41 The Behavior Analyst 1980, 3, 41-48 No. 2 (Fall)

Natural Concepts and Generalization ClassesNathan Stemmer

Bar-Ilan University, Ramat-Gan, Israel

(Abstract)The results of an experiment performed by Herrnstein, Loveland, and Cable (1976) show that pigeons

possess concepts that are not based on common elements. The conclusion they draw from these results is thatthese concepts are largely determined by genetic factors. In the present paper it is argued that, at least forsome of the concepts, the conclusion is incorrect. These concepts-called here uncommon generalizationclasses-were acquired by the pigeons as a consequence of certain experiences through which they went dur-ing the experiment. The discussion will also suggest the fruitfulness of replacing the vague and misleading no-tion of concept by the more adequate notion of generalization class.

In an article on natural concepts, Herrn-stein, Loveland, and Cable (1976)describe an experiment that seems toestablish the innate character of certainconcepts in pigeons. In the experiment,the pecking of pigeons was reinforcedwhen pictures showing a person, as well asparts of this person, in different settings,were projected onto a small screen withinan operant chamber. The response wasnot reinforced when the pictures showed adifferent person. The experiment in-dicated a significant transfer of theresponse to other pictures showing thesame person or the same parts of the per-son in other settings, and also to otherparts of the person, in several kinds of set-tings. Similar results were obtained withpictures containing one or more trees orparts of a tree, and also with picturesshowing water in several forms.

In the discussion of the experiment,Herrnstein et al. point out that thesegeneralizations cannot be explained interms of discriminable common elements."Having looked at the hundreds of in-stances used here . . . we cannot begin todraw up a list of common elements" (p.297/8). They then propose an explanationin terms of genetic factors: ". . . trees,bodies of water, and people have longbeen both important and common in thepigeon's natural environment. By now,these objects may have had enough evolu-tionary significance to be somehowrepresented in the genes" (p. 299). Thisdoes not imply, however, that pictures of

trees, water, and persons are innatecategories for pigeons. The innate in-gredient is less specific. "Given a finite setof varying stimuli, the pigeon activates aparticular category out of the limitlessnumber of categories more or less equallywell determined by the same set. It is inthis narrowing of the range of possiblecategories that innateness seems to be ex-pressed" (p. 300).

Still, even this conclusion seems to betoo strong. Although a basic type of thegeneralization classes of an organism isindeed strongly determined by innatedispositions, I will discuss evidence sug-gesting that at least some of the gen-eralizations classes discussed by Herrns-tein et al. are only indirectly determinedby such predispositions.The discussion will also clarify a

number of fundamental aspects of acapacity that plays a crucial role in humanlife, namely, the capacity to learn togeneralize according to so-called uncom-mon generalization classes. Finally, I willargue that a terminology based on the no-tion of generalization class has variousadvantages over alternative ter-minologies.

Normal and UncommonGeneralization Classes

The results of several experiments pointto the need for distinguishing between twotypes of concepts, or to use a terminologywhich will be seen to have certain advan-tages, between two types of generalization

41

42 NATHAN STEMMER

classes: classes that are normal and classesthat are not normal generalization classesfor a species. Consider, for example,Razran's well-known experiment onsemantic generalization (1939). The ex-periment shows that speakers of Englishgeneralize from utterances of 'urn' to ut-terances of 'vase.' That is to say, they"use" the class containing sounds of'urn' and of 'vase' as a generalizationclass. This class is not a normalgeneralization class for humans, sinceonly certain people generalize accordingto this class. On the other hand, the classcontaining similar utterances of 'urn' is anormal generalization class for humans,or very close to such a class. Most normalpeople generalize from one utterance of'urn' to other utterances of 'urn.'l

Generalization classes that are not nor-mal generalization classes (for a species)will be called uncommon generalizationclasses.The transformation of a "neutral"

class-i.e. of a class of entities that is notyet a generalization class for anorganism-into an uncommon generaliza-tion class for the organism can usually beattributed to specific experiences whichthe organism has undergone with respectto the elements of the class. Thus, thetransformation of the class containing ut-terances of 'urn' and 'vase' into ageneralization class for speakers ofEnglish is a consequence of the fact thatthese persons have gone through certainexperiences in which they heard (or read)utterances of the terms. On the otherhand, the generalization class containingonly utterances of 'urn' does not dependon specific experiences with this term. It isnot necessary for people to have heard ut-terances of 'urn' before they become able

'By speaking of similar utterances of 'urn,' I amusing the notion of similarity in an intuitive, non-technical way. I return to this topic in section 5,where I also discuss some uses of the notion ofsalience. Regarding the claim that most normal peo-ple generalize from one utterance of 'urn' to otherutterances of this term, see fn. 2.

to generalize from one utterance of thisterm to another.2To give another example, consider the

class of similar giraffes. This class is anormal generalization class for humans,or very close to such a class. Therefore, achild, who has had no previous ex-periences with giraffes, can neverthelessgeneralize from one giraffe to another (ifsome behavior is reinforced in thepresence of a giraffe). This explains whychildren can learn to apply the term'giraffe' to the whole class just by hearingan utterance of the term while seeing agiraffe.3 On the other hand, the class con-taining toys-e.g. balls, dolls, and littleblocks-is not a normal generalizationclass for humans. Therefore, a child can-not learn to apply the term 'toy' to thewhole class just by hearing an utterance ofthe term while seeing an element of theclass. The child must previously gothrough the experiences that transformthis class into a generalization class forhim.With respect to normal generalization

classes, two questions can be asked. (1)Why do (normal) organisms generalize ac-cording to the normal generalizationclasses of their species? (2) Do normalgeneralization classes possess a propertythat distinguishes them from other kindsof classes? I shall deal only very brieflywith these questions, since I am interestedhere mainly in uncommon generalizationclasses. Regarding the first question, theanswer given by Herrnstein et al. seems tobe the most plausible. It is very likely thatthe disposition of an organism togeneralize according to the normal

21t may be necessary that for people to generalizefrom one utterance of 'urn' to another, they mustalready be acquainted with English or with a similarlanguage. But the important point is that they neednot to have heard utterances of the particular term'urn.' A general exposure to the language is suffi-cient. On the other hand, the generalization from'urn' to 'vase' occurs only after the person has heard(or read) utterances of the specific terms.

30f course, the class will not always be strictlyidentical with the class to which adults apply theterm. On this topic see, e.g. Anglin (1977), Reich(1976), Stemmer (1979).

NATURAL CONCEPTS & GENERALIZATION CLASSES 43

generalization classes of its species derivesprincipally from genetic factors, and thatthe disposition has been evolutionarilyselected because of its survival value.4Regarding the second question, thediscussion in Quine (1973, p. 16-27) sug-gests that most, perhaps even all normalgeneralization classes have the followingproperty. The elements of these classeshave a number of (broadly defined)features that are possessed by the originalentities, i.e. by the entities whose percep-tion by the organism gives origin to itsgeneralizing behavior. Moreover, thesefeatures are salient for the normalmembers of the corresponding species.(However, it does not seem to benecessary that all the elements of the classshare the same features. It probably issufficient that each of them possesses partof the salient features of the original en-tities. See, e.g. Posner (1973) and Cerella(1979) on this "clustering" effect.)The two questions also arise in connec-

tion with uncommon generalizationclasses. Now, the answer to the first ques-tion is clearly different from the one givenabove. The disposition to generalize ac-cording to a particular uncommongeneralization class C does not derivefrom genetic causes. Thus the generaliza-tion from the sounds of 'urn' to thesounds of 'vase' is not genetically deter-mined, since most normal humans do not"use" the class containing these sounds asa generalization class. This suggests thatthe disposition to generalize according tosuch classes is acquired by an organismwith the help of certain experiences,which brings us to the following question.What are the experiences that transform aclass that is not a normal generalizationclass (for a species) into an uncommongeneralization class (for some members ofthe species)? Clearly, it is not yet possibleto give a complete answer to this question.

4The implications of the latter conclusion for thetopic of induction have been investigated in Quine(1969), Quine and Ullian (1970), and Stemmer(1971a, 1978a, 1978b).

However, for certain elementary cases,Hull's analysis (1943, p. 191-194) of whathe calls logical or abstract similarityenables us to give an answer. Consider theexperiences that transformed the classcontaining utterances of 'urn' and 'vase'into a generalization class. Generallyspeaking, they were those that enablespeakers of English to learn the meaningsof these terms. More specifically, andonly for elementary cases, the experienceswere like the following.5 The child heard(at least once) an utterance of 'urn' whileseeing an object a, and he also heard (atleast once) an utterance of 'vase' whileseeing an object b. Moreover, and this iscrucial here, the class containing a andb-that is, the urn and the vase-is a nor-mal generalization class for humans.6 Asa consequence of these experiences, theclass containing utterances of both 'urn'and 'vase' became an uncommongeneralization class for the child.

In general, the union of two (or more)normal generalization classes X and Ymay become an uncommon generalizationclass for an organism, if (but only if) theorganism is exposed to at least two pairingsituations. In these situations, an elementofX is paired with an entity a and an ele-ment of Y is paired with an entity b, and aand b belong to a normal generalizationclass Z7.

This conclusion is also supported by the

5Here under elementary cases I refer to cases inwhich a person learns the meaning of a term osten-sively-i.e. by hearing the term-while observing thecorresponding object-rather than verbally-i.e. byhearing the term in a verbal context withoutobserving the object.

6Notice that its elements share a number offeatures that are likely to be salient for humans, inparticular, the features that determine the shape of aandb.

71t is possible that Z does not have to be a normalgeneralization class. It may be sufficient that it be anuncommon (acquired) generalization class for theorganism. But this hypothesis has to be confirmedempirically. Notice, moreover, that I have statedonly that the described experiences may transform aneutral class into an uncommon generalization class.It is the task of the experimental psychologist todiscover the conditions which will allow us to predictthat the transformation will indeed take place.

44 NATHAN STEMMER

following experiment described by Pavlov(1927, p. 55-56). A dog was exposed toseveral pairing situations in which thesound of a buzzer, the sound of ametronome, and a tactile stimulus werepaired with the reception of acid. Then,one of the stimuli was inhibited and it wasobserved that the inhibition generaliza-tion to the other stimuli. This shows thatthe exposure to the pairing situations hadtransformed the class containing the threekinds of stimuli into an uncommongeneralization class for this dog.The conclusion also seems to account

for the transformation of the class con-taining balls, dolls, and little blocks into ageneralization class for children who haveplayed with these objects. These childrenhave been exposed to at least three pairingsituations in which an element of the classballs, of the class dolls, and of the classblocks were paired with "playing ac-tivities." As a consequence, the union ofthe three classes became an uncommongeneralization class for these children.8Once this had taken place, the childrencould learn to apply the term 'toy' to theelements of all the classes by hearing ut-terances of the term while seeing elementsofsome of the classes.Our analysis enables us to give a partial

answer to the question regarding the ex-periences that transform a class of entitiesinto an uncommon generalization classfor an organism. One of the experiencesthat has this effect is exposure to the kindof pairing situations we have describedhere.

Some Results of the Herrnsteinet al. Experiment

Let us now return to the Herrnstein etal. experiment. The interesting results ofthe experiment show that a response

81 am assuming here that "playing activities" con-stitutes a normal generalization class for humans.But if this is not the case, then perhaps thehypothesis of fn. 7 may account for thetransformation of the union of the classes into ageneralization class for the children (provided onecan show that the class of playing activities is at leastan uncommon generalization class for children whohave engaged in such activities).

which was reinforced in the presence of apicture of one part of an objecttransferred to pictures of other parts ofthe object, say, from the picture of thehead of a woman to a picture of her legs.Concentrating on this example, the ex-periment shows that the class containingpictures of the head and of the legs of thesame person had the status of ageneralization class for the pigeons. Thisgeneralization is surprising because theabsence of shared features that are salientfor pigeons suggests that the class is not anormal generalization class for pigeons.Now, our conclusions make it possible toattribute this generalization to previousexperiences undergone by the pigeons.Among the pictures that were presented tothem were pictures of the complete per-son. That is, the pigeons had observed atleast one pairing of the head with thecomplete person, and at least one pairingof the legs with this person. Hence, wehave here a process in which elements of anormal generalization class X were pairedwith an object a, and elements of a nor-mal generalization class Y were pairedwith an object b. Moreover, a and bbelong to a normal generalization class Z:pictures of the complete person. As a con-sequence, the union of X and Y becamean uncommon generalization class for thepigeons, which therefore explains theirgeneralization from certain parts of theperson to other parts.According to this interpretation of the

results of Herrnstein et al. 's experiment, abehavior that apparently derives from in-nate behavior mechanisms turns out to belearned. The behavior derives fromspecific experiences, just like thegeneralization from 'urn' to 'vase' (whichclearly is not genetically determined).Since the pigeons of the experiment in-deed had these experiences, the interpreta-tion obtains a high degree of plausibilitybecause it reduces the number of their in-herited behavior mechanisms.9

9Cf. Skinner (1953, p. 26): ". . . the doctrine of'being born that way' has little to do withdemonstrated facts. It is usually an appeal to ig-norance."

NATURAL CONCEPTS & GENERALIZATION CLASSES 45

The present interpretation is also direct-ly confirmed by some other results obtain-ed in the experiment. Herrnstein et al.report that three (out of four) pigeonsalso generalized from the pictures of theperson to pictures of her child, and theymention that this child occasionally ap-peared with the mother in the positivestimuli. This result cannot be accountedfor in their interpretation, which compelsthem to call it a misclassification (eventhough 75% of the subjects were in-volved). However, it can be explainedeasily in our interpretation. Since thechild appeared together with his mother insome of the pictures, the pigeons had beenexposed to the pairing of the child withthe complete person, i.e. with elements ofclass Z. Therefore, the class of pictures ofthe child-say, the class V-also became apart of the uncommon generalizationclass that had become operative for thepigeons.

InnatenessOur analysis suggests the following

conclusions regarding innateness. Thedisposition to generalize according to thenormal generalization classes of a speciesis largely determined by genetic factors.Moreover, the capacity to acquire uncom-mon generalization classes by having ex-periences like those described above isalso likely to be innate. However, the par-ticular classes that have become uncom-mon generalization classes for certainmembers of a species have no geneticbasis. They arise from the particular pair-ing situations to which the organisms areexposed like those that give origin to theuncommon generalization class contain-ing utterances of 'urn' and 'vase.' Still,there is an innate factor even in uncom-mon generalization classes. Since theseclasses consist of the union of several nor-mal generalization classes, and since thelatter classes are strongly determined bygenetic factors, uncommon generalizationclasses are partially determined by innate

predispositions. (This explains, for exam-ple, why all people who have learnedEnglish can generalize from utterances of'urn' to utterances of 'vase,' in spite ofthe fact that the class containing these ut-terances is clearly not represented in theirgenes.)

These conclusions point to the possibili-ty of using an alternative terminology forspeaking about the two types ofgeneralization classes. Instead of normalwe might use the term innate generaliza-tion classes to stress their strong geneticbasis. Instead of uncommon we mightspeak of acquired generalization classes inorder to express the fact that the tendencyto generalize in accordance with theseclasses is acquired as a consequence ofcertain experiences.

Generalization Classes VersusConcepts and Categories

Throughout this paper I have used theterm generalization class rather than themore customary terms concept orcategory. I prefer this term for tworeasons. Firstly, the term generalizationclass expresses in a very suggestive man-ner the basic data that are obtained in ex-periments on concept possession or con-cept formation. In paradigm experimentsof this type, the organism is stimulated mtimes by m entities al, . . ,a. and responsesare reinforced. The m entities can be in-stances of different stimuli, such as m pic-tures of different triangles, or instances ofthe same stimulus such as m utterances of'urn.' (Since there may be minor dif-ferences between the different utterancesof 'urn,' I prefer to consider them as dif-ferent entities.) After the training period,it is then observed that the organismgeneralizes from these m entities to n en-tities bl, . . ,bn. For example, when beingpresented n pictures of differentgeometrical figures or n utterances of'urn,' the organism responds without be-ing reinforced. Hence, our basic datum isthe class containing the m + n entities al, .

46 NATHAN STEMMER

. ,am,bI, . . ,bn. This class I then callgeneralization class because the namepoints to the generalization from the mreinforced to the n nonreinforced entities.The second reason derives from the

usual connotations of the term conceptand category. Many psychologists con-ceive concepts or categories as havingsome kind of independent existence. Thusthey speak of the concept "animal" or"dog" as if these concepts somehow existin the mind of humans and perhaps ofother organisms, too. Now, maybe con-cepts and categories indeed have such anindependent existence, or, to use Quine'sterminology (1970, p. 8), there existsperhaps a 'gallery of ideas.' However,with respect to experiments like thosediscussed here, this view is misleading,since the generalization classes-i.e. theconcepts or categories-that are obtainedare always relative, relative to original en-tities. Thus, the generalization class thatoriginates in a black poodle who is sleep-ing will often be different from the onethat originates in a white San Bernardodog who is running, in spite of the factthat both animals fall under the concept"dog" (as well as under many other con-cepts, e.g. "quadruped," "mammal,""animal," "three dimensional object,"etc.) Therefore, the use of terms such asconcept and category (also to classify),which do not have relativistic connota-tions, may bring the psychologist to ig-nore the dependence of a particulargeneralizing behavior on the nature of theoriginal entities. (This may be the reasonwhy some of the treatments of so-calledcognitivist psychologists aremethodologically unsatisfactory.) Thissuggests the usefulness of adopting a ter-minology which clearly points to the factthat every generalizing behavior startsfrom original entities, and that generaliza-tion classes are relative to these entities.

Similarity and SalienceWith the help of the notion of

generalization class it is possible to clarifycertain uses of our intuitive notion of

similarity. Thus, suppose that the child,who heard an utterance of 'giraffe' whilepaying attention to a giraffe, generalizesfrom this giraffe to the elements of classG. We might then say that for the childthe elements of G are similar to theoriginal giraffe.

This conclusion shows the circularity ofmy earlier statement that normal peoplegeneralize from one utterance of 'urn' tosimilar utterances of this term. It is theother way around. It is because normalpeople make this generalization that weconclude that the utterances are similarfor them.The notion of generalization class also

stands behind one of our intuitive notionsof salience. Thus, let us make the plausi-ble assumption that most of the elementsof G are (normal) giraffes; that is,animals with four legs, with a long neck,with a head, etc. We would then concludethat features such as the quadrupednessof the original giraffe, his long neck, hishaving a head, etc. were salient featuresfor the child.Moreover, from the fact that the

generalization class was G rather than,say, the class containing pieces of dust, weobtain another conclusion about salience,namely, that when the child heard the ut-terance of the term 'giraffe,' the giraffewas for him a salient aspect of thestimulus situation, at least more salientthan the dust of the giraffe's cage. I refer-red to this type of salience when sayingthat the child paid attention to the giraffe.

These conclusions point to themethodological primacy of the notion ofgeneralization class. It is from the dataabout the generalization class that we con-jecture hypotheses concerning thesimilarity and salience conditions that areoperative in a particular generalizationprocess.

Notice, finally, that by applying thedistinction between normal and uncom-mon generalization classes to the notionof similarity we obtain two subnotions:normal similarity which is strongly deter-

NATURAL CONCEPTS & GENERALIZATION CLASSES 47

mined by genetic factors and uncommonsimilarity which is only indirectly deter-mined by such factors. I have in-vestigated this topic in Stemmer (1971b,1973, 1978c), where I used the terms in-nate and acquired similarity instead ofnormal and uncommon similarity. Seealso Quine's extensive treatment of thenotion of similarity in his book (1973).With respect to the notion of salience thesituation is somewhat different. The dif-ference between the two types ofgeneralization classes is not directlyreflected in the conditions of salience. ButI shall not enter into this here. The readeris referred t6 Quine's chapter on salience(1973, p. 24-27) in which he formulates anumber of important conclusions aboutthis topic, including some about the in-nate character of certain salience condi-tions.

ConclusionsA distinction has been made between

normal and uncommon generalizationclasses. Whereas the former are stronglydetermined by genetic factors, the latterare to a great extent determined byspecific experiences. A basic type of theexperiences that enable organisms to ac-quire uncommon generalizationclasses-namely, exposure to certainkinds of pairing situations-was thendiscussed. The conclusions that were ob-tained suggest that at least some of theclasses that are operative in the Herrnsteinet al. experiment are uncommongeneralization classes, which means thatthey are largely determined by specific ex-periences. Finally, arguments have beenpresented which suggest that a ter-minology that is based on the notion ofgeneralization class has certain advan-tages over other terminologies.

Let me close with a word about the im-portance of a comprehensive investigationof the capacity to acquire uncommongeneralization classes, which seems tohave been neglected somewhat. I believethat such an investigation is indeed highly

important, because of the fundamentalrole played by such capacity in humanlife. Especially in language acquisition itperforms a fundamental function. Notonly is the learning of terms such as 'toy,''clothes,' or 'furniture' based on thiscapacity but, as I have argued in Stemmer(1971b, 1973, 1978c), it is very likely thatit also underlies the creative use oflanguage. It is this capacity which enablesus to understand and to produce sentenceswe have never heard before.10 Hence, asystematic investigation of the capacitymight enable us to give an adequate"behaviorist" account of language ac-quisition.

l0ln particular, this capacity appears to standbehind the psychological reality of syntacticcategories such as the category of noun or of verbphrases. As has been shown by several scholars, suchsyntactic categories must be available for children inorder to enable them to learn a generative grammar(see especially Chomsky, 1965). Now, in that syntac-tic categories are uncommon generalization classes.These classes become generalization classes for thechildren as a consequence of the experiences throughwhich they go when learning the language of theircommunity.

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