Intelligence_ Knowns and Unknowns

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Selected Portions of

Intelligence: K nowns and U nknowns

Report of a Task Force established by the Board of Scientif ic Af f airs of the American Psychological

Association

Released August 7, 1!

"lric #eisser, Ph$, %hair& 'mory "ni(ersity

As presented by ST ALK I NG T H E W I LD T ABOO

at http)**+++lrainccom*s+taboo*taboos*apa-.1html

/A slightly edited (ersion +as published in the American Psychologist , Feb 10 f f icial 2ournal of the

APA3

...

I. CONCEPTS OF INTELLIGENCE

4ndi(iduals dif f er f rom one another in their ability to understand comple5 ideas, to adapt ef f ecti(ely to the

en(ironment, to learn f rom e5perience, to engage in (arious f orms of reasoning, to o(ercome obstacles by

taking thought Although these indi(idual dif f erences can be substantial, they are ne(er entirely consistent) a

gi(en person6s intellectual perf ormance +ill (ary on dif f erent occasions, in dif f erent domains, as udged by

dif f erent criteria %oncepts of 8intelligence8 are attempts to clarif y and organi9e this comple5 set of

phenomena Although considerable clarity has been achie(ed in some areas, no such conceptuali9ation has

yet ans+ered all the important :uestions and none commands uni(ersal assent 4ndeed, +hen t+o do9en

prominent theorists +ere recently asked to def ine intelligence, they ga(e t+o do9en some+hat dif f erent

def initions ;Sternberg < $etterman, 1=0> Such disagreements are not cause f or dismay Scientif ic research

rarely begins +ith f ully agreed def initions, though it may e(entually lead to them

This f irst section of our report re(ie+s the approaches to intelligence that are currently inf luential, or that seem

to be becoming so ?ere ;as in later sections> much of our discussion is de(oted to the dominant

ps ychometric approach, +hich has not only inspired the most research and attracted the most attention ;upto this time> but is by f ar the most +idely used in practical settings #e(ertheless, other points of (ie+ deser(e

serious consideration Se(eral current theorists argue that there are many dif f erent 8intelligences8 ;systems of

abilities>, only a f e+ of +hich can be captured by standard psychometric tests thers emphasi9e the role of

culture, both in establishing dif f erent conceptions of intelligence and in inf luencing the ac:uisition of intellectual

skills $e(elopmental psychologists, taking yet another direction, of ten f ocus more on the processes by +hich

all children come to think intelligently than on measuring indi(idual dif f erences among them There is also a

ne+ interest in the neural and biological bases of intelligence, a f ield of research that seems certain to e5 pand

in the ne5t f e+ years

4n this brief report, +e cannot do f ull ustice to e(en one such approach Rather than trying to do so, +e

f ocus here on a limited and rather specif ic set of :uestions)

@hat are the signif icant conceptuali9ations of intelligence at this time ;Section 4>



@hat do intelligence test scores mean, +hat do they predict, and ho+ +ell do they predict it ;Section

44>

@hy do indi(iduals dif f er in intelligence, and especially in their scores on intelligence tests ur

discussion of these :uestions implicates both genetic f actors ;Section 444> and en(ironmental f actors

;Section 4>

$o (arious ethnic groups display dif f erent patterns of perf ormance on intelligence tests, and if so +hat

might e5plain those dif f erences ;Section >

@hat signif icant scientif ic issues are presently unresol(ed ;Section 4>

Public discussion of these issues has been especially (igorous since the 1C publication of ?ermstein and

Durray6s The Bell Curve, a contro(ersial (olume +hich stimulated many e:ually contro(ersial re(ie+s and

replies #e(ertheless, +e do not directly enter that debate ?ermstein and Durray ;and many of their critics>

ha(e gone +ell beyond the scientif ic f indings, making e5plicit recommendations on (arious aspects of public

policy ur concern here, ho+e(er, is +ith science rather than policy The charge to our Task Force +as to

prepare a dispassionate sur(ey of the state of the art) to make clear +hat has been scientif ically established,

+hat is presently in dispute, and +hat is still unkno+n 4n f ulf illing that charge, the only recommendations +e

shall make are f or f urther research and calmer debate

The Psychometric Approach

'(er since Alf red Binet6s great success in de(ising tests to distinguish mentally retarded children f rom those

+ith beha(ior problems, psychometric instruments ha(e played an im portant part in 'uropean and American

lif e Tests are used f or many purposes, such as selection, diagnosis, and e(aluation Dany of the most +idely

used tests are not intended to measure intelligence itself but some closely related construct) scholastic

aptitude, school achie(ement, specif ic abilities, etc Such tests are especially im portant f or selection purposes

For preparatory school, it6s the SSAT& f or college, the SAT or A%T& f or graduate school, the ER'& f or

medical school, the DAT& f or la+ school, the SAT& f or business school, the EDAT Scores on

intelligenceGrelated tests matter, and the stakes can be high

Intelligence tests. Tests of intelligence itself ;in the psychometric sense> come in many f orms Some use

only a single type of item or :uestion& e5amples include the Peabody Picture ocabulary Test ;a measure of

children6s (erbal intelligence> and Ra(en6s Progressi(e Datrices ;a non(erbal, untimed test that re:uires

inducti(e reasoning about perceptual patterns> Although such instruments are usef ul f or specif ic purposes, the

more f amiliar measures of general intelligence, such as the @echsler tests and the Stanf ordGBinet, include

many dif f erent types of items, both (erbal and non(erbal TestGtakers may be asked to gi(e the meanings of

+ords, to complete a series of pictures, to indicate +hich of se(eral +ords does not belong +ith the others,and the like Their perf ormance can then be scored to yield se(eral subscores as +ell as an o(erall score

By con(ention, o(erall intelligence test scores are usuall( con(erted to a scale in +hich the mean is 1.. and

the standard de(iation is 1! ;The standard de(iation is a measure of the (ariability of the distribution of

scores> Appro5imately !H of the population has scores +ithin t+o standard de(iations of the mean, ie

bet+een 7. and 1I. For historical reasons, the term 84J8 is of ten used to describe scores on tests of

intelligence 4t originally ref erred to an 8intelligence Juotient8 that +as f ormed by di(iding a soGcalled mental

age by a chronological age, but this procedure is no longer used

Intercorrelations among Tests. 4ndi(iduals rarely perf orm e:ually +ell on all the dif f erent kinds of

items included in a test of intelligence ne person may do relati(ely better on (erbal than on spatial items, f or

e5am ple, +hile another may sho+ the opposite pattern #e(ertheless, subtests measuring dif f erent abilities

tend to be positi(ely correlated) people +ho score high on one such subtest are likely to be abo(e a(erage on



others as +ell These comple5 patterns of correlation can be clarif ied by f actor analysis, but the results of

such analyses are of ten contro(ersial themsel(es Some theorists ;eg, Spearman, 1K7> ha(e emphasi9ed

the importance of a general f actor, g, +hich represents +hat all the tests ha(e in common& others ;eg,

Thurstone, 1I=> f ocus on more specif ic group f actors such as memory, (erbal comprehension, or number

f acility As +e shall see in Section K, one common (ie+ today en(isages something like a hierarchy of f actors

+ith g at the ape5 But there is no f ull agreement on +hat g actually means) it has been described as a mere

statistical regularity ;Thompson, 1I>, a kind of mental energy ;Spearman, 1K7>, a generali9ed abstract

reasoning ability ;Eustaf sson 1=C>, or an inde5 measure of neural processing speed ;R eed < 2ensen, 1K>

There ha(e been many disputes o(er the utility of 4J and g Some theorists are critical of the entire

psychometric approach ;eg, %eci, 1.& Eardner, 1=I& Eould, 17=>, +hile others regard it as f irmly

established ;eg, %arroll, 1I& 'ysenck, 17I& ?ermstein < Durray, 1C& 2ensen, 17K> The critics do

not dispute the stability of test scores, nor the f act that they predict certain f orms of achie(ementGespecially

school achie(ementGGrather ef f ecti(ely ;see Section K> They do argue, ho+e(er, that to base a concept of

intelligence on test scores alone is to ignore many important aspects of mental ability Some of those aspects

are em phasi9ed in other approaches re(ie+ed belo+

!ltiple Forms of Intelligence

Gardner"s Theory. A relati(ely ne+ approach is the theory of 8multiple intelligences8& proposed by

?o+ard Eardner ;1=I> n this (ie+ conceptions of intelligence should be inf ormed not only by +ork +ith

normal children and adults but also by studies of gif ted indi(iduals ;including soGcalled 6sa(ants8>, of persons

+ho ha(e suf f ered brain damage, of e5perts and (irtuosos, and of indi(iduals f rom di(erse cultures These

considerations ha(e led Eardner to include musical, bodlilyGkinesthetic, and (arious f orms of personal

intelligence as +ell as more f amiliar spatial, linguistic, and logical mathematical abilities in the scope of his

theory ?e argues that psychometric tests address only linguistic and logical plus some aspects of spatial

intelligence& other f orms ha(e been entirely ignored Doreo(er, the paper andGpencil f ormat of most tests

rules out many kinds of intelligent perf ormance that matter in e(eryday lif e, such as gi(ing an e5temporaneous

talk ;linguistic> or being able to f ind one6s +ay in a ne+ to+n ;spatial> @hile Eardner6s arguments ha(e

attracted considerable interest, the stability and (alidity of perf ormance tests in these ne+ domains has yet to

be conclusi(ely demonstrated 4t is also possible to doubt +hether some of these abilitiesGbodilyGkinesthetic,8

f or e5ampleGGare appropriately described as f orms of intelligence rather than as special talents

Stern#erg"s Theory. Robert Sternberg6s ;1=!> triarchic theory proposes three f undamental aspects of

intelligenceGanalytic, creati(e, and practicalGGof +hich only the f irst is measured to any signif icant e5tent by

mainstream tests ?is in(estigations suggest the need f or a balance bet+een analytic intelligence, on the onehand, and creati(e and especially practical intelligence on the other The distinction bet+een analytic ;or

8academic8> and practical intelligence has also been made by others ;eg, #eisser, 170> Analytic problems,

of the type suitable f or test construction, tend to ;a> ha(e been f ormulated by other people, ;b> be clearly

def ined, ;c> come +ith all the inf ormation needed to sol(e them, ;d> ha(e only a single right ans+er, +hich

can be reached by only a single method, ;e> be disembodied f rom ordinary e5perience, and ;f > ha(e little or

no intrinsic interest Practical problems, in contrast, tend to ;a> re:uire problem recognition and f ormulation,

;b> be poorly def ined, ;c> re:uire inf ormation seeking, ;d> ha(e (arious acceptable solutions, ;e> be

em bedded in and re:uire prior e(eryday e5perience, and ;f > re:uire moti(ation and personal in(ol(ement

As part of their study of practical intelligence, Sternberg and his collaborators ha(e de(eloped measures of

8tacit knowledge8 in (arious domains, especially business management 4n these measures, indi(iduals are

gi(en +ritten scenarios of (arious +ork related situations and then asked to rank a number of options f or

dealing +ith the situation presented The results sho+ that tacit kno+ledge predicts such criteria such as ob



perf ormance f airly +ell, e(en though it is relati(ely independent of intelligence test scores and other common

selection measures ;Sternberg < @agner, 1I& Sternberg, @agner, @illiams < ?or(ath, in press> This

+ork, too, has its critics ;2ensen, 1I& Schmidt < ?unter, 1I>

$elated Findings. ther in(estigators ha(e also demonstrated the relati(e independence of academic

and practical intelligence Bra9ilian street children, f or e5ample, are :uite capable of doing the math re:uired

f or sur(i(al in their street business e(en though they ha(e f ailed mathematics in school ;%arraher, %arraher,

and Schliemann, 1=!> Similarly, +omen shoppers in %alif ornia +ho had no dif f iculty in comparing product(alues at the supermarket +ere unable to carry out the same mathematical operations in paperGand pencil

tests ;a(e, 1==> 4n a study of e5pertise in +agering on harness races, %eci and iker ;1=0> f ound that

the skilled handicappers implicitly used a highly comple5 interacti(e model +ith as many as se(en (ariables&

the ability to do this successf ully +as unrelated to scores on intelligence tests

C!lt!ral %ariation.

4t is (ery dif f icult to com pare concepts of intelligence across cultures 'nglish is not alone in ha(ing many

+ords f or dif f erent aspects of intellectual po+er and cogniti(e skill ;wise, sensible, smart, bright, clever;cunning, etc.>& if another language has ust as many, +hich of them shall +e say corresponds to its speakers6

8concept of intelligence8 The f e+ attempts to e5amine this issue directly ha(e typically f ound that, e(en +ithin

a gi(en society, dif f erent cogniti(e characteristics are em phasi9ed f rom one situation to another and f rom one

subculture to another;Serpell, 17C& Super, 1=I& @ober, 17C> These dif f erences e5tend not ust to

conceptions of intelligence but to +hat is considered adapti(e or appropriate in a broader sense

These issues ha(e occasionally been addressed across subGcultures and ethnic groups in America 4n a study

conducted in San 2ose %alif ornia, kagaki and Sternberg ;1I> asked immigrant parents f rom %ambodia,

De5ico, the Philippines and ietnam, as +ell as nati(eGborn AngleGAmericans and De5icanGAmericans,

about their conceptions of childGrearing, appropriate teaching, and children6s intelligence Parents f rom all

groups e5cept AngleGAmericans indicated that such characteristics as moti(ation, social skills, and practical

school skills +ere as or more important than cogniti(e characteristics f or their conceptions of an intelligent

f irstGgrade child

?eath ;1=I> f ound that dif f erent ethnic groups in #orth %arolina ha(e dif f erent conceptions of intelligence

To be considered as intelligent or adapti(e, one must e5cel in the skills (alued by one6s o+n group ne

particularly interesting contrast +as in the importance ascribed to (erbal (s non(erbal communication skillsGG

to saying things e5plicitly as opposed to using and understanding gestures and f acial e5pressions #ote that

+hile both these f orms of communicati(e skill ha(e their uses, they are not e:ually +ell represented in psychometric tests

?o+ testing is done can ha(e dif f erent ef f ects in dif f erent cultural grou ps This can happen f or many reasons,

including dif f erential f amiliarity +ith the test materials themsel(es Serpell ;17>, f or e5am ple, asked Lam bian

and 'nglish children to reproduce patterns in three media) +ire models, clay models, or pencil and paper The

Lambian children e5celled in the +ire medium +ith +hich they +ere f amiliar, +hile the 'nglish children +ere

best +ith pencil and paper Both groups perf ormed e:ually +ell +ith clay

&e'elopmental Progressions

Piaget"s Theory. The bestGkno+n de(elopmentallyGbased conception of intelligence is certainly that of

the S+iss psychologist 2ean Piaget ;17K> "nlike most of the theorists considered here, Piaget had relati(ely

little interest in indi(idual dif f erences 4ntelligence de(elops in all children through the continually shif ting



balance bet+een the assimilation of ne+ inf ormation into e5isting cogniti(e structures and the accommodation

of those structures themsel(es to the ne+ inf ormation To inde5 the de(elopment of intelligence in this sense,

Piaget de(ised methods that are rather dif f erent f rom con(entional tests To assess the understanding of

8conser(ation8 f or e5ample, ;roughly, the principle that material :uantity is not af f ected by mere changes of

shape>, children +ho ha(e +atched +ater being poured f rom a shallo+ to a tall beaker may be asked if there

is no+ more +ater than bef ore ;A positi(e ans+er +ould suggest that the child has not yet mastered the

principle of conser(ation> Piaget6s tasks can be modif ied to ser(e as measures of indi(idual dif f erences& +hen

this is done, they correlate f airly +ell +ith standard psychometric tests ;f or a re(ie+ see 2ensen, 1=.>

%ygots(y"s Theory. The R ussian psychologist e( ygotsky ;17=> argued that all intellectual abilities

are social in origin anguage and thought f irst appear in early interactions +ith parents, and continue to

de(elop through contact +ith teachers and others Traditional intelligence tests ignore +hat ygotsky called

the 89one of pro5imal de(elopment8 ie, the le(el of perf ormance that a child might reach +ith appropriate

help f rom a su pporti(e adult Such tests are 8static8 measuring only the intelligence that is already f ully

de(eloped 8$ynamic8 testing, in +hich the e5aminer pro(ides guided and graded f eedback, can go f urther to

gi(e some indication of the child6s latent potential These ideas are being de(eloped and e5tended by a

number of contemporary psychologists ;Bro+n < French, 17& Feuerstein, 1=.& PascualGeone < 4 a9,1=>

)iological Approaches

Some in(estigators ha(e recently turned to the study of the brain as a basis f or ne+ ideas about +hat

intelligence is and ho+ to measure it Dany aspects of brain anatomy and physiology ha(e been suggested as

potentially rele(ant to intelligence) the arbori9ation of cortical neurons ;%eci, 1.>, cerebral glucose

metabolism ;?aier 1I>, e(oked potentials ;%aryl, 1C>, ner(e conduction (elocity ;R eed < 2ensen,

1K>, se5 hormones ;see Section C>, and still others ;cf ernon, 1I> Ad(ances in research methods,

including ne+ f orms of brain imaging such as P'T and DR4 scans, +ill surely add to this list 4n the notGtooG

distant f uture it may be possible to relate some aspects of test perf ormance to specif ic characteristics of brain

f unction

This brief sur(ey has re(ealed a +ide range of contem porary conceptions of intelligence and of ho+ it should

be measured The psychometric approach is the oldest and best established, but others also ha(e much to

contribute @e should be open to the possibility that our understanding of intelligence in the f uture +ill be

rather dif f erent f rom +hat it is today

II INTELLIGENCE TESTS AN& T*EI$ CO$$ELATES

Tests as Predictors

School Performance. 4ntelligence tests +ere originally de(ised by Alf red Binet to measure children6s

ability to succeed in school They do in f act predict school perf ormance f airly +ell) the correlation bet+een 4S

scores and grades is about !. They also predict scores on school achie(ement tests, designed to measure

kno+ledge of the curriculum #ote, ho+e(er, that correlations of this magnitude account f or only about K!Hof the o(erall (ariance Successf ul school learning depends on many personal characteristics other than

intelligence, such as persistence, interest in school, and +illingness to study The encouragement f or academic

achie(ement that is recei(ed f rom peers, f amily and teachers may also be im portant, together +ith more

general cultural f actors ;see Section !>



The relationship bet+een test scores and school perf ormance seems to be ubi:uitous @here(er it has been

studied, children +ith high scores on tests of intelligence tend to learn more of +hat is taught in school than

their lo+erGscoring peers There may be styles of teaching and methods of instruction that +ill decrease or

increase this correlation, but none that consistently eliminates it has yet been f ound ;%ronbach and Sno+,

177>

@hat children learn in school depends not only on their indi(idual abilities but also on teaching practices and

on +hat is actually taught R ecent comparisons among pupils attending school in dif f erent countries ha(emade this especially ob(ious %hildren in 2apan and %hina, f or e5am ple, kno+ a great deal more math than

American children e(en though their intelligence test scores are :uite similar ;see Section !> This dif f erence

may result f rom many f actors, including cultural attitudes to+ard schooling as +ell as the sheer amount of time

de(oted to the study of mathematics and ho+ that study is organi9ed ;Ste(enson < Stigler, 1K> 4n

principle it is :uite possible to impro(e the school learning of American childrenGGe(en (ery substantiallyG

+ithout changing their intelligence test scores at all

+ears of Ed!cation. Some children stay in school longer than others& many go on to college and

perhaps beyond T+o (ariables that can be measured as early as elementary school correlate +ith the total

amount of education indi(iduals +ill obtain) test scores and social class background %orrelations bet+een 4J

scores and total years of education are about !!, implying that dif f erences in psychometric intelligence

account f or about I.H of the outcome (ariance The correlations of years of education +ith social class

background ;as inde5ed by the occupation* education of a child6s parents> are also positi(e, but some+hat

lo+er

There are a num ber of reasons +hy children +ith higher test scores tend to get more education They are

likely to get good grades, and to be encouraged by teachers and counselors& of ten they are placed in 8college

preparatory8 classes, +here they make f riends +ho may also encourage them 4n general, they are likely to

f ind the process of education re+arding in a +ay that many lo+Gscoring children do not ;R ehberg andRosenthal, 17=> These inf luences are not omnipotent) some high scoring children do drop out of school

Dany personal and social characteristics other than psychometric intelligence determine academic success

and interest, and social pri(ilege may also play a role #e(ertheless, test scores are the best single predictor

of an indi(idual6s years of education

4n contemporary American society, the amount of schooling that adults complete is also some+hat predicti(e

of their social status ccupations considered high in prestige ;eg, la+, medicine, e(en corporate business>

usually re:uire at least a college degreeG10 or more years of educationGas a condition of entry 4t is partly

because intelligence test scores predict years of education so +ell that they also predict occupational status,and e(en income to a smaller e5tent, ;2encks, 17> Doreo(er, many occupations can only be entered

through prof essional schools +hich base their admissions at least partly on test scores) the D%AT, the

EDAT, the SAT, etc 4ndi(idual scores on admissionGrelated tests such as these are certainly correlated

+ith scores on tests of intelligence

Social Stat!s and Income. ?o+ +ell do 4J scores ;+hich can be obtained bef ore indi(iduals enter

the labor f orce> predict such outcome measures as the social status or income of adults This :uestion is

comple5, in part because another (ariable also predicts such outcomes) namely, the socioeconomic status

;S'S> of one6s parents "nsurprisingly, children of pri(ileged f amilies are more likely to attain high social

status than those +hose parents are poor and less educated These t+o predictors ;4J and parental S'S> are by no means independent of one another& the correlation bet+een them is around II ;@hite, 1=K>

ne +ay to look at these relationships is to begin +ith S'S According to 2encks ;17>, measures of

parental S'S predict about oneGthird of the (ariance in young adults6 social status and about oneGf if th of the



(ariance in their income About half of this predicti(e ef f ecti(eness depends on the f act that the S'S of

parents also predicts children6s intelligence test scores, +hich ha(e their o+n predicti(e (alue f or social

outcomes& the other half comes about in other +ays

@e can also begin +ith 4J scores, +hich by themsel(es account f or about oneGf ourth of the social status

(ariance and oneGsi5th of the income (ariance Statistical controls f or parental S'S eliminate only about a

:uarter of this predicti(e po+er ne +ay to conceptuali9e this ef f ect is by comparing the occupational status

;or income> of adult brothers +ho gre+ u p in the same f amily and hence ha(e the same parental S'S 4n suchcases, the brother +ith the higher adolescent 4J score is likely to ha(e the higher adult social status and

income ;2encks, 17> This ef f ect, in turn, is substantially mediated by education) the brother +ith the higher

test scores is likely to get more schooling, and hence to be better credentialled as he enters the +orkplace

$o these data im ply that psychometric intelligence is a maor determinant of social status or income That

depends on +hat one means by maor 4n f act, indi(iduals +ho ha(e the same test scores may dif f er +idely in

occupational status and e(en more +idely in income %onsider f or a moment the distribution of occu pational

status scores f or all indi(iduals in a population, and then consider the conditional distribution of such scores

f or ust those indi(iduals +ho test at some gi(en 4= 2encks ;17> notes that the standard de(iation of the

latter distribution may still be :uite large& in some cases it amounts to about ==H of the standard de(iation f or

the entire population ie+ed f rom this perspecti(e, psychometric intelligence appears as only one of a great

many f actors that inf luence social outcomes

,o# Performance. Scores on intelligence tests predict (arious measures of ob perf ormance) super(isor

ratings, +ork sam ples, etc Such correlations, +hich typically lie bet+een rMI. and rM!., are partly

restricted by the limited reliability of those measures themsel(es They become higher +hen ris statistically

corrected f or this unreliability) in one sur(ey of rele(ant studies ;?unter, 1=I>, the mean of the corrected

correlations +as !C This im plies that, across a +ide range of occu pations, intelligence test perf ormance

accounts f or some KH of the (ariance in ob perf ormance

Although these correlations can sometimes be modif ied by changing methods of training or aspects of the ob

itself , intelligence test scores are at least +eakly related to ob perf ormance in most settings Sometimes 1

scores are described as the 6best a(ailable predictor8 of that perf ormance 4t is +orth noting, ho+e(er, that

such tests predict considerably less than half the (ariance of obGrelated measures ther indi(idual

characteristics such as interpersonal skills, aspects of personality, etc, are probably of e:ual or greater

im portance, but at this point +e do not ha(e e:ually reliable instruments to measure them

Social O!tcomes. Psychometric intelligence is negati(ely correlated +ith certain socially undesirable

outcomes For e5ample, children +ith high test scores are less likely than lo+erGscoring children to engage in

u(enile crime in one study, Dof f itt, Eabrielli, Dednick < Schulsinger ;1=1> f ound a correlation of G1

bet+een 4J scores and number of u(enile of f enses in a large $anish sam ple& +ith social class controlled, the

correlation dropped to G 17 The correlations f or most 8negati(e outcome8 (ariables are ty pically smaller than

K., +hich means that test scores are associated +ith less than CH of their total (ariance 4t is im portant to

reali9e that the causal links bet+een psychemetric ability and social outcomes may be indirect %hildren +ho

are unsuccessf ul inGand hence alienated f romGschool may be more likely to engage in delin:uent beha(iors f or

that (ery reason, compared to other children +ho en oy school and are doing +ell

4n summary, intelligence test scores predict a +ide range of social outcomes +ith (arying degrees of success%orrelations are highest f or school achie(ement, +here they account f or about a :uarter of the (ariance They

are some+hat lo+er f or ob perf ormance, and (ery lo+ f or negati(ely (alued outcomes such as criminality 4n

general, intelligence tests measure only some of the many personal characteristics that are rele(ant to lif e in

contemporary America Those characteristics are ne(er the only inf luence on outcomes, though in the case of



school perf ormance they may +ell be the strongest

III. T*E GENES AN& INTELLIGENCE

4n this section of the report +e f irst discuss indi(idual dif f erences generally, +ithout ref erence to any particular

trait @e then f ocus on intelligence, as measured by con(entional 4J tests or other tests intended to measuregeneral cogniti(e ability The dif f erent and more contro(ersial topic of group dif f erences +ill be considered in

Section

@e f ocus here on the relati(e contributions of genes and en(ironments to indi(idual dif f erences in particular

traits To a(oid misunderstanding, it must be em phasi9ed f rom the outset that gene action al+ays in(ol(es an

en(ironmentGGat least a biochemical en(ironment, and of ten an ecological one ;For humans, that ecology is

usually interpersonal or cultural> Thus all genetic ef f ects on the de(elopment of obser(able traits are

potentially modif iable by en(ironmental input, though the practicability of making such modif ications may be

another matter %on(ersely, all en(ironmental ef f ects on trait de(elopment in(ol(e the genes or structures to

+hich the genes ha(e contributed Thus there is al+ays a genetic aspect to the ef f ects of the en(ironment ;cf

Plomin < Bergeman, 11>

So!rces of Indi'id!al &if f erences

Partitioning the %ariation. 4ndi(iduals dif f er f rom one another on a +ide (ariety of traits) f amiliar

e5am ples include height intelligence, and aspects of personality Those dif f erences are of ten of considerable

social im portance Dany interesting :uestions can be asked about their nature and origins ne such :uestion

is the e5tent to +hich they ref lect dif f erences among the genes of the indi(iduals in(ol(ed, as distinguished

f rom dif f erences among the en(ironments to +hich those indi(iduals ha(e been e5posed The issue here is not

+hether genes and en(ironments are both essential f or the de(elopment of a gi(en trait ;this is al+ays the

case>, and it is not about the genes or en(ironment of any particular person @e are concerned only +ith the

obser(ed (ariation of the trait across indi(iduals in a gi(en population A f igure called the 8heritability8 ;hK> of

the trait represents the proportion of that (ariation that is associated +ith genetic dif f erences among the

indi(iduals The remaining (ariation ;1 G hK3 is associated +ith en(ironmental dif f erences and +ith errors of

measurement These proportions can be estimated by (arious methods described belo+

Sometimes special interest attaches to those aspects of en(ironments that f amily members ha(e in common

;f or e5ample, characteristics of the home> The part of the (ariation that deri(es f rom this source, called

8shared8 (ariation or cK, can also be estimated

A high heritability does not mean that the en(ironment has no impact on the de(elopment of a trait, or that

learning is not in(ol(ed ocabulary si9e, f or e5ample, is (ery substantially heritable ;and highly correlated

+ith general intelligence> although e(ery +ord in an indi(idual6s (ocabulary is learned 4n a society in +hich

plenty of +ords are a(ailable in e(eryone6s en(ironment, especially f or indi(iduals +ho are moti(ated to seek

them out, the num ber of +ords that indi(iduals actually learn depends to a considerable e5tent on their genetic

predispositions

*o- Genetic Estimates are ade. 'stimates of the magnitudes of these sources of indi(idual

dif f erences are made by e5ploiting natural and social 6e5periments8 that combine genotypes and en(ironments



in inf ormati(e +ays Dono9ygotic ;DL> and dy9ygotic ;$L> t+ins, f or e5ample, can be regarded as

e5 periments ofnature DL t+ins are paired indi(iduals of the same age gro+ing u p in the same f amily +ho

ha(e all their genes in common& $L t+ins are other+ise similar pairs +ho ha(e only half their genes in

common Adoptions, in contrast, are e5periments of society They allo+ one to compare genetically unrelated

persons +ho are gro+ing up in the same f amily as +ell as genetically related persons +ho are gro+ing up in

dif f erent f amilies They can also pro(ide inf ormation about genotypeGen(ironment correlations) in ordinary

f amilies genes and en(ironments are correlated because the same parents pro(ide both, +hereas in adopti(e

f amilies one set of parents pro(ides the genes and another the en(ironment An e5periment in(ol(ing bothnature and society is the study of mono9ygotic t+ins +ho ha(e been reared apart ;Bouchard, ykken,

DcEue, Segal < Tellegen, 1.& Pedersen, Plomin, #esselroade < Dc%learn, 1K> Relationships in the

f amilies of mono9ygotic t+ins also of f er uni:ue possibilities f or analysis ;eg, Rose, ?arris, %hristian, <

#ance, 17> Because these comparisons are su bect to dif f erent sources of potential error, the results of

studies in(ol(ing se(eral kinds of kinship are of ten analy9ed together to arri(e at robust o(erall conclusions

;For general discussions of beha(ior genetic methods, see Plomin, $eFries, < Dc%learn, 1., or ?ay,

1=!>

$es!lts f or I scores

Parameter Estimates. Across the ordinary range of en(ironments in modern @estern societies, a si9able

part of the (ariation in intelligence test scores is associated +ith genetic dif f erences among indi(iduals

Juantitati(e estimates (ary f rom one study to another, because many are based on small or selecti(e

samples 4f one simply combines all a(ailable correlations in a single analysis, the heritability ;hK> +orks out to

about !. and the bet+eenGf amily (ariance ;cK> to about K! ;eg, %hipuer, R o(ine, < Plomin, 1.&

oehlin, 1=> These o(erall f igures are misleading, ho+e(er, because most of the rele(ant studies ha(e

been done +ith children @e no+ kno+ that the heritability of 4J changes +ith age) hK goes up and cK goes

do+n f rom inf ancy to adulthood ;Dc%artney, ?arris, < Bernieri, 1.& DcEue, Bouchard, 4acono, <

ykken, 1I> 4n childhood hK and %K f or 4J are of the order of C! and I!& by late adolescence hK is

around 7! and cK is :uite lo+ ;9ero in some studies> Substantial en(ironmental (ariance remains, but it

primarily ref lects +ithinGf amily rather than bet+eenGf amily dif f erences

Implications. 'stimates of hK and cK f or 4J ;or any other trait> are descripti(e statistics f or the

populations studied ;4n this respect they are like means and standard de(iations> They are outcome

measures, summari9ing the results of a great many di(erse, intricate, indi(idually (ariable e(ents and processes, but they can ne(ertheless be :uite usef ul They can tell us ho+ much of the (ariation in a gi(en trait

the genes and f amily en(ironments e5plain, and changes in them place some constraints on theories of ho+

this occurs n the other hand they ha(e little to say about specif ic mechanisms, ie about ho+ genetic and

en(ironmental dif f erences get translated into indi(idual physiological and psychological dif f erences Dany

psychologists and neuroscientists are acti(ely studying such processes& data on heritabilities may gi(e them

ideas about +hat to look f or and +here or +hen to look f or it

A common error is to assume that because something is heritable it is necessarily unchangeable This is +rong

?eritability does not imply immutability As pre(iously noted, heritable traits can depend on learning, and they

may be subect to other en(ironmental ef f ects as +ell



I%. EN%I$ONENTAL EFFECTS ON INTELLIGENCE

The 6en(ironment8 includes a +ide range of inf luences on intelligence Some of those (ariables af f ect +hole

populations, +hile others contribute to indi(idual dif f erences +ithin a gi(en group Some of them are social,

some are biological& at this point some are still mysterious 4t may also happen that the proper interpretation

of an en(ironmental (ariable re:uires the simultaneous consideration of genetic ef f ects #e(ertheless, a good

deal of solid inf ormation is a(ailable

Social %aria#les

4t is ob(ious that the cultural en(ironment G ho+ people li(e, +hat they (alue, +hat they do G has a signif icant

ef f ect on the intellectual skills de(eloped by indi(iduals Rice f armers in iberia are good at estimating

:uantities of rice ;Eay < %ole, 107>& children in Bots+ana, accustomed to storytelling, ha(e e5cellent

memories f or stories ;$ube, 1=K> Both these groups +ere f ar ahead of American controls on the tasks in

:uestion n the other hand Americans and other @esterni9ed groups ty pically outperf orm members of

traditional societies on psychometric tests, e(en those designed to be 8cultureGf air8

%ultures typically dif f er f rom one another in so many +ays that particular dif f erences can rarely be ascribed to

single causes '(en com parisons bet+een subpopulations are of ten dif f icult to interpret 4f +e f ind that groups

li(ing in dif f erent en(ironments ;eg, middleGclass and poor Americans> dif f er in their test scores, it is easy to

suppose that the en(ironmental dif f erence causes the 4J dif f erence But there is also an opposite direction of

causation) indi(iduals may come to be in one en(ironment or another because of dif f erences in their o+n

abilities, including the abilities measured by intelligence tests @aller ;171> has sho+n, f or e5ample, that sons

+hose 4J scores are abo(e those of their f athers also tend to achie(e a higher social class status& con(ersely,

those +ith scores belo+ their f athers6 tend to achie(e lo+er status Such an ef f ect is not surprising, gi(en the

relation bet+een 4J scores and years of education re(ie+ed in Section 44

Occ!pation. 4n section 44 +e noted that intelligence test scores predict occu pational le(el, not only

because some occupations re:uire more intelligence than others but also because admission to many

prof essions depends on test scores in the f irst place There can also be an ef f ect in the opposite direction, ie

+orkplaces may af f ect the intelligence of those +ho +ork in them Nohn and Schooler ;17I>, +ho

inter(ie+ed some I... men in (arious occupations ;f armers, managers, machinists, porters>, argued that

more 8comple58 obs produce more 8intellectual f le5ibility8 in the indi(iduals +ho hold them Although the

issue of direction of ef f ects complicates the interpretation of their study, this remains a plausible suggestion

Among other things, Nohn < Schooler6s hypothesis may help us understand urban*rural dif f erences Ageneration ago these +ere substantial in the "nited States, a(eraging about si5 4J points or .C standard

de(iations ;Terman < Derrill, 1I7& Seashore, @esman < $oppelt, 1!.> 4n recent years the dif f erence

has declined to about t+o points ;Nauf man < $oppelt, 170& Reynolds, %hastain, Nauf man < Dcean,

1=7> 4n all likelihood this urban* rural con(ergence primarily ref lects en(ironmental changes) a decrease in

rural isolation ;due to increased tra(el and mass communications>, an im pro(ement in rural schools, the

greater use of technology on f arms All these changes can be regarded as increasing the 8comple5ity8 of the

rural en(ironment in general or of f arm +ork in particular ;?o+e(er, processes +ith a genetic component,

eg, changes in the selecti(ity of migration f rom f arm to city, cannot be completely e5cluded as contributing

f actors>

Schooling. Attendance at school is both a dependent and an independent (ariable in relation to

intelligence n the one hand, children +ith higher test scores are less likely to drop out, more likely to be

promoted f rom grade to grade and then to attend college Thus the number of years of education that adults



complete is roughly predictable f rom their childhood scores on intelligence tests n the other hand schooling

itself changes mental abilities, including those abilities measured on psychometric tests This is ob(ious f or

tests like the SAT that are e5plicitly designed to assess school learning, but it is almost e:ually true of

intelligence tests themsel(es

The e(idence f or the ef f ect of schooling on intelligence test scores takes many f orms ;%eci, 11> @hen

children of nearly the same age go through school a year apart ;because of birthdayGrelated admission

criteria>, those +ho ha(e been in school longer ha(e higher mean scores %hildren +ho attend schoolintermittently score belo+ those +ho go regularly, and test perf ormance tends to drop o(er the summer

(acation A striking demonstration of this ef f ect appeared +hen the schools in one irginia county closed f or

se(eral years in the 10.s to a(oid integration, lea(ing most Black children +ith no f ormal education at all

%ompared to controls, the intelligenceGtest scores of these children dropped by about .C standard

de(iations ;0 points> per missed year of school ;Ereen et al, 10C>

Schools af f ect intelligence in se(eral +ays, most ob(iously by transmitting inf ormation The ans+ers to

:uestions like 8@ho +rote ?amlet8 and 8@hat is the boiling point of +ater8 are typically learned in school,

+here some pupils learn them more easily and thoroughly than others Perhaps at least as important are

certain general skills and attitudes) systematic problemGsol(ing, abstract thinking, categori9ation, sustained

attention to material of little intrinsic interest, repeated manipulation of basic symbols and operations There is

no doubt that schools promote and permit the de(elopment of signif icant intellectual skills, +hich de(elop to

dif f erent e5tents in dif f erent children 4t is because tests of intelligence dra+ on many of those same skills that

they predict school achie(ement as +ell as they do

To achie(e these results, the school e5perience must meet at least some minimum standard of :uality 4n (ery

poor schools, children may learn so little that they f all f arther behind the national 4J norms f or e(ery year of

attendance @hen this happens, older siblings ha(e systematically lo+er scores than their younger

counterparts This pattern of scores appeared in at least one rural Eeorgia school system in the 17.s;2ensen, 177> Bef ore desegregation, it must ha(e been characteristic of many of the schools attended by

Black pupils in the South 4n a study based on Black children +ho had mo(ed to Philadelphia at (arious ages

during this period, ee ;1!1> f ound that their 4J scores +ent up more than half a point f or each year that

they +ere enrolled in the Philadelphia system

Inter'entions. 4ntelligence test scores ref lect a child6s standing relati(e to others in his or her age cohort

ery poor or interru pted schooling can lo+er that standing substantially& are there also +ays to raise it 4n

f act many inter(entions ha(e been sho+n to raise test scores and mental ability 6in the short run8 ;ie +hile the

program itself +as in progress>, but longGrun gains ha(e pro(ed more elusi(e ne note+orthy e5ample of ;atleast shortGrun> success +as the ene9uelan 4ntelligence Proect ;?ermstein et al, 1=0>, in +hich hundreds

of se(enthGgrade children f rom underpri(ileged backgrounds in that country +ere e5posed to an e5tensi(e,

theoretically based curriculum f ocused on thinking skills The inter(ention produced substantial gains on a

+ide range of tests, but there has been no f ollo+Gu p

%hildren +ho participate in 8?ead Start8 and similar programs are e5 posed to (arious schoolGrelated

materials and e5periences f or one or t+o years Their test scores of ten go up during the course of the

program, but these gains f ade +ith time By the end of elementary school, there are usually no signif icant 4 or

achie(ementGtest dif f erences bet+een children +ho ha(e been in such programs and controls +ho ha(e not

There may, ho+e(er, be other dif f erences Follo+Gu p studies suggest that children +ho participated in such programs as preschoolers are less likely to be assigned to special education, less likely to be held back in

grade, and more likely to f inish high school than matched controls ;%onsortium f or ongitudinal Studies,

1=I& $arlington, 1=0& but see ocurto, 11>



Dore e5tensi(e inter(entions might be e5pected to produce larger and more lasting ef f ects, but f e+ such

programs ha(e been e(aluated systematically ne of the more successf ul is the %arolina Abecedarian

Proect ;%ampbell < Ramey, 1C>, +hich pro(ided a group of children +ith enriched en(ironments f rom

early inf ancy through preschool and also maintained appropriate controls The test scores of the enrichmentG

group children +ere already higher than those of controls at age t+o& they +ere still some f i(e points higher at

age t+el(e, se(en years af ter the end of the inter(ention 4mportantly, the enrichment group also

outperf ormed the controls in academic achie(ement

Family en'ironment. #o one doubts that normal child de(elopment re:uires a certain minimum le(el of

responsible care Se(erely depri(ed, neglectf ul, or abusi(e en(ironments must ha(e negati(e ef f ects on a great

many aspects of de(elopment, including intellectual aspects Beyond that minimum, ho+e(er, the role of

f amily e5perience is no+ in serious dispute ;Baumrind, 1I& 2ackson, 1I& Scarr, 1K, 1I>

Psychometric intelligence is a case in point $o dif f erences bet+een children6s f amily en(ironments ;+ithin the

normal range> produce dif f erences in their intelligence test perf ormance The problem here is to disentangle

causation f rom correlation There is no doubt that such (ariables as resources of the home ;Eottf ried, 1=C>

and parents6 use of language ;?art < Risley, 1K, in press> are correlated +ith children6s 4J scores, but

such correlations may be mediated by genetic as +ell as ;or instead of > en(ironmental f actors

These f indings suggest that dif f erences in the lif e styles of f amilies +hate(er their importance may be f or many

aspects of children6s li(es make little longGterm dif f erence f or the skills measured by intelligence tests @e

should note, ho+e(er, that lo+Gincome and nonG+hite f amilies are poorly represented in e5isting adoption

studies as +ell as in most t+in samples Thus it is not yet clear +hether these surprisingly small (alues of

;adolescent> cK apply to the population as a +hole 4t reGmains possible that, across the f ull range of income

and ethnicity, bet+eenGf amily dif f erences ha(e more lasting conse:uences f or psychometric intelligence

)iological %aria#les

'(ery indi(idual has a biological as +ell as a social en(ironment, one that begins in the +om b and e5tends

throughout lif e Dany aspects of that en(ironment can af f ect intellectual de(elopment @e no+ kno+ that a

number of biological f actors, including malnutrition, e5posure to to5ic substances, and (arious prenatal and

perinatal stressors, result in lo+ered psychometric intelligence under at least some conditions

N!trition. There has been only one maor study of the ef f ects of prenatal malnutrition ;ie malnutrition of

the mother during pregnancy> on longGterm intellectual de(elopment Stein et al ;17!> analy9ed the testscores of $utch 1GyearGold males in relation to a +artime f amine that had occurred in the +inter of 1CCG

C!, ust bef ore their birth 4n this (ery large sample ;made possible by a uni(ersal military induction

re:uirement>, e5 posure to the f amine had no ef f ect on adult intelligence #ote, ho+e(er, that the f amine itself

lasted only a f e+ months& the sub ects +ere e5posed to it prenatally but not af ter birth

4n contrast, prolonged malnutrition during childhood does ha(e longGterm intellectual ef f ects These ha(e not

been easy to establish, in part because many other unf a(orable socioeconomic conditions are of ten

associated +ith chronic malnutrition ;R icciuti, 1I& but cf Sigman, 1!> 4n one inter(ention study,

ho+e(er, preGschoolers in t+o Euatemalan (illages ;+here undernourishment is common> +ere gi(en ad lib

access to a protein dietary supplement f or se(eral years A decade later, many of these children ;namely,

those f rom the poorest socioGeconomic le(els> scored signif icantly higher on school related achie(ement tests

than comparable controls ;Pollitt et al, 1I> 4t is +orth noting that the ef f ects of poor nutrition on

intelligence may +ell be indirect Dalnourished children are typically less responsi(e to adults, less moti(ated



to learn, and less acti(e in e5ploration than their more ade:uately nourished counterparts

...

Lead. %ertain to5ins ha(e +ell established negati(e ef f ects on intelligence '5posure to lead is one such

f actor 4n one longGterm study ;DcDichael et al, 1==& Baghurst et al, 1K>, the blood lead le(els of

children gro+ing up near a lead smelting plant +ere substantially and negati(ely correlated +ith intelligence

test scores throughout childhood #o 8threshold dose8 f or the ef f ect of lead appears in such studies Althougham bient lead le(els in the "nited States ha(e been reduced in recent years, there is reason to belie(e that

some American children G especially those in inner cities G may still be at risk f rom this source ;cf

#eedleman, Eeiger < Frank, 1=!>

Alcohol '5tensi(e prenatal e5posure to alcohol ;+hich occurs if the mother drinks hea(ily during

pregnancy> can gi(e rise to f etal alcohol syndrome, +hich includes mental retardation as +ell as a range of

physical symptoms Smaller 8doses8 of prenatal alcohol may ha(e negati(e ef f ects on intelligence e(en +hen

the f ull syndrome does not appear Streissguth et al ;1=> f ound that mothers +ho reported consuming more

than 1! o9, of alcohol daily during pregnancy had children +ho scored some f i(e points belo+ controls at

age f our Prenatal e5posure to aspirin and antibiotics had similar negati(e ef f ects in this study

Perinatal Factors. %omplications at deli(ery and other negati(e perinatal f actors may ha(e serious

conse:uences f or de(elopment #e(ertheless, because they occur only rarely, they contribute relati(ely little

to the population (ariance of intelligence /Broman et al, 17!> $o+n6s syndrome, a chromosomal

abnormality that produces serious mental retardation, is also rare enough to ha(e little im pact on the o(erall

distribution of test scores

The correlation bet+een birth +eight and later intelligence deser(es particular discussion 4n some cases lo+

birth +eight simply ref lects premature deli(ery& in others, the inf ant6s si9e is belo+ normal f or its gestationalage Both f actors apparently contribute to the tendency of lo+GbirthG+eight inf ants to ha(e lo+er test scores

in later childhood ;ubchenko, 170> These correlations are small, ranging f rom .! to 1I in dif f erent

groups ;Broman et al, 17!> The ef f ects of lo+ birth +eight are su bstantial only +hen it is (ery lo+ indeed

;less than 1!.. gm> Premature babies born at these (ery lo+ birth +eights are behind controls on most

de(elopmental measures& they of ten ha(e se(ere or permanent intellectual def icits ;R osetti, 1=0>

Contin!o!sly $ising Test Scores

Perhaps the most striking of all en(ironmental ef f ects is the steady +orld+ide rise in intelligence test

perf ormance Although many psychometricians had noted these gains, it +as 2ames Dynn ;1=C, 1=7> +ho

f irst described them systematically ?is analysis sho+s that perf ormance has been going u p e(er since testing

began The 8Flynn 'f f ect8 is no+ (ery +ell documented, not only in the "nited States but in many other

technologically ad(anced countries The a(erage gain is about three 4J points per decade& more than a f ull

standard de(iation since, say, 1C.

Although it is sim plest to describe the gains as increases in population 4J, this is not e5actly +hat happens

Dost intelligence tests are 8reGstandardi9ed8 f rom time to time, in part to keep up +ith these (ery gains As

part of this process the mean score of the ne+ standardi9ation sample is ty pically set to 1.. again, so the

increase more or less disappears f rom (ie+ 4n this conte5t, the Flynn ef f ect means that if t+enty years ha(e passed since the last time the test +as standardi9ed, people +ho no+ score 1.. on the ne+ (ersion +ould

probably a(erage about 1.0 on the old one

The sheer e5tent of these increases is remarkable, and the rate of gain may e(en be increasing The scores of



nineteenGyearGolds in the #etherlands, f or e5am ple, +ent up more than = pointsGGo(er half a standard

de(iationGbet+een 17K and 1=K @hat6s more, the largest gains appear on the types of tests that +ere

specif ically designed to be f ree of cultural inf luence ;Flynn, 1=7> ne of these is Ra(en6s Progressi(e

Datrices, an untimed nonG(erbal test that many psychometricians regard as a good measure of g

These steady gains in intelligence test perf ormance ha(e not al+ays been accom panied by corresponding

gains in school achie(ement 4ndeed, the relation bet+een intelligence and achie(ement test scores can be

comple5 This is especially true f or the Scholastic Aptitude Test ;SAT>, in part because the ability range of the students +ho take the SAT has broadened o(er time That change e5plains some portion, but not all, of

the prolonged decline in SAT scores that took place f rom the mid nineteenGsi5ties to the early eighties, e(en

as 4J scores +ere continuing to rise;Flynn, 1=C> Dean+hile, ho+e(er, other more representati(e measures

sho+ that school achie(ement le(els ha(e held steady or in some cases actually increased /?ermstein <

Durray, 1C> The #ational Assessment of 'ducational Progress ;#A'P>, f or e5ample, sho+s that the

a(erage reading and math achie(ement of American 1IG and l7GyearGolds im pro(ed some+hat f rom the early

nineteenGse(enties to 1. ;Erissmer, Nirby, Berends < @illiamson, 1C> An analysis of these data by

ethnic group, reported in Section !, sho+s that this small o(erall increase actually ref lects (ery substantial

gains by Blacks and atinos combined +ith little or no gain by @hites

The consistent 4J gains documented by Flynn seem much too large to result f rom simple increases in test

sophistication Their cause is presently unkno+n, but three interpretations deser(e our consideration Perhaps

the most plausible of these is based on the striking cultural dif f erences bet+een successi(e generations $aily

lif e and occupational e5 perience both seem more 8comple58 ;Nohn < Schooler, 17I> today than in the time

of our parents and grandparents The population is increasingly urbani9ed& tele(ision e5poses us to more

inf ormation and more perspecti(es on more topics than e(er bef ore& children stay in school longer& almost

e(eryone seems to be encountering ne+ f orms of e5perience These changes in the comple5ity of lif e may

ha(e produced corresponding changes in com ple5ity of mind, and hence in certain psychometric abilities

A dif f erent hypothesis attributes the gains to modern impro(ements in nutrition ynn ;1.> points out that

large nutritionallyGbased increases in height ha(e occurred during the same period as the 4J gains) perhaps

there ha(e been increases in brain si9e as +ell As +e ha(e seen, ho+e(er, the ef f ects of nutrition on

intelligence are themsel(es not f irmly established

The third interpretation addresses the (ery def inition of intelligence Flynn himself belie(es that real

intelligenceG+hate(er it may beGGcannot ha(e increased as much as these data +ould suggest %onsider, f or

e5am ple, the number of indi(iduals +ho ha(e 4J scores of 1C. or more ;This is slightly abo(e the cutof f

used by D Terman ;1K!> in his f amous longitudinal study of 8genius8> 4n 1!K only .I=H of $utch test

takers had 4Js o(er 1C.& in 1=K, scored by the same norms, 1KH e5ceeded this f igureO 2udging by these

criteria, the #etherlands should no+ be e5periencing 8a cultural renaissance too great to be o(erlooked8

;Flynn, 1=7, p1=7> So too should France, #or+ay, the "nited States, and many other countries Because

Flynn ;1=7> f inds this conclusion im plausibie or absurd, he argues that +hat has risen cannot be intelligence

itself but only a minor sort of 8abstract problem sol(ing ability8 The issue remains unresol(ed

Indi'id!al Lif e E/periences

Although the en(ironmental (ariables that produce large dif f erences in intelligence are not yet +ell

understood, genetic studies assure us that they e5ist @ith a heritability +ell belo+ 1.., 4J must be subect

to substantial en(ironmental inf luences Doreo(er, a(ailable heritability estimates apply only +ithin the range

of en(ironments that are +ellGrepresented in the present population @e already kno+ that some relati(ely

rare conditions, like those re(ie+ed earlier, ha(e large negati(e ef f ects on intelligence @hether there are



;no+ e:ually rare> conditions that ha(e large positi(e ef f ects is not kno+n

As +e ha(e seen, there is both a biological and a social en(ironment For any gi(en child, the social f actors

include not only an o(erall cultural* social*school setting and a particular f amily but also a uni:ue 8microG

en(ironment8 of e5 periences that are shared +ith no one else The adoption studies re(ie+ed in Section I

sho+ that f amily (ariables, such as dif f erences in parenting style, in the resources of the home, etc, ha(e

smaller longGterm ef f ects than +e once su pposed At least among people +ho share a gi(en S'S le(el and a

gi(en culture, it seems to be uni:ue indi(idual e5perience that makes the largest en(ironmental contribution toadult 4J dif f erences

@e do not yet kno+ +hat the key f eatures of those microGen(ironments may be Are they biological Social

%hronic Acute 4s there something especially im portant in the earliest relations bet+een the inf ant and its

caretakers @hate(er the critical (ariables may be, do they interact +ith other aspects of f amily lif e f

culture At this point +e cannot say, but these :uestions of f er a f ertile area f or f urther research

4n this contentious arena, our most usef ul role may be to remind our readers that many of the critical :uestionsabout intelligence are still unans+ered ?ere are a f e+ of those :uestions)

1 $if f erences in genetic endo+ment contribute substantially to indi(idual dif f erences in ;psychometric>

intelligence, but the path+ay by +hich genes produce their ef f ects is still unkno+n The im pact of

genetic dif f erences appears to increase +ith age, but +e do not kno+ +hy

K 'n(ironmental f actors also contribute substantially to the de(elopment of intelligence, but +e do not

clearly understand +hat those f actors are or ho+ they +ork Attendance at school is certainly

important, f or e5ample, but +e do not kno+ +hat aspects of schooling are critical

I The role of nutrition in intelligence remains obscure Se(ere childhood malnutrition has clear negati(eef f ects, but the hy pothesis that particular 8microGnutrients8 may af f ect intelligence in other+ise

ade:uatelyGf ed populations has not yet been con(incingly demonstrated

C There are signif icant correlations bet+een measures of inf ormation processing speed and psychometric

intelligence, but the o(erall pattern of these f indings yields no easy theoretical interpretation

! Dean scores on intelligence tests are rising steadily They ha(e gone up a f ull standard de(iation in the

last f if ty years or so, and the rate of gain may be increasing #o one is sure +hy these gains are

happening or +hat they mean

0 The dif f erential bet+een the mean intelligence test scores of Blacks and @hites ;about one standard

de(iation, although it may be diminishing> does not result f rom any ob(ious biases in test construction

and administration, nor does it simply ref lect dif f erences in socioGeconomic status '5planations based

on f actors of caste and culture may be appropriate, but so f ar ha(e little direct em pirical support

There is certainly no such support f or a genetic interpretation At present, no one kno+s +hat causes

this dif f erential

7 4t is +idely agreed that standardi9ed tests do not sample all f orms of intelligence b(ious e5amples

include creati(ity, +isdom, practical sense and social sensiti(ity& there are surely others $espite the

importance of these abilities +e kno+ (ery little about them) ho+ they de(elop, +hat f actors inf luence

that de(elopment, ho+ they are related to more traditional measures

4n a f ield +here so many issues are unresol(ed and so many :uestions unans+ered, the conf ident tone thathas characteri9ed most of the debate on these topics is clearly out of place The study of intelligence does not

need politici9ed assertions and recriminations& it needs self Grestraint, ref lection, and a great deal more

research The :uestions that remain are socially as +ell as scientif ically important There is no reason to think

them unans+erable, but f inding the ans+ers +ill re:uire a shared and sustained ef f ort as +ell as the



commitment of substantial scientif ic resources, 2ust such a commitment is +hat +e str ongly r ecommend,

Documents

Intelligence_ Knowns and Unknowns