who discovered twin studies

8/10/2019 Who Discovered Twin Studies

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Behavior Genetics VoL 20 No. 2 1990

W h o D i s c o v e r e d t h e T w i n M e t h o d

R i c h a r d D . R e n d e 1 R o b e r t P l o m i n 1 a n d S t e v e n G . V a n d e n b e r g 2

Rece ived 17F eb. 1989- -Final 23A pr . J989

The twin method is usually credited to Francis Galton's 1875 article on twins.

However, Galton did not propose the comparison between identical and frater-

nal twin resemblance which is the essence of the twin method. Although the

twin method was in the air in the mid-1920s, the first descriptions of the

method appeared in an article by Curtis Merriman and in a book by Hermann

Siemens, both in 1924, 50 years after Galton's paper.

K E Y W O R D S : t w i n m e t h o d ; h is t o ry ; G a l to n ; M e r r im a n ; S i e m e n s .

I N T R O D U C T I O N

The classical twin method that compares the resemblance of identical and fra-

ternal twins is the most widely used method in the armamentarium of human

behavioral genetics. Textbooks on behavioral genetics usually credit Francis

Galton with its discovery. Although Galton deserves to be called the father of

human behavioral genetics for many reasons, discovering the twin method is

not one of them. The first part of this paper examines what Galton said and did

not say about twins, and the second part traces the origins of the twin method,

which did not emerge for 50 years after Galton.

F R A N C I S G A L T O N A N D T W I N S

The discovery of the twin method is usually accorded to Galton's 1875

article in Fraser's Magazine entitled, The History of Twins, as a Criterion of

Prepara tion of thi s a r ticle w as supported in par t by gran ts f rom the N at ional Ins t itute of C hi ld Heal th

and Hum an Dev e l opment (HD-10333 and HD-184 26) , t he Na t iona l Sc ience Founda t i on (BSN-

8806589), and t he Success fu l Agi ng program of t he John D. and Ca t he r i ne T . M acA r t hur Foun-

dat ion.

i The C ent e r fo r Deve l opm ent a l and Hea l t h Gene t i c s , Col lege of Hea l t h and Hum an D eve l opment ,

The Pennsy l van i a S ta t e Uni ve rs i t y , Uni ve rs i t y Pa rk , P ennsy l van i a 16802.

2 Ins t i tute for Beh aviora l Genet ics , The U nivers i ty of Colorado, Bou lder , Colorado 80309-0447.

277

0001-8244/90/0300-0277506.00/0 9 199 0 Plenu m Pub lishing Corporation


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278 Rende Plomin and Vandenberg

t h e R e l a ti v e P o w e r s o f N a t u r e a n d N u r t u r e . T h i s a rt ic le w a s r e p ri n te d i n 1 8 7 6

w i t h s l i g h t r e v i s i o n i n t h e

Journal of the Anthropological Institute

( G a l t o n ,

1 8 7 6 ) . G a l t o n ' s f ir st m e n t i o n o f tw i n s w a s i n 1 8 7 4 i n h i s b o o k , English M en

of Science:

There are twins of the sam e sex so alike in b ody and m ind that not even their own

mo thers can distinguish them . The ir features, voice , and expressions are similar; they

see thing s in the s am e light, and the ir ideas follow the sam e law s of association. Th is

close resemblance necessarily gives way under the gradually accumulated influences

of difference of nu rture, but it often lasts till manho od. (pp. 13-14)

T h i s e a r l y q u o t a t i o n p r e s a g e s G a l t o n ' s u s e o f tw i n s t o t es t t h e p o w e r o f th e

e n v i r o n m e n t t o c h a n g e t w i n s , t h a t i s , t o m a k e i n i t i a l l y s i m i l a r t w i n s d i f f e r e n t

a n d t o m a k e i n i t i a l l y d i f f e r e n t t w i n s s i m i l a r . H e d i d n o t p r o p o s e t h a t t h e r e s e m -

b l a n c e o f id e n t i c a l tw i n s b e c o m p a r e d t o th e r e s e m b l a n c e o f f r a te r n a l p a i rs t o

a s s e s s g e n e t i c i n f l u e n c e .

G a l t o n b e g a n h i s 1 8 7 5 a r t i c l e b y n o t i n g a n o b j e c t i o n t o h i s s t u d i e s o f

f a m i l i e s i n Hereditary Genius ( 1 8 6 9 ) t h a t c o m p a r e d r e s e m b l a n c e f o r f ir s t- , s e c -

o n d - , a n d t h i r d - d e g r e e r e l a t i v e s . H i s a p p r o a c h h a d c o n s i d e r e d e n v i r o n m e n t a l

f a c t o r s s u c h a s s o c i a l c o n d i t i o n s a n d e d u c a t i o n t h a t le a d t o fa m i l i a l r e s e m b l a n c e .

T h e o b j e c t i o n w a s t h a t s u c h s y s t e m a t i c e n v i r o n m e n t a l f a c t o r s

are o nly a small part of those that determine the future of each m an's life. It is to

trifling accidental circumstances that the b ent o f his disposition and his su ccess are

mainly due, and these you leave wholly out o f accoun t. (p. 566)

G a l t o n i n d i ca t e d th a t h e a t t a c k e d t h e p r o b l e m f r o m t h e o p p o s i t e s i d e :

The life history of tw ins supplies what I wan ted. W e m ight begin by enquiring

about twins who we re closely alike in boyho od and yo uth , and who w ere educated

together for many years, and learn whether they subsequently grew unlike . . . . We

can enquire into the history of twins wh o w ere ex ceedingly unlike in childhood, and

learn how far th ey bec am e assimilated unde r the influence of the ir identical nurtures.

(p. 566)

T h u s , i n m o d e r n t e r m s , G a l t o n p r o p o s e d i n th e fi rs t c a s e to s t u d y n o n s h a r e d

e n v i r o n m e n t a l f a c t o r s i n a d u l t h o o d b y f o l l o w i n g t h e li fe h i s t o r y o f i d e n t ic a l

t w i n s a f t e r le a v i n g t h e i r fa m i l i e s . H e v i e w e d t h e s e c o n d c a s e a s a t e s t o f s h a r e d

e n v i r o n m e n t a l i n f l u e n c e , ' to s e e h o w f a r an i d e n t i t y o f n u r t u re i n c h i l d h o o d

a n d y o u t h t e n d e d to a s s im i l a te t h e m ( 1 8 7 5 , p . 5 7 5 ) . T r a c k i n g l if e h i s t o r y

c h a n g e s o f tw i n s w a s a n o v e l i d e a , a n d o n l y r e c e n t ly h a v e b e h a v i o r a l g e n e t ic i s ts

t a k e n s e r i o u s l y t h e n e e d t o s t u d y d e v e l o p m e n t a l c h a n g e . H o w e v e r , t h e o r i g i n s

o f th e tw i n m e t h o d t h a t c o m p a r e s i d e n t ic a l a n d f r a te r n a l t w i n s c a n n o t b e c o n -

s t r u e d f r o m G a l t o n ' s a r t i c l e .

C o n f u s i o n c o n c e r n i n g G a l t o n ' s a r t i c l e c a n a r i s e b e c a u s e i t i n c l u d e s a p a r -

a g r a p h a c k n o w l e d g i n g o n e - e g g a n d t w o - e g g t w in s , a n d a s t u d y is re p o r te d t h at

i n v o l v e d 3 5 t w i n p a ir s o f c l o s e s i m i l a r i t y a n d 2 0 p a i rs o f g r e a t d i si m i la r -

i t y . T h e 3 5 s im i l a r p a i r s a p p e a r t o b e p r i m a r i l y i d e n t ic a l t w i n s . G a l t o n s a w


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W h o D i s c ov e r ed t h e T w i n

M e t h o d ? 7 9

some change and som e con t inu i t y i n t he l if e h i st o r ie s o f t hese twins . He found

the resu l t s for the 20 pai rs of d i ss imi lar twins more conclus ive: " I have not a

s ing le case i n wh ich m y co r responden ts speak o f o r i g ina l ly d is s imi la r cha rac te r s

hav ing becom e as s imi l a t ed t h rough i den t i t y o f nu r tu re" (p. 575 ) . Th e l a t te r

da t a , no t a compar i son be tween t he two types o f twins , l ed t o Ga l ton ' s o f t -

quo t ed s t a t emen t t h a t " t he re i s no escape f rom the conc lus ion tha t na tu re p revai ls

enorm ous ly over nu r tu re " (p . 576 ).

Three o the r po in t s bu t t r e s s t he a rgumen t t ha t , a l t hough aware o f t he two

types o f twins , Ga l ton d id no t sugges t t ha t ev idence fo r t he i n f l uence o f na tu re

cou ld be adduced b y t he com par i son be tween i den t ica l and f ra te rna l twins . F i r s t,

f rom ano the r paper i n t he s ame i s sue o f t he

Journal of the Anthropological

Institute

i t appears t ha t Ga l ton t houg h t t ha t h is s ame -sex d i s s imi la r twins were

ident ica l twins In th i s paper on a theo ry of hered i ty , he refers to twins f rom

the same ovum as " t rue twins , " and i n a foo tno t e , he s t a t e s ,

I h a d t w e n t y c a s e s o f s t r o n g d i s s i m i l a r i ty in t w i n s , a n d i n a l l t h e c a s e s t h e t w i n s

w e r e o f t h e s a m e s e x . N o w i t a p p e a r s t o b e a r u l e w i t h o u t e x c e p t i o n t h a t w h a t I h a v e

a b o v e t e rm e d t r u e t w i n s a re o f t h e s a m e s e x . S u c h t w i n s ar e b y n o m e a n s u n c o m -

m o n . . . . H e n c e t h e re is m u c h p r o ba b i li t y t h a t m y c a s e s o f s tr o n g d i s s im i l a r it y w e r e

u s u a l l y , i f n o t i n v a r i a b ly , c a s e s o f t r u e t w i n s . B u t I h a v e n o d i r ec t e v i d e n c e o n e w a y

or t he o the r . ( Ga l ton , 1876 b , p . 337 )

W hy wou ld twins f rom the s ame ovum be so d i f f e ren t ? Ga l ton sugges t ed t ha t

i f d iv i s i on o f t he ovum was de l ayed beyond the po in t a t wh ich d i f fe ren t i a t i on

occur red , t he twin ha lves " wou ld be s t rong ly con t ras t ed" (p . 337) .

S econd , Ga l ton ' s (1908) au tob iog raphy bo l s te r s t he a rgum en t tha t Ga l ton

did not propose the compar i son between ident ica l and f ra ternal twins :

I t o cc u r r e d t o m e t h a t t h e a f t e r - h i s to r y o f t h o s e t w i n s w h o h a d b e e n c l o s e l y a l ik e

a s c h i l d r e n , a n d w e r e a f t e r w a r d s p a r t e d , o r w h o h a d b e e n o r i g i n a l l y u n l i k e a n d

a f t e rw a r d s r e a re d t o g e t h e r , w o u l d s u p p l y m u c h o f w h a t w a s w a n t e d . ( p . 2 9 4 )

I t is no tew orthy that a s ingle paragraph i s a ll tha t G al ton sa id about tw ins in h i s

323-page au tob iog raphy . I f Ga l ton had rea l i zed t he va lue o f comp ar ing the two

types of twins in order to es t imate genet ic inf luence, i t i s reasonable to expect

t ha t he wo u ld hav e condu c t ed o the r r e sea rch wi th twins a f t e r 1875 a nd t ha t he

wou ld have had someth ing more t o s ay abou t t he twin method i n h i s 1908

au tob iog raphy .

F ina l l y , Kar l P ea r son ' s fou r -vo lum e b iog raphy o f G a l ton con t a ins on ly fou r

pages abo u t Ga l ton ' s twin work . Th i s d i s cus s ion i s in t he s econd vo lu m e o f t he

b iog raphy , pub l i shed in 1924 . I t m ere ly sum mar i zes Ga l ton ' s 1875-1876 papers

and makes no re fe rence t o t he compar i son o f i den t i ca l and f ra t e rna l twins .

F u r the rmo re , he i n t roduces th i s s ec t ion on G a l ton ' s twin work w i th t he fo l l owing

s ta tement :

I t s e e m s b e s t t o c o n s i d e r h e r e tw o p a p e r s o n t h e s u b j e c t o f t w i n s , b e c a u s e a l t h o u g h


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280 Rend e Plomin and Vandenberg

they to som e extentwere associatedwith Galton's ideas on heredity,yet they sprung,

I think, from his w ork on the influenceof the environment. (Pearson, 1924, p. 126)

I n summ a r y , Ga l ton ' s de l igh t i n d i s c ove r ing tw ins wa s to a s se ss t he a b i l it y

o f t he e nv i r onm e n t t o ma k e in i t i a ll y s imi l a r tw ins d i f f e r e n t a nd to ma k e in i t i a l ly

d i f f e r en t tw ins s imi l a r . G a l ton though t t ha t a ll o f h is tw ins - - b o th the s imi l ar

a nd the d is s imi l a r pa i r s - - w e r e one - e g g tw ins , wha t we n ow c a l l i de n t ic a l tw ins .

H e d id no t sugge s t c om pa r ing one - e g g a nd two - e gg tw ins . Th us , i t is no t c o r r e c t

to c l a im tha t Ga l ton p r opose d the tw in me thod .

W H O D I S C O V E R E D T H E T W I N M E T H O D ?

I f Ga l ton d id no t d i s c ove r t he tw in me thod , who d id? A l though Ga l ton ' s

a r ti c le wa s wr i t t e n in 1875 , no o the r stud i es o f tw in r e se mb la nc e we r e pub l i she d

un t i l 1905 , whe n E . L . Thor nd ike , t he l e a r n ing p syc ho log i s t f a mous f o r h i s

wor k on t r i a l - a nd - e r r o r l e a r n ing , r e po r t e d tw in r e se mbla nc e f o r younge r a nd

o lde r tw ins on a ba t t e r y o f c ogn i t i ve t e st s . T hor nd ike d id no t c om pa r e i de n t i ca l

a nd fr a t e rna l tw ins . I n de e d , T hor nd ike , t oge the r w i th R . A . F i she r ( 1919) , wa s

a l e a d ing p r opone n t o f t he v i e w , ba se d on a na lyse s o f t he d i s t r i bu t ion o f tw in

d i f f e r e nc e s , t ha t t he r e we r e no t two k inds o f tw ins . F o r e xa mple , Thor nd ike

( 1905) s ta t e s t ha t t h e e v ide nc e in the c a se o f t he t h i rt y - n ine pa i rs o f tw ins f r om

w h o m w e h a v e e x t e n d e d p h y s ic a l m e a s u r e m e n t s g i v e s n o r e a so n f o r a c c e p t a n c e

o f t he hypo the s i s o f two suc h d is t inc t g r oups o f tw ins ( p . 44 ) . Thor nd ike

e x te nde d Ga l ton ' s tw in r e se a r c h by u s ing ob je c t ive t e s t s o f c ogn i t i ve a b i l i t i e s

a nd by f o r mu la t ing a dd i t i ona l t e s t s o f e nv i r onme n ta l e f f e c t s . F o r e xa mple ,

Thor nd ike a r gue d tha t t he e nv i r onme n ta l hypo the s i s p r e d i c t s t ha t tw ins on a v -

e r a ge g r ow m or e s imi l a r du r ing c h i ldhood a nd tha t h igh ly t r a ine d t a sks suc h a s

a r i thm e t i c a b i li t y shou ld show g r e a t e r tw in r e se mb la nc e tha n t a sks l e s s sub je c t

to t r a in ing suc h a s qu ic kne s s i n ma r k ing A ' s on a she e t o f c a p i t a l l e t t e r s . As

w i t h G a l t o n ' s w o r k , n a t u re i s s u g g e st e d o n l y b y d e f a u l t. T h a t i s , T h o r n d i k e

a t t r i bu t e d pe r f o r ma nc e on h i s t a sks t o he r e d i ty be c a use he f ound tha t tw ins do

no t g r ow mor e s imi l a r du r ing c h i ldhood a nd tha t tw ins do no t r e se mble e a c h

o the r mor e f o r h igh ly - t r a ine d t a sks . L ike Ga l ton , Thor nd ike g r e a t ly ove r s t a t e d

h i s c a se f o r h e r e d i t a r y i n f lue nc e :

The facts then are e asily, simply, and com pletelyexplained by one simple hy-

pothesis: nam ely, that the natures of the germ ce lls--the conditionsof conception--

cause w hatever similarities and differencesexis t in the original natures of m en, that

these conditions influence body and m ind equ ally, and that in lif e the differences

producedby such differencesas obtain between the environmentsof present day New

York C ity pub lic scho ol children are slight. (p. 16)

N e a r l y 2 0 y e a r s a f te r T h o r n d i k e ' s t w i n s t u d y a n d 5 0 y e a r s a f te r G a l t o n ' s ,

a tw in s tudy pub l i she d in Psychological onographspr ov ide s wha t a ppe a r s t o

be the f ir s t e xp l i c it de sc r ip tion o f t he tw in m e thod :


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W h o D i s c o v e r ed t h e T w i n M e t h o d ? 2 8

Since the tw o distinct species theory is the more widely accepted, let us assume

that it is the co rrect theory and then list the principal claim s that it mak es, and the

results that should fol low . . . .

1. Th ere are two distinct types of twins , fraternal and dup licate.

2. The fraternal, being of the two-egg origin, should show n o greater resem-

blance than ordinary siblings, since each individual of the pair develops from

a w holly independent arrangement of the factors for heredity in the g erm

cells.

3. The duplicate, being of the one-egg origin, should show a very much

higher degree of resemblance than the fraternal because each mem ber of the

pair d evelops from substantially the s am e arrangem ent of the factors for

heredity in the ge rm cells.

T h i s e x e r p t i s f r o m a 1 9 2 4 r e p o r t i n Psychological onographs ( p p . 2 6 - 2 7 ) o f

a t w i n s t u d y o n m e n t a l a b i l i t i e s b y C u r t i s M e r r i m a n , A s s i s t a n t P r o f e s s o r o f

E d u c a t i o n a t t h e U n i v e r s i t y o f W i s c o n s i n . M e r r i m a n ' s a rt ic le c o n t a i n s n o c l u e s

c o n c e r n i n g t h e c o n c e p t u a l o r i g in s o f t h e tw i n m e t h o d , a l th o u g h it s h o u l d b e

n o t e d t h a t a s e a rl y a s 1 9 0 1 W e i n b e r g h a d p o i n t e d o u t th a t th e n u m b e r o f li ke -

s e x e d D Z t w i n s o u g h t to b e t h e s a m e a s t h e n u m b e r o f b o y - g i r l p a ir s , a n d b y

s u b t r a c t i o n t h is a l l o w e d e s t i m a t e s o f t h e i n c i d e n c e o f M Z t w i n s ( S t e rn , 1 9 6 2 ) .

M e r r i m a n d e s c r i b e s G a l t o n ' s p a p e r o n s i m i l a r a n d d i s s i m i l a r t w i n s i n a s e c t i o n

a p p r o p ri a te l y e n ti tl ed T h e E f fe c t s o f E n v i r o n m e n t :

The excess of difference in the first case, and o f resemblance in the second, was

thought to give a measure of the influence of environment. The persistence of sim -

ilarities in the first case and of differences in the sec ond wa s take n as a m easure of

the influence of nature. (p. 8)

M e r r i m a n a l so d i s c u ss e s T h o r n d i k e ' s s t u d y in th e s a m e s e c t i o n o n e n v i r o n m e n t a l

e f fe c t s. H e n o t e s w e a k n e s s e s i n t h e s tu d i es o f G a l t o n a n d T h o r n d i k e b u t d o e s

n o t p o i n t o u t t h e m a i n o n e - - t h a t t h e y d i d n o t c o m p a r e r e s e m b l a n c e s f o r t h e t w o

t y p e s o f t w i n s .

M e r r i m a n ' s s t u d y w a s c o n d u c t e d a s h is d i s se r ta t io n r e s e ar c h a t S t a n fo r d

U n i v e r s i t y , u n d e r t h e s u p e r v i s io n o f L e w i s T e r m a n ( M e r r i m a n , 1 9 2 2 ) . T e r m a n ' s

r o l e in M e r r i m a n ' s s t u d y is n o t cl e ar . T e r m a n w a s i n t e re s te d i n G a l to n ( T e r m a n ,

1 9 1 7 ) a n d h e d e v e l o p e d t h e A m e r i c a n t ra n s la t io n a n d r e v is i o n o f th e B i n e t I Q

t es t t h a t w a s u s e d in M e r r i m a n ' s t w i n s t u d y ( T e r m a n , 1 9 1 6 ) . A l t h o u g h T e r m a n

d i d n o t w r i t e a b o u t g e n e t i c s o r c o n d u c t tw i n s t u d i e s , h e d i s c u s s e d t h e r e l e v a n c e

o f t h e n a t u r e - n u r t u r e i s s u e w i t h r e g a r d t o i n t e ll i g e n c e ( T e r m a n , 1 9 2 8 ) , a n d h e

i s c o n s i d e r e d t o h a v e h a d a n i m p l i c i t i n t e r e s t i n g e n e t i c a n d e n v i r o n m e n t a l

c o n t r i b u t i o n s t o in t e l l ig e n c e t h r o u g h o u t m o s t o f h i s r e s e a r c h c a r e e r ( B o r i n g ,

1 9 5 9 ) . I n a d d i t i o n , h e a l s o s u p e r v i s e d t h e d i s s e r ta t i o n r e s e a r c h o f B a r b a r a B u r k s ,

w h i c h w a s a c la s s i c a d o p t i o n s t u d y o f I Q ( B u r k s , 1 9 2 8 ) .

C o m m u n i c a t i o n s w i t h t h e U n i v e r s i t y A r c h i v e s a t W i s c o n s i n r e v e a l e d t h a t

M e r r im a n w a s o n t h e f a c u l t y o f t h e S c h o o l o f E d u c a t i o n f r o m 1 9 2 3 t o 1 9 3 6 ,

w h e r e h i s p r i m a r y t e a c h i n g a n d s c h o l a r l y i n t e re s t s w e r e e d u c a t i o n a n d s ta t is t ic s .

I n a d d i t i o n , h e s e r v e d a s re g i s tr a r o f th e U n i v e r s i t y f r o m 1 9 3 6 t o h i s r e t i re m e n t


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282 Rende PIomin and Vandenberg

in 1945. Merriman remained an active figure on the campus from 1945 to his

death, at the age of 100, in 1975. In 1972, the U niv ersity A rch ive s at W isconsin

conducted an oral his tory on Merriman's l ife (Lowe, 1972). During the inter-

view , Merriman com m ented that he unde rtook his twin research to sh o w that

there w as a difference betw een the identical twins and the non iden tical but

that he didn ' t follow up the w ork, as his dissertation research g o t what [he]

wa s af ter (p .3).

In addit ion to outlining the twin m ethod , M errim an's s tudy i tself deserves

attention. First, recently- de ve lop ed intelligence tests wer e used that continue

to be use d tod ay , including the individually adm inistered Stanford-Binet and the

group-administered A rm y Be ta. Sec ond , Merriman wa s conce rned abou t sam-

pling issues and took pains to test all twins in a given school, an effort that paid

off becau se the proport ion of twins in his s tudy agr ees c losely wi th the ob-

served frequenc y in the general pop ula tion (p. 4) , as does the relative nu m ber

of like and unlike sex pairs. Finally, his stu dy used physic al similarity to identify

same-sex twins who are identical:

E a c h e x a m i n e r w a s a s k e d t o r e p o r t w h e t h e r t h e m e m b e r s o f t h e t w i n p a i r b e i n g

s t u d i e d r e s e m b l e d e a c h o t h e r c l o s e l y e n o u g h t o f r e q u e n t l y c a u s e c o n f u s i o n o f i d e n t it y .

p . 4 3 )

Bec ause no t every examiner made this rep or t , the num ber of identica l twin

pairs w as o nly 22. T hese tw ins yielded correlations of .98 for Stanford-Binet

IQ, .88 for Beta scores , .98 for NIT scores , and .94 for teacher ratings. Mer-

r iman concludes,

T h e s e a r e v e r y h i g h c o r r e l a t i o n s . N o t o n l y a r e t h e y h i g h c o r r e l a t i o n s , b u t w i t h o n e

e x c e p t i o n t h e y a r e m a t e r i a l l y h i g h e r t h a n t h e r e s u l t s t h a t w e r e f o u n d i n e a r l i e r p a r t s

o f t h e s t u d y f o r t h e r e s e m b l a n c e i n t h e e n ti r e t w i n p o p u l a t i o n . p . 4 3 )

Merriman did not, how eve r, identify a group o f fraternal twins and thus he did

not com pare identical tw in correlations to fraternal tw in correlations. Th e first

authors wh o actually comp ared the correlations of MZ and DZ twins for IQ

were Tallman (1928) and Wingfield (1928) (see von Bracken, 1969).

After a 50-y ear hiatus in the use of twins, a spate of twin s tudies appeared

in the second ha lf of the roaring twenties. Several twin s tudies we re pun ish ed

in Am erica after M errim an's (Hirsch, 1930 ; 'Holzinger, 1928; Lauterbach, 1925;

Tallman, 1928; W ing field, 1928). Surprisingly, these studies do not usu ally

refer to the origins of the twin m ethod. W hen they do , they vag uely credit

Galton fo r first stud ying twins and mention in passing th e twin studies of Thorn-

dike and M erriman. On ly La uterb ach (1925) give s explicit credit to M erriman ,

and that is for providing s tatis tical eviden ce for tw o type s of twins:

M e r r i m a n h a s s h o w n s t a t i s t ic a l l y th a t a d i s t ri b u t i o n o f t h e i n t e ll i g e n c e q u o t i e n t s o f

a t w i n p o p u l a t i o n r e p r e s e n t s t w o t y p e s o f p o p u l a t i o n a n d h e c o n c l u d e s t h a t t h e s e t w o

t y p e s a r e d e t e r m i n e d b y o n e - e g g a n d tw o - e g g g e n e s i s . p . 5 6 8 )


7/9

W h o D i s c o v e r e d t h e T w i n M e t h o d ? 2 8 3

O n e r e a s o n t h a t M e r r i m a n ' s w o r k d i d n o t r e c e i v e m o r e a t t e n t i o n m a y b e

t h a t h e d i d n o t i n d i c a t e t h a t t h e c o m p a r i s o n b e t w e e n i d e n t i c a l a n d f r a t e r n a l t w i n

r e s e m b l a n c e c o u l d b e u s e d t o a s s e s s h e r e d i t a r y i n f l u e n c e o n a t r a i t . I n s t e a d , i n

a c c o r d w i t h t h e z e it g e is t , M e r r i m a n a s s u m e d t h a t h e r e d i ty w a s a l l- i m p o r t an t f o r

p e r f o r m a n c e o n i n t e l l i g e n c e t e s t s a n d i n t e r p r e t e d h i s r e s u l t s o n l y i n t e r m s o f

t h e ir s u p p o r t f o r th e e x i s t e n c e o f t w o t y p e s o f t w i n s .

D u r i n g th e s a m e y e a r th a t M e r r i m a n ' s a r ti cl e a p p e a r e d i n A m e r i c a , a b o o k

p u b l i s h e d i n G e r m a n y p r o v i d e d t h e fi rs t e x p li c it d e s c r ip t io n o f t h e tw i n m e t h o d .

H e r m a n n S i e m e n s , a d e r m a t o l o g i s t , p r o p o s e d t h a t h e r e d i t a r y i n f l u e n c e o n f e a -

t u re s s u c h a s s k in d i s o r d e rs c o u l d b e a s s e s s e d b y c o m p a r i n g t h e o c c u r r e n c e o f

t h e f e a t u r e i n i d e n t i c a l t w i n s w i t h t h e o c c u r r e n c e i n f r a t e r n a l t w i n s :

If an illness is regu larly dominant, then both o f the identical twins either suffer

from it or are free from i t . . 9 the nonidentical twins correlate as the siblings

of a two-child family . . . . W ith the help of twin pathology, we found a possible

w ay to judge hereditary influence on the investigated features . . . . The assessment

is based on the com parison of the findings in iden tical and noniden tical twins (p. 21;

translated from the o riginal Germ an).

I n h i s b o o k , S i e m e n s d i s c u s s e s r e s u l t s o f h i s m e t h o d a s a p p l i e d t o i d e n ti c a l

a n d n o n i d e n t i c a l t w i n p a ir s . A l t h o u g h t h e e m p h a s i s w a s o n s k i n d i s o r d e r s , p s y -

c h o l o g i c a l fe a t u re s w e r e a l so e x a m i n e d . F o r e x a m p l e , S i e m e n s r e p o r ts t h a t 3 7

o f 4 8 i d e n t i c a l tw i n p a i r s h a d t h e s a m e p e r f o r m a n c e i n s c h o o l , w h i l e 8 d i f f e re d

s l i g h t ly . I n c o n t r a s t , o f 2 9 n o n i d e n t i c a l p a i rs , 1 7 p e r f o r m e d c o n s i d e r a b l y d if -

f e r e n t i n s c h o o l , 6 d i f f e r e d s l i g h t l y , a n d 6 h a d s i m i l a r p e r f o r m a n c e s . O n t h e

b a s i s o f t h e s e re s u l ts , S i e m e n s c o n c l u d e s th a t

9 .

. it is reasonab le to assum e that the one g rou p w ith its similarities of bod y

features and men tal capacities derives it from a h ereditary poo l (p. 32).

I n E u r o p e , S i e m e n s i s o f te n c r e d it e d w i th i n t r o d u c i n g t h e t w i n m e t h o d

( e . g . , v o n B r a c k e n , 1 9 6 9 ) , a l t h o u g h s o m e r e s e a r c h e rs i n t h e 1 9 2 0 s d i d n o t

c o m p l e t e l y e m b r a c e S i e m e n s ' p r o p o s a l , b e c a u s e o f w h a t t h e y in t e rp r e te d a s a

" s t r o n g c l a i m " t h a t i d e n ti c a l t w i n s h a d t o d e m o n s t r a t e n e a r l y t o ta l c o n c o r d a n c e

i n o r d e r f o r a t ra i t t o b e c o n s i d e r e d h e r e d i t a r y ( e . g . , D a h l b e r g , 1 9 2 6 ) . H o w e v e r ,

i n h i s b o o k S i e m e n s a r g u e s t h a t t h e c o m p a r i s o n o f i d e n t ic a l a n d f r a te r n a l t w i n s

c a n b e u s e d t o a s s e s s h e r e d i t a r y i n f l u e n c e o n f e a t u r e s w h i c h a r e n o t t o t a l l y

d e t e r m i n e d b y h e r e d i t y :

On the ba sis of the similarities betw een identical and no nidentical twins, a nother

question can be tackled w hich has b een neg lected so f ar: tha t is the hereditary dis-

position of non-hereditary features [such as birthmarks] (p. 23) . . . . Ev en there,

wh ere the con ditions are not as clear, one can find pro of of an idiotypical disposition

of paratypical features w ith the he lp of a suitable metho d. It was fou nd , tha t the

correlation of the am ount of birthmarks on bo th identical twins was 0.40 (-+ 9

the correlation between the non identical twins was only half o f it 0 .20 (-+0.19), and

that am ong siblings the correlation is dub ious 0.10 (-+ 09149 T hese results are exactly

as on e w ould anticipate in an idiotypical nevus disposition; one may not assum e that

there are no hereditary differences in the d evelopm ent of the nev us. (p. 29)


8/9

284 Rende Plomin and Vandenberg

T h i s i n v e s t i g a t i o n o f t h e o r i g in s o f th e t w i n m e t h o d l e a d s t o m o r e q u e s t i o n s

t h a n a n s w e r s . H o w d i d M e r r i m a n a n d S i e m e n s b o t h a r r i v e a t a c l e a r s t a t e m e n t

a b o u t t h e tw i n m e t h o d ? T o a n s w e r t h is , i t m a y b e f r u it fu l t o e x p l o r e t h e g a p

b e t w e e n 1 9 0 5 a n d 1 9 2 4 . W h y d i d t w o d e c a d e s p a s s w i t h n o t w i n s t u d i e s a f t e r

T h o r n d i k e ' s 1 9 0 5 s t u d y ? C o u l d i t b e t h a t T h o r n d i k e ' s f o r c e f u l d e n i al t h a t t h e re

a re t w o t y p e s o f t w i n s s l o w e d t h e d i s c o v e r y o f th e t w i n m e t h o d u n ti l th e 1 9 2 0 s

w h e n t h e b i o lo g i c a l e v i d e n c e f o r t h e t w o t y p e s b e c a m e o v e r w h e l m i n g ? A r e la t ed

q u e s t io n i s w h y n e a r l y 5 0 y e a r s t r an s p i re d b e t w e e n G a l t o n ' s a r ti cl e o n t w i n s

a n d th e u s e o f t h e t w i n m e t h o d . C o u l d i t b e th a t G a l t o n ' s c o n c l u s i o n th a t n a t u r e

p r e v a il s e n o r m o u s l y o v e r n u r t u r e ( 1 8 7 5 , p . 5 7 6 ) w a s s o w i d e l y a c c e p t e d t h a t

it s t if le d t h e n e e d f o r fu r t h e r r e s e a r c h o n t h e i m p o r t a n c e o f h e r e d i t y ? M e r r i m a n ,

f o r e x a m p l e , s t at es t h at i n s p it e o f t h e im p e r f e c t i o n s o f G a l t o n ' s m e t h o d , h i s

g e n e r a l c o n c l u s i o n a s t o th e ~ p e rs is te n c e o f n a t u r e h a s b e e n f a i l y w i d e l y a c -

c e p t e d ( p . 8 ) .

A r e a s o n a b l e h y p o t h e s i s is t h a t t h e t w i n m e t h o d w a s i n t h e a i r i n t h e

1 9 2 0 s , w h e n i t b e c a m e c l e a r t h a t t w i n s w e r e e i t h e r f r o m t h e s a m e e g g o r f r o m

t w o s e p a r a t e l y f e r t i l i z e d e g g s . T h e t w i n m e t h o d f i r s t m a t e r i a l i z e d , h o w e v e r , i n

1 9 2 4 , i n th e a r ti cl e b y C u r t is M e r r i m a n a n d i n th e b o o k b y H e r m a n n S i e m e n s .

A C K N O W L E D G M E N T

T h e a u t h o r s t h a n k I l s e - R o s e W a r g f o r a s s i s t a n c e w i t h t r a n s l a t i o n s .

R E F E R E N C E S

Boring, E. G. (1959). Lew is M adison Terma n. Biograph. Mere. 33:414-461.

Burks, B. (1928). The relative influence of nature and nurture upon m ental development: A com -

parative s tudy o f fo ster parent-foster-child resemblance and true parent-true child resemblance.

Twenty-Seventh Yearbook of the N ational Society for the Study o f Education, Part 1, pp. 219 -

316.

Dahlberg, G. (1926).

Tw in Births and Twins from a H ereditary Point of View,

Tidens Tryckery,

Stockholm.

Fischer, R. A . (1919). The gen esis of twins. Genetics 4:489-499.

Galton, F. (1869). Hereditary Genius. An Inquiry into Its Laws and Consequences, Macmillan,

London.

Galton, F. (1874). English Men o f Science: Their Nature and Nurture, Clarke, DoN e & B rendon,

London.

Galton, F. (1875 ). The history of twins, as a criterion of the relative pow ers of na ture and nurture.

Fraser's Mag. No v. 566-576.

Galton, F. (1876). The h istory of tw ins, as a criterion o f the relative pow ers of nature and nurture.

J. Anthropol. Inst. 5:391--406.

Galton, F. (1908).

Memories o f My Li fe ,

Methuen , London .

Hirsch, N. D . M. (1930).

Twins: Heredity and Environment,

Harvard U niversity Press, Cambridge,

MA.

Holzinger, K. J. (1929 ). The relative effects of nature and nurture influences on twin differences.

J. Educ. Psychol. 20:141-148.

Lauterbach, C. E. (192 5). Studies in tw in resemb lance.

Genetics

10:525-568.


9/9

W h o D i s co v e re d t h e T w i n M e t h o d ? 8 5

Low e, S. 1972).

Curt is Merr iman: An In terview

Unpublished document, University Archives oral

history project, University of Wisconsin, Madison.

M erriman, C. 1922).

The In te l lectual Resemblance o f Twins

Unpublished doctoral dissertation,

Stanford University, Palo Alto.

Me rriman, C. 1924). The intellectual resemblance of twins. Psychol . Monogr . 33:1-58 .

Pearson, K. 1924).

The Li fe Let ters and Labours o f Francis Gal ton

Vol. 2. Cambridge University

Press, Cambridge.

Siemens, H. 1924).

D ie Zwil l ingspathologie

Springer-Verlag, Berlin.

Stern, C. 1962). W ilhelm W einberg 1862-1937). Genet ics 17:1-5 .

Tallman, G . G. 1928). A com parative study of identical and nonidentical twins with respect to

intelligence resemblances.

Twenty-Seventh Yearbook o f the Nat ional Society fo r S tudies in

Educat ion pp. 83-86.

Terman, L. M. 1916).

The M easurement o f ln teUigence

Houghton Mifflin, Boston.

Terma n, L. M . 1917). The intelligence quotient of Francis Galton in childhood. Am. J . Psychol .

28:208-215.

Terma n, L. M . 1928). Introduction. Twenty-Seventh Yearbook o f the Nat ional So ciety for the S tudy

o f Educa t ion Part 1, pp. 1-7.

Thorndike, E. L. 1905). Measurement of twins.

Arch. Phi los . Psychol . Sci Meth .

1:1-64 .

yo n Bracken, H. 1969). Human genetische Psychologic. In Becker, P. E. ed.),

Humangenet ik: e in

kur z es H andbuch in fu n f banden

Band I/II, pp. 409-561.

W ingfield, A . H. 1928).

Twins and Orphans: The Inher itance o f In te l ligence

Dent, London.

Edited by H. H. Goldsmith

who discovered twin studies

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