centered around referents, then the more referents a sen­ The test sentence (the fifth) varied only in the plurality tence includes, the more processing it should require to of the noun and in verb agreement (when the noun was beunderstood. An important consideration here is to con­ the subject of the sentence). The two versions of this sen­ trol for the total amount of information presented-that tence seem to contain the same number of propositions is, not to present twice as many propositions when two (as measured, e.g., by methods used by Kintsch & elements are referred to. van Dijk, 1978). Experiment 3 attempted to investigate this hypothesis One might argue that syntactic or morphological differ­ by comparing sentences with plural and singular noun ences between plurals and singulars could lead to slower phrases that evoked either one or two discourse entities, comprehension ofthe plurals. It seems unlikely that such 494 MURPHY factors would have effects of much magnitude, but Ex­ They were just several blocks away from the train station. periment 4 provided more substantial evidence for this The houses were far too expensive, however. assumption. She decided to stay in her old place after all.

Method Notice that the crucial (fifth) sentence is identical in the Subjects. Sixteen Stanford undergraduates fulfilled acourse require­ two versions, but that they differ in their interpretation ment by participating in the experiment. of the houses. In (9), the individuals condition, it refers Materials andProcedure. Two versions of20 new stories were con­ structed. One version mentioned asingle object and predicated (at least) to the two houses just mentioned; in (10), the set condi­ two propositions about that object, and the other mentioned two objects tion, it refers to a group of similar houses, their exact and predicated (at least) one ofthose propositions about each one. The number unspecified. Note also that the sentencespreced­ stories were six lines long, with the test sentence always on the fifth ing the crucial one in the two versions are as identical line. The test sentences were identical inthe two versions except for the plurality changes. Each subject read only one version ofeach story, as possible. and half of his or her stories were singular, half plural. If the results of Experiment 3 were due to the length, The procedure was the same as in the previous experiments. morphological characteristics, etc., of the plural, then Ex­ periment 4 would have been expected to find no differ­ Results and Discussion ences between the two versions. If readers are sensitive The plurality change did affect the reading times of the to the number of discourse elements being referred to, target sentences. For singular sentences, the mean read­ then the individuals condition shouldproducelonger read­ ing time was 1,728 msec; for the plural sentences, it was ing times than the set condition, even though the test sen­ 1,919 msec. This difference was highly significant over tences are identical. subjects [F(I,15) = 14.82, P < .003, MSe = .0197] and just at the significancelevel over items [F(1,19) = 3.97, Method P = .058, MSe = .0918]. The lack of full significance Subjects. Eighteen Brown University undergraduate and graduate stu­ probably reflects the smaller number of items used here dents performed the task for pay. than in Experiments 1-2, which had two test sentences Materials. The 19 stories were modified from those used in Experi­ in each story. The resultssuggestthat accessingtwo refer­ ment 3 to generate a set and individuals plural version of each, as described above. Because ofthe preponderance ofplurals in these stories, ents in interpreting a noun phrase does take more time 15filler stories involving a single person orobject were added topre­ than accessing one. Experiment 4 ruled out some alter­ vent subjects from noticing this fact. native explanations for this effect. Procedure. The procedure and apparatus were identical tothose of Experiment I. For each story, half ofthe subjects received the set ver­ sion, and half the individuals version. EXPERIMENT 4

As discussed earlier, not all plurals refer to more than Results and Discussion one element of a discourse model. Some of them appear The plural sentences with a set interpretation took to engender a "set" interpretation, in which a single ele­ 1,812 msec to read, whereas those with an individuals in­ ment refers to a set of individuals that share a single terpretation took 1,953 msec. This 141-msecdifference description. This representationseemsnecessaryto avoid was reliable [F(1,17) = 4.58, P < .05, MSe = 4,362, memoryoverload; if theobjects can all be describedby the over subjects; F(1,18) = 5.23, P < .05, MSe = 3,858, same propositions, there is little need to represent each over items]. separately. Apparently, the plurals referring to differentiated in­ The explanationI suggestedfor Experiment 3 was that dividuals did generatemultiplediscourseelements, which referring to more than one discourse element would take took longer to access in understanding. Sincethe test sen­ longer than referring to one. However, alternative ac­ tences were identical in the two conditions, these results countsfocus on other differences betweenpluralsand sin­ cannotbe attributed to linguistic or perceptual differences, gulars. Experiment 4 avoided those possible confounds but must be due to what the sentences refer to. by using only plural noun phrases, varying the context These results seemingly conflict with the claims of so that they sometimes referred to a set of identical ob­ Johnson-Laird (1981, 1983), who argued that statements jects and sometimes to two differentiated objects. I call like "All the beekeepers are gardeners" are represented these conditionsthe set and individuals interpretations of by a numberof separateelementsrepresentingbeekeepers the plural, respectively. The individuals condition was and gardeners. Johnson-Lairdusedthis assumptionto ex­ based on the plural versionsof the stories used in Experi­ plain subjects' performance in evaluatingsyllogisms with ment 3. The setversionswereconstructed so thatthe same quantifiers. But since the beekeepers are presented as an propositions were expressed about a group of objects. undifferentiated set, the results of Experiment 3 suggest Compare (10) below with the individuals version in (9).· that they should be represented as a single discourse ele­ ment. However, there need not be a conflict here: In (10) Linda was looking for a new house. Johnson-Laird's task, subjectsrealized that they were be­ There were a lot of houses for sale in her town. ing given a reasoning test. His results may be true for She visited several houses with fireplaces in every room. tasks setupassyllogistic reasoning tests, and the results REFERENTS IN DISCOURSE 495 presented here may hold for more neutral reading situa­ order to get a clear distance effect, Clark and Sengul tions. (Also,the needto establish many discourse elements (1979) were careful to eliminate such implicitreferences to explain reasoning is crucial only when quantifierslike in their experimentson the distance of antecedents. The some are used to distinguish members of a class. In this stories in the current experimentsdid make such implicit case, the two theories agree that separate elements must references, and therefore probably eliminated thedistance be created.) Similarly, the elaborate mental modelsused effect. This reasoning suggests that the number-of­ in solving physics problems (e.g., Gentner & Gentner, referents explanation is more likely here. However, in 1983; Larkin, 1983)may not be employedin understand­ natural texts, both factors probablyaffect understanding. ing nontechnical discourse involving those concepts. These possibledifferencesbetween casual discourse and GENERAL DISCUSSION structured reasoning situations deserve more empirical research. All four experiments demonstrated the importance of The stories in Experiments3 and4 were carefullycon­ reference in comprehension. Readers attend to separate trolled so that equivalent information was presented in referents and establish new referents when they are in­ each condition before the test sentence. This was done troduced, and these processes take measurable amounts so that the differencesbetween the versions could not be of time. Referentialprocesses cannot be cued by syntac­ explained as the difficulty in accessing the propositions tic features of the sentence: Whether an expression es­ of previous sentences or the like. However,there is a pos­ tablishes a new referent or refers to one, two, or more sible discourse-level explanation different from the one discourse elements can be computed only by relating it suggested earlier, based on a "distance effect." In the to the previous discourse and the context. Even the lin­ pluralstoriesreferring to twoobjects [e.g., (10)],thethird guistic markers of referential status are not perfect sentenceintroducesone referent, the fourth sentence in­ indicators-plurals can sometimesrefer to one discourse troduces another, and the fifth sentence refers to both of element, and the definite article may sometimes be used them. In the singularand set versions, however, only one to introduce a new element (Haviland & Clark, 1974). referentis introduced and discussed in thethird and fourth These considerations suggestthat peoplemust use some­ sentences. Thus, the differencebetweenthese conditions thing like a discourse model to keep track of the refer­ could have either of two causes: It could take longer to ents when interpreting discourse. accesstwodiscourse elements thanone (asI have argued), Experiments 1 and 2 showed that understanding in­ or it could take longer to access a referent that has not definite noun phrases and pronounstook longer than un­ been mentioned for a sentence (the first referent in the derstanding definite references. This finding suggests that individuals condition)(see Clark & Sengul, 1979). Note when a new referent is mentioned, the reader must es­ that the two explanations have similar assumptions about tablish a new element for it and connect it with the other discourse referents-they differ primarily in the mecha­ elements in the model. Furthermore, Experiment 2 found nisms that access referents. thatestablishing an elaborated newelementtook no more These two explanations make nearly identical predic­ time than establishing a simple one (varied through tions for normal stories, becauseany text that introduces pronoun antecedents). This can be explained by the effi­ two separate referents is likely to haveone mentioned later cient use of descriptions already in memory-apparently, than another, whereas a similar text about one referent peoplecan establisha new referent withouthaving to re­ will not have this problem. In spiteofthis similarity, Ex­ write all of the information associated with it, if that in­ periments 3 and 4 gave some evidence that favors the formation is already in memory. number-of-elements explanation. In the plural versionof Experiment 4 confirmed the use of such an efficient (9), the following two sentences preceded the test sen­ representation for sets of objects. Whentwo objectswith tence: She visited a house with a fireplace in every room. different descriptions wereevokedin thetext, twoseparate Another was only a block away from the train station. elements were created. When a group of objects having Although the twosentences focus on twodifferenthouses, identical descriptionswere evoked, they were represent­ noticethat the secondone actually refers to bothof them. ed by one element. That is, the elliptical Another means another house. In Experiments 3 and4 alsoprovided insight intoprocesses order to understand this sentence, the reader had to ac­ of understanding definite reference. Their resultsshowed cess a representation of the first house. Therefore, even thatsentences that referredto twodiscourse elements took though this sentence is not about both houses, readers longer to understand than those that referred to one ele­ probably had access to both discourse elements when ment. This occurred when the same number of proposi­ processing it. Other stories were very similar in this tions was expressed in each sentence and even when the regard: The second sentence used pronouns or elliptical sentences were identical. This resultgives moreevidence NPs that referred to the first element while establishing that people structure their memory for discourse around a new one. referents, and thattheyaccess the referents in understand­ The point of this discussion is to show that the first ing the sentence. referent was probably not as "distant" as might seem at Considered together, the experiments suggest the fol­ first glance, since readers probably evoked it to under­ lowing account of reference by pluralnoun phrases. When stand the sentence with the second referent. In fact, in a noun phrase refers to a homogeneous set of objects, 496 MURPHY reference is similar to that of a singular noun phrase. That Theoretical issuesin reading comprehension (pp. 385-4(7). Hillsdale, is, the single discourse element that represents the objects NJ: Erlbaum. is picked out. When a noun phrase picks out differentiat­ DE VILLIERS, P. A. ([974). Imagery and themein recall ofconnected dis­ course. Journal of Experimental Psychology, 103, 263-268. ed entities, the reader accesses each ofthem and connects EHRLICH, K., & JOHNSON-LAIRD, P. N. ([982). Spatial descriptions and them to the new predicate expressed in the sentence. It referential continuity. Journalof l4>rbal Learning and verbal Behavior, seems most likely that this predicate is represented only 21, 296-306. once in memory but is linked to all referents. FREDERIKSEN,1. R. (1981). Understanding anaphora: Rules usedby read­ Natural texts include more complex discourse elements, ers inassigning pronominal referents. Discourse Processes, 4, 323-347. GARNHAM, A. (1981). Mental modelsas representations of text.Memory such as plurals that refer to multiple sets. For example, & Cognition, 9, 560-565. a text might discuss 50 senators, distinguishing in some GARROD, S., & SANFORD, A. (1977). Interpreting anaphoric relations: The detail many groups of senators (e.g., Southern Democrats, integration of semanticinformation while reading. Journal of l4>rbal right-wing Republicans, industrial-state senators, etc.). Learning and l4>rbal Behavior, 16, 77-90. GENTNER, D., & GENTNER, D. R. (1983). Flowing waters or teeming It might be impossible to keep each group separate in a crowds: Mentalmodelsof electricity. In D. Gentner & A. L. Stevens discourse model. Perhaps, in such cases, readers construct (Eds.), Mental models (pp. 99-120). Hillsdale, NJ: Erlbaum. a complex memory network of which only part is in ac­ GENTNER, D., &: STEVENS, A. L. (Eds.) 0983}. Mental models. Hills­ tive memory at any time. In this example, there might dale, NJ: Erlbaum. be a single element for all the senators, which is connect­ HAVILAND, S. E., & CLARK, H. H. (1974). What'snew?Acquiring new information as a processin comprehension. Journalof Verbal Learn­ ed to elements representing the various subgroups (which ing and l4>rbal Behavior, 13, 512-521. themselves may be connected by various relations). When IRWIN, D. E., BOCK, 1. K., & STANOVICH, K. E. (1982). Effectsof in­ the reader encounters a sentence about the senators as a formation structure cues on visual word processing. Journal of 14>r­ whole, the single element for all the senators may be ac­ bal Learning and 14>rbal Behavior, 21, 307-325. JOHNSON-LAIRD, P. N. (l981). Mental models of meaning. In A. K. Joshi, cessed, rather than the element for every subgroup. This B. L. Webber, & I. A. Sag(Eds.), Elementsofdiscourseunderstand­ account seems compatible with many theories of ing (pp. 106-126). Cambridge: Cambridge University Press. knowledge representation (e.g., Anderson, 1976; Nor­ JOHNSON-LAIRD, P. N. (1983). Mental models. Cambridge, MA: Har­ man & Rumelhart, 1975; Schank, 1982), but those the­ vard University Press. ories have not addressed the implications of such JOHNSON-LAIRD, P. N., & GARNHAM, A. (1980). Descriptionsand dis­ course models. Linguistics and Philosophy, 3, 371-391 representations for reference in discourse. Clearly, more KiNTSCH, w., &: VAN DUK, T. A. (1978). Toward a model of textcom­ experimentation is required to find evidence for this ac­ prehensionand production. Psychological Review, 85, 363-394. count and to flesh out the details of how reference is made LARKIN, 1. H. (1983). The role of problem representation in physics. to complexly related sets of objects. In D. Gentner & A. L. Stevens (Eds.), Mental models (pp. 75-98). Hillsdale, NJ: Erlbaum. REFERENCES LASNIK, H. ([976). Remarks on coreference. Linguistic Analysis, 2, 1-22. MARSLEN-WILSON, w., & TYLER, L. K. (1980). Towards a psychologi­ ANDERSON, 1. R. (1976). Language, memoryand thought. Hillsdale, NJ: calbasisfora theory ofanaphora. In1.Kreiman & A. E. Ojeda(Eds.), Erlbaum. Papers from the parasession on pronouns and anaphora. Chicago: BRANSFORD, J. D., BARCLAY, J. R., &: FRANKS, J. J. (l972). Sentence Chicago Linguistic Society. memory: A constructive versusinterpretive approach. Cognitive Psy­ MURPHY, G. L. (1982). Understanding anaphora. Unpublished doctoral chology, 3, 193-209. dissertation, Stanford University, Stanford, CA. CLARK, H. H., & CARLSON, T. B. (1981). Context for comprehension. MURPHY, G. L. (in press). Processes of anaphoric understanding. In 1. Long & A. Baddeley (Eds.), Attention and performance IX Journal of Verbal Learning and Verbal Behavior. (pp. 313-330). Hillsdale, NJ: Erlbaum. NORMAN, D. A., & RUMELHART, D. E. (1975). Explorations in cogni­ CLARK, H. H., & MARSHALL, C. R. (1981). Definite reference andmutual tion. San Francisco: Freeman. knowledge. In A. K. Joshi, B. L. Webber, & l. A. Sag (Eds.), Ele­ SCHANK, R. C. (1982). Dynamic memory: A theory of reminding and ments of discourse understanding. Cambridge: Cambridge Universi­ learningin computers and people. Cambridge: Cambridge Universi­ ty Press. ty Press. CLARK, H. H., & MURPHY, G. L. (1982). Audience designin meaning SHWARTZ, S. P. (1981). In searchofpronominalreference (Ya[e Cogni­ and reference. In1. F.LeNy& W. Kintsch (Eds.), Language and com­ tive ScienceTech. Rep. 10). New Haven, CT: Yale University. prehension (pp. 287-299). Amsterdam: North-Holland. WEBBER, B. L. (1979). Aformal approachto discourseanaphora. New CLARK, H. H., &: SENGUL, C. J. (1979). In searchof referents for nouns York: Garland Press. and pronouns. Memory & Cognition, 7, 35-41. YEKOVICH, F. R., WALKER, C. H., & BLACKMAN, H. S. (1979). The role COLLINS, A., BROWN, 1. S., & LARKIN, K. M. (1980). Inference in text of presupposed and focal information in integrating sentences. Jour­ understanding. In R. 1. Spiro, B. C. Bruce, & W. F. Brewer(Eds.), nal of 14>rbal Learning and l4>rbal Behavior, 18, 535-548.

Appendix A Two Versions of a Story From Experiment 1 Barry was in training for the US Olympic bicycle racing team. He spent all his free time riding up and down nearby hills, and all his money on bicycles and bike equipment, as repairs were common. Once his bicycle broke down. Fortunately, he had a bicycle ready in time for the race. He was glad to have brought an extra racing bike along. REFERENTS IN DISCOURSE 497

Many races were unpleasant because of poor conditions. Often the weather was bad and the course lousy. He once rode a rainy race. He won the race in spite of the difficulty, partly because there were accidents caused by the traffic.

Barry was in training for the US Olympic bicycle racing team. He spent all his free time riding up and down nearby hills, and all his money on bicycles and bike equipment, as repairs were common. Once his bicycle broke down. Fortunately, he had the bicycle ready in time for the race. He was glad to have brought spare parts for his racing bike. Many races were unpleasant because of poor conditions. Often the weather was bad and the course lousy, He once rode a rainy race. He won a race in spite of the difficulty, partly because there were accidents caused by the traffic. Note - The crucial lines are the 5th and 10th lines, although other lines differ slightly in the two versions to ensure consistency.

Appendix B Two Versions From Experiment 2 Brian and Gordon were housesitting for a professor of theirs. They had to stay for a week, but, after a day, they were bored. They began to rollerskate through the halls before going outside. Gordon slipped and hit a rare, wide, stained-glass window. Brian slipped and hit one too. The one Gordon hit broke, and he spent all afternoon trying to fix it. They were still bored, so that night they raided the liquor cabinet. They sat around the living room, drinking, smoking and watching t.v. Brian's cigarette set a couch on fire. Soon, Gordon's cigarette set it on fire as well. Fortunately, Brian was sober enough to put out the fire.

Brian and Gordon were housesitting for a professor of theirs. They had to stay for a week, but, after a day, they were bored. They began to rollerskate through the halls before going outside. Gordon slipped and hit a window. Brian slipped and hit it too. It broke when Gordon hit it, and he spent all afternoon trying to fix it. They were still bored, so that night they raided the liquor cabinet. They sat around the living room, drinking, smoking and watching t.v. Brian's cigarette set a plush, modernistic couch from Denmark on fire. Soon, Gordon's cigarette set one on fire as well. They put Brian's fire out before long, but the other couch was ruined. Note - The second version is the reverse of the first, in that the long antecedent is changed to a short one, and one is changed to it and vice versa. However, there are four such versions in the whole design, tofill out the combinations offactors. The crucial lines are again the 5th and 10th ones.

(Manuscript received February 2, 1984; revision accepted for publication May 20, 1984.)