Sandra Scarr, Andrew J Pakstis, Solomon H Katz and William B Barker - Absence of a relationship between degree of white ancestry and intellectual skills within a black population (1977)

8/14/2019 Sandra Scarr, Andrew J Pakstis, Solomon H Katz and William B Barker - Absence of a relationship between degree …

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Hum. Genet. 39, 69--86 (1977)

© by Springer-Verlag 1977

A b s e n c e o f a R e l a ti o n sh i p b e t w e e n D e g r e e o f W h i te A n c e s t r y

a n d I n t e l l e c t u a l S k i l l s w i t h i n a B l a c k P o p u l a t i o n

S a n d r a S c a r r 1 *, A n d r e w J . P a k s t is l, S o l o m o n H . K a t z 2, a n d W i l li a m B . B a r k e r 2

1University of Minnesota, USA2University of Pennsylvania, USA

Introduct ion

G e n e t i c d i f f e re n c e s h a v e b e e n o f f e r ed a s a n h y p o t h e s i s t o e x p l a i n t h e a v e r a g e I Q

d i f fe r e n c e u s u a l ly f o u n d b e t w e e n U S b l a c k a n d w h i t e p o p u l a t i o n s ( J e n s en , 1 973 ;

S h o c k l e y , 1 97 2). W h i l e m o s t b e h a v i o r a l s c ie n t is t s w o u l d c h o o s e t o i g n o r e t h e

h y p o t h e s i s a s d i s t a s t e f u l , t h e r e i s l i tt l e d i r e c t e v i d e n c e a g a i n s t it ( S c a r r a n d

W e i n b e r g , 1 9 7 6 ) . T h o s e w h o p r e f e r a n e n v i r o n m e n t a l h y p o t h e s i s t o a c c o u n t f o r

t h e a v e r a g e d i f f e r e n c e b e t w e e n b l a c k a n d w h i t e g r o u p s o n i n t e ll e c t u a l t e s ts h a v e

n o t s u c c e e d e d i n a c c o u n t i n g f o r th e m a g n i t u d e o f th e e ff e ct , n o r h a v e t h o se w h o

h o l d a g e n e t i c h y p o t h e s i s b e e n a b l e to r e f u t e a n e n v i r o n m e n t a l s t a n c e . N o d i r e c tc o m p a r i s o n s o f b l a c k a n d w h i t e s a m p l e s w i ll s e t tl e th e i s su e o f p o s s i b l e g e n e t i c

d i ff e re n c e s, b e c a u s e o b v i o u s e n v i r o n m e n t a l d i f fe r e n ce s a r e c o n f o u n d e d w i t h a n y

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

a n d w h i t e ( S c a r r - S a l a p a t e k , 1 9 7 1 a , 1 9 7 1 b , 1 9 7 2 , 1 9 7 3 , 1 9 7 4 ; S c a r r a n d W e i n b e r g ,

1976).

T h e f a c t t h a t U S b l a c k s a r e a h y b r i d p o p u l a t i o n 1 m a k e s t h e s t u d y o f

a d m i x t u r e a p o t e n t i a l m e t h o d t o e v a l u a t e t h e e f fe c t s o f r a c ia l g e n e ti c d i f fe r e n c e s.

T h o s e e n v i r o n m e n t a l d i f fe r e n ce s b e t w e e n t h e r ac e s th a t a f f e c t a l l b l a c k s e q u a l l y ,

b u t n o w h i t e s , w i l l n o t c o n t a m i n a t e t h e p o s s i b l e r e l a t i o n s h i p b e t w e e n g e n e t i c

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

g e n e t ic , r a c i a l d i f f e r e n c e s d o c o n t r i b u t e t o a v e r a g e i n t e ll e c t u a l d i f fe r e n c e s

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

a n c e s t r y s h o u l d p e r f o r m b e t t e r o n i n t e l le c t u a l t es t s t h a n t h o s e w i t h l e ss e r d e g r e e s

o f a d m i x t u r e ( J e n s e n , 1 97 3; S h u e y , 1 96 6).

E v e n w i t h in t h e h y b r i d g r o u p , t h e e f fe c ts o f e n v i r o n m e n t a l d i ff e re n c e s c a n n o t

b e i g no r e d . T h e a m o u n t o f r a c ia l d i s c r i m i n a t io n m a y b e r e l a t e d t o th e d e g r e e o f

* A d d r e s s o f f p r i n t r e q u e s t s t o: D r. S. Scarr, Dept. of Psychology, Yate University, New H aven ,Conn. 06520, USA

In the United States, an y pe rson with visible signs of African ancestry is socially classifiedas black. This social classification o f race provides the basis for Reed's (1969a; 196 9b; 1973)rep ort that blacks in the Oakland, California are a have abo ut 22% Europe an ancestry, whereassocially classified whites in the sam e a rea have less than 0.1% African ancestry



70 S. Scarr et al.

A f r i c a n a n c e s t r y ( K l i n e b e r g , 19 63 ). I n v i s i b l e m a r k e r s f o r a n c e s t r y , s u c h a s b l o o d

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

s u c h a s s k i n c o l o r , n o s e a n d l i p w i d t h , h a i r t e x t u r e , a n d t h e li k e. G r e a t c a r e m u s t

b e t a k e n t o s e p a r a t e t h e e f fe c ts o f c o r r e la t e d g e n e ti c a n d e n v i r o n m e n t a l v a r i a b l e s

o n i n t e l l e c t u a l p e r f o r m a n c e .T h i s s t u d y w i ll e v a l u a t e t h e h y p o t h e s i z e d e f f e c ts o f g e n e ti c r a c i a l d i f fe r e n c e s ,

e s t i m a t e d f r o m b l o o d g r o u p a n d s e r u m p r o t e i n l o ci , o n i n te l le c tu a l p e r f o r m a n c e

i n a s a m p l e o f b l a c k t w i n s . S o c i a l, e n v i r o n m e n t a l e f f e c ts w i ll b e c o n s i d e r e d a p a r t

f r o m g e n e t ic d i f f er e n c e s b e t w e e n t h e r a c i a l g r o u p s . T h e c o n s t r u c t v a l i d i ty o f tw o

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

Admixture Estimates for US Blacks. E s t i m a t e s o f t h e d eg r e e t o w h i c h c o n t e m p o -

r a r y b l a c k g e n e f re q u e n c i e s d e r i v e f r o m a n c e s t ra l A f r i c a n a n d C a u c a s i a n

p o p u l a t i o n s v a r y , d e p e n d i n g u p o n t h e r e g i o n o f t h e c o u n t ry , t h e g en e lo c i u s e d to

e s t i m a t e a d m i x t u r e , a n d t h e s a m p l i n g p r o c e d u r e s ( A d a m s a n d W a r d , 1 97 3; C l a s sa n d L i , 1 9 53 ; P o l l i t z e r , 1 97 2; R e e d , 1 9 6 9 a , 1 9 6 9 b , 19 73 ). T h e r e i s g e n e r a l a g r e e -

m e n t , h o w e v e r , t h a t t h e D u f f y ( F y ) l o c us o f f e rs t h e b e st e s ti m a t e s o f a b o u t 2 2 %

o f C a u c a s i a n a d m i x t u r e i n N o r t h e r n u r b a n p o p u l a t io n s .

P o p u l a t i o n a d m i x t u r e e s t im a t e s a re a n average o f th e i n d i v i d u a l s i n t h e h y b r i d

p o p u l a t i o n . I n d i v i d u a l a n c e s t r y c a n v a r y f r o m n e a r z e ro t o n e a r o n e w h e n t h e

a d m i x t u r e h a s c o n t i n u e d o v e r te n g e n e r at io n s . I n d e p e n d e n t a s s o r t m e n t a n d

m a t i n g t h a t is r a n d o m w i t h r e s p e c t t o a d m i x t u r e s e r v e t o d is t ri b u t e C a u c a s i a n

g e n e s t h r o u g h o u t t h e h y b r i d p o p u l a t i o n ; a s s o r t a t i v e m a t i n g w i t h r e s p e c t t o

a d m i x t u r e t e n d s t o r e s t r ic t g e n e f lo w w i t h in t h e p o p u l a t i o n . B e c a u s e t h e r e ar e n o

a c c u r a t e p e d i g r e e s o v e r t e n g e n e r a t i o n s a n d b e c a u s e t h e p o p u l a t i o n p a r a m e t e r st h a t a f f e c t t h e d i s t r i b u t i o n o f a d m i x t u r e c a n n o t b e m e a s u r e d h i s to r i ca l ly , it is

v e r y d if f ic u l t to e s t i m a t e t h e d i s t r i b u ti o n o f C a u c a s i a n g e n e s a m o n g i n d i v id u a l s

i n t h e c o n t e m p o r a r y b l a c k p o p u l a t i o n . R e c e n t ly , M a c L e a n a n d hi s c o l le a g u e s

( 19 7 4) e s t i m a t e d t h e d i s t r i b u t io n o f a d m i x t u r e i n a l a rg e b l a c k s a m p l e f r o m

u p s t a t e N e w Y o r k . T h e i n d i v i d u a l C a u c a s i a n a d m i x t u r e v a l u e s r a n g e d f r o m l es s

t h a n 1 0% t o m o r e t h a n 6 0 % , w i th a m e a n a r o u n d 2 0 % . T h e s t a n d a r d e r ro r o f

e s t i m a t e f o r t h e i r a d m i x t u r e v a l u e ( 0 ), b a s e d o n n i n e b l o o d g r o u p s y s t e m s , is s o

l a r g e (_+ 2 S E . 2 + 0 .1 6 ) t h a t p o i n t e s t i m a t e s o f a d m i x t u r e w e r e n o t r e a l l y a c h i e v e d .

T h e y d i d f in d , h o w e v e r , a s i gn i f ic a n t r e la t i o n s h i p b e t w e e n 0 a n d h y p e r t e n s i o n i n

t h e b l a c k g r o u p .

Correlates of Ancestry and Intellectual Skills. A n y p o s i ti v e o r n e g a t i v e r e la t i o n s h i p

b e t w e e n b l o o d g r o u p e s t i m a t e s o f a n c e s t r y a n d i n t e ll e c t u a l sk i ll s w i ll b e

c o n f o u n d e d w i t h c o r r e la t e s o f t h e tw o v a r i a b l es . F o r e x a m p l e , b l o o d g r o u p

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

g e n e t i c m a r k e r s . I f s k in c o l o r d e p e n d s u p o n a f e w g e n e l oc i ( S t e rn , 1 97 0 ), t h e n a

l ar g e s et o f i n d e p e n d e n t b l o o d g r o u p m a r k e r s d r a w n f r o m t h e s a m e a n ce s tr a l

p o p u l a t i o n s h o u l d b e c o r r e l a t e d w i t h s k in c o l o r. T h e m a g n i t u d e o f t h e c o r r e l a t i o n

w i ll d e p e n d u p o n t h e d e g r e e o f r e a s s o r t m e n t a n d t h e d i s p e rs i o n o f a d m i x t u r e i n t h e

b l a c k p o p u l a t i o n . B u t s k i n c o l o r is a l s o a v i s ib l e m a r k e r f o r r a c i a l d i s c r i m i n a t i o na n d h a s i n t h e p a s t b e e n a s s o c i a t e d w i th s o c i o e c o n o m i c s t a tu s w i t h i n th e U S b l a c k

p o p u l a t i o n . T h e d a r k e r o n e i s , t h e l o w e r o n e ' s s o c i a l s t a t u s ( K l i n e b e r g , 1 96 3 ).

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



Racial Mixture and Cognition 71

I BLOODGROUPMARKERSBY ANCESTRY

I GENERALINTELLECTUALSKILLS

I KINCOLORMARKERS

Fig. 1. A model for the effects of ancestry and socioeconomic status o n the intellectual skillsof US blacks

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Fig. 2. Th e distribution of ancestral and sample odds coefficients in the sample of Philadelphiablacks

m e n t s . T h u s , s k i n c o l o r a n d s o c i o e c o n o m i c c h a r ac t e ri s ti c s m u s t b e c o n s i d e re d a s

c o r r e l a t e s o f a n y e s t i m a t e o f a n c e s t r y a n d i n t e ll e c t u a l sk i ll s i n t h e b l a c k p o p u l a t i o n .

T h e p a t h m o d e l s h o w n i n F i g u r e 1 s p ec if ic s t h e g e n e ti 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 io n s o f a n c e s t r y ( b l o o d g r o u p s a n d s k in c o l o r ) a n d l if e c h a n c e s ( s o ci o -

e c o n o m i c s t a t u s a n d s k i n c o l o r ) t o i n t e l l e c t u a l s k i l l s .

Blood Group Markers. B l o o d g r o u p l o c i , i n c l u d i n g r e d c e ll a n t i g e n s a n d s e r u m

p r o t e i n s , a r e t h e m o s t r e li a bl e m a r k e r s o f p o p u l a t i o n d i ff e re n c es . W h i le 9 0 % o f t h e

v a r i a n c e i n b l o o d g r o u p p h e n o t y p e s o c c u r s w i t h in p o p u l a t i o n s , s o m e 1 0 % o c c u r s

b e t w e e n p o p u l a t i o n s ( N e i a n d R o y c h o u d h u r y , 1 9 72 ). A t s o m e l oc i, s u c h as D u f f y

a n d G m , a l t e r n a t e a l le le s a re f o u n d i n s o m e p o p u l a t i o n s a n d n o t o t h e rs ; fo r m o s t

l oc i, h o w e v e r , o n l y a l le li c fr e q u e n c ie s v a r y a m o n g p o p u l a t i o n s .

K n o w n b l o o d g r o u p l o ci a r e a v e r y s m a l l s a m p l e o f t h e t o t a l g e n o m e , w h i c h is

e s t i m a t e d t o c o n t a i n f r o m 1 5 0 0 0 to 1 0 0 0 0 0 l o c i . I t i s p o s s i b l e t h a t b l o o d g r o u p

m a r k e r s d o n o t s a m p l e t h o s e s e g m e n t s o f t h e g e n o m e a s s o c i a t e d w i th c o g n i ti v ea b i li t ie s ( L o e h l i n e t a l. , 1 9 73 ). I f i n d e p e n d e n t a s s o r t m e n t h a s o c c u r r e d r e p e a t e d l y

o v e r m a n y g e n e r a t i o n s , o n e m i g h t n o t e x p e c t a n y a s s o c ia t i o n a m o n g g e n e s f r o m

t h e s a m e a n c e s t r a l p o p u l a t i o n , e x c e p t t h o s e t h a t a r e c l o s e ly l i n k e d .



72 S . S ca r r e t a l .

O n t h e o t h e r h a n d , i f r e a s s o r t m e n t i s l im i t e d b y lo w c r o s s o v e r ra t e s a n d b y

a s s o r t a t i v e m a t i n g w i t h r e s p e c t to a d m i x t u r e , g e n e t i c m a r k e r s f r o m a n a n c e s t r a l

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

g e n e r a ti o n s . E v i d e n c e o n t h e n o n i n d e p e n d e n c e o f p o p u l a t i o n m a r k e r s c a n b e

f o u n d i n t h e c o r re l a t io n s a m o n g b l o o d g r o u p s a n d b e t w e e n b l o o d g r o u p s a n do t h e r g e n e t i c m a r k e r s , s u c h a s s k i n c o l o r . I f u n l i n k e d g e n e t ic m a r k e r s a r e s ti ll

a s s o c i a t e d , t h e n t h e r e i s s o m e r e a s o n t o b e l i e ve t h a t o t h e r g e n e s ( t h o s e a s so c i a t e d

w i t h c o g n i t i v e s k i l l s ) f r o m t h e s a m e a n c e s t r a l p o p u l a t i o n a r e s t i l l a s s o c i a t e d w i t h

t h e b l o o d g r o u p m a r k e r s .

I t s h o u l d b e c l e a r n o n e t h e l e s s t h a t e v e n w i t h o u t a s s o r ta t i v e m a t i n g f o r

a d m i x t u r e t o m a i n t a i n a h i g h d e g r e e o f a s s o c i a t i o n o f C a u c a s i a n g e n e s , t h e u se o f

g e n e ti c lo c i t o i n d e x th e p r o p o r t i o n o f C a u c a s ia n a n c e s t r y - - c o n t r a r y to w h a t

s o m e a u t h o r s s e e m t o s u g g e s t ( L o e h l i n e t a l . , 1 9 7 3 ) - - i s n o t i n v a l i d a t e d . F r o m a

s a m p l i n g p e r s p e c t i v e , if 4 0 % o f a n i n d i v i d u a l 's g e n o m e d e r i v e s f r o m t h e w h i t e

p o p u l a t i o n , t h e n a n a d e q u a t e , r a n d o m s a m p l e o f g e n e ti c m a r k e r s d i s c ri m i n a ti n ga n c e s t r a l o r i g in o f t h e a l le l es s h o u l d on the average r e f l e c t t h a t a c t u a l p e r c e n t a g e .

T h i s i n t u r n s h o u l d a l s o b e t h e b e s t e s t im a t e o f t h e p r o p o r t i o n o f g e n e s

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

A n e v e n g r e a t e r p r o b l e m i n p r o d u c i n g r e l i a b le e s t im a t e s o f i n d i v i d u a l

a d m i x t u r e i s t h e h i g h d e g r e e o f o v e r l a p in g e n e f re q u e n c ie s a m o n g c o n t e m p o r a r y

h u m a n p o p u l a t io n s . M a c L e a n a n d W o r k m a n ( 1 9 7 3 ) p r o p o s e d a m e t h o d b y

w h i c h i n d i v i d u a l a d m i x t u r e e s t i m a t e s in a h y b r i d p o p u l a t i o n c a n b e c a l c u la t e d .

R e e d ( 1 9 7 3 ) n o t e d , h o w e v e r , t h a t a t l e a s t 1 8 l o c i t h a t d i s c r i m i n a t e p e r f e c t l y

b e t w e e n t h e t w o a n c e s tr a l p o p u l a t i o n s w o u l d b e r e q u i r e d t o o b t a i n p o i n t

e s t im a t e s o f in d i v i d u a l a d m i x t u r e w i t h a c c e p t a b l e s t a n d a r d e r r o r s . T h e r e a r e o n l y

t w o n e a r l y p e r f e c t l o c i a n d m a n y w h i c h y i e ld f a r le ss i n f o r m a t i o n a b o u t a n c e s t r y .

G i v e n t h e p a u c i t y o f b l o o d g r o u p l oc i t h a t d i s c r im i n a t e A f r i c a n f r o m C a u c a s i a n

p o p u l a t i o n s , w e r e j e c t t h e p o s s i b i l i t y o f p o i n t e s t im a t e s o f a d m i x t u r e a t t h e

p r e s e n t t i m e .

W e p r o p o s e i n s t e a d a n odds coe f f ic ienf l t h a t e s t a b l i s h e s a r a n k o r d e r o f

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

o f t w o p o p u l a t i o n s . B y u s i n g p h e n o t y p e 3 f r e q u e n c y e s ti m a te s f o r p o p u l a t i o n s A

a n d B , th e o d d s t h a t a n i n d i v id u a l 's p h e n o t y p e c a m e f r o m p o p u l a t i o n A c a n b e

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

[ A 1 A 2 A 3 . _ ._ . A n ]O A = log [ Bl B2 B3. ~~ - ,

2 O t h e r c o e f f i c i e n ts w e r e a s s e s s e d f o r t h e i r e f fi c i e n c y i n d i s c r i m i n a t i n g i n d i v i d u a l d i f f e r e n c e s i n

[ A 1 - B 1 + A 2 - B 2 A n - B n 1a n c e s tr y w i t h i n t h e b l a c k p o p u l a t i o n . A n a d d i ti v e m o d e l [ ~ A ---7~-B 2 + " " " + A n + B ~ J'

a m o d e l t h a t w e i g h t e d th e l o c i b y t h e c o m b i n e d p h e n o t y p i c f r e q u e n c i e s f o r p o p u l a t i o n s A a n d B

[ ( l o g A I A 2 . A ~A 2 ...A n:~_A n~__B _~2 ...B n1 a n d s e v e r a l o t h e r s w e r e tr i e d . T h e c o e f f i c i e n ts w e r el l ,

h i g h l y c o r r e l a t e d ( > 0 .9 ) u n l e s s t h e i r d i s t r i b u t io n s w e r e v e r y p o o r . W e c h o s e t h e s im p l e s t o d d s

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

3 P h e n o t y p e f r e q u e n c i e s w e r e c h o s e n i n s te a d o f g e n o t y p e s b e c a u s e , f o r m a n y l o c i, e s t i m a t -i n g in d i v i d u a l g e n o t y p e s i s a n o t h e r t e n t a t i v e st e p a w a y f r o m t h e d a t a . F o r l o c i w i t h o u t c o -

d o m i n a n t a l l e l e s , p a r t i c u l a r l y c o m p l e x o n e s s u c h a s R h e s u s , e s t i m a t i n g i n d i v i d u a l g e n o t y p e s

s e e m e d u n n e c e s s a r il y c o m b e r s o m e . T h e u s e o f in d i v id u a l p h e n o ty p e s a n d p h e n o t y p i c f r e q u e n -

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




w h e r e A1 is th e f r e q u e n c y o f th e i n d i v i d u a l 's p h e n o t y p e a t l o cu s 1 i n p o p u l a t i o n A ,

B1 i s t h e f r e q u e n c y o f h is p h e n o t y p e a t l o c u s 1 i n p o p u l a t i o n B , et c .

T h e o d d s c o e f f i c ie n t is n o t a n a d m i x t u r e e s t i m a t e ; it i s m e r e l y a n e x p r e s s i o n o f

t h e c o m b i n e d p r o b a b i l i t i e s a c r o s s n l o c i t h a t a n i n d i v i d u a l ' s b l o o d g r o u p a l le le s

c o m e f r o m p o p u l a t i o n A , g i v e n t h e f re q u e n c y t o th o s e a ll el es i n p o p u l a t i o n s Aa n d B . T h e s iz e o f t h e o d d s c o e f f i c ie n t , w h e n c o n s i d e r i n g o n l y o n e m a r k e r l o c u s ,

d e p e n d s o n h o w d i s si m i l ar th e p h e n o t y p e f r e q u e n ci e s a re in th e p r e s u m e d

a n c e s t r a l p o p u l a t i o n s . T h u s , p e r f e c t d i s c r i m i n a t i o n o c c u r s w h e n a g e n e e x i st s i n

o n e a n c e s t r a l p o p u l a t i o n b u t n o t t h e o t h e r . T h e m o r e s i m i l a r t h e f r e qu e n c i es a r e ,

t h e l es s r e l i a b l y th e o r i g i n o f th e g e n e t i c m a r k e r c a n b e p r e d i c t e d a n d c o n -

s e q u e n t l y t h e s m a l l e r t h e o d d s . T h e f o r m u l a c a n b e u s e d to e x p r e s s t h e r e l a ti v e

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

i n g p h e n o t y p e f r e q u e n c i e s .

African and Caucasian Populations: The Putative Parents. M a n y A f r i c a n g r o u p sc o n t r i b u t e d t o t h e U S b l a c k g e n e p o o l o v e r s e v e ra l c e n tu r ie s . T h e e s t i m a t i o n o f

a n h i s to r i c a l a n c e s tr a l p o p u l a t i o n f r o m c o n t e m p o r a r y A f r i c a n p o p u l a t i o n s is

f r a u g h t w i t h p i t fa l l s. T h e r e i s c o n s i d e r a b l e h e t e r o g e n e i t y i n g e n e f r e q u e n c i e s

a m o n g c o n t e m p o r a r y A f r i c a n e th n ic g r o u p s ; n o o n e k n o w s t h e e x a ct p r o p o r t i o n s

o f s la v e s t h a t w e r e b r o u g h t t o t h e U S A f r o m t h es e v a r i e d g r o u p s , o r ev e n i f t h e

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

s l av e s w e r e b r o u g h t . F u r t h e r , n o o n e k n o w s i f t h e s la v e s w e r e a n r a n d o m s a m p l e

o f t h e A f r i c a n p o p u l a t i o n s , o r i f s u r v iv a l a n d r e p r o d u c t i v e r a te s w e r e e q u a l a c r o s s

t h e A f r i c a n g r o u p s t h a t b e c a m e s l a v e s . P o s s i b l e s e l e c t i o n t r e n d s a n d g e n e t i c d r i f t

c o m p l i c a t e t h e e s t i m a t i o n p r o b l e m s til l m o r e . T h u s , t h e g e n e f re q u e n c i es f o r a n yp u t a t iv e A f r ic a n ' p a r e n t ' p o p u l a t i o n f o r c o n t e m p o r a r y U S b la c k s m a k e s m a n y

t e n u o u s a s s u m p t i o n s ( A d a m s a n d W a r d , 1 973 ).

E s t i m a t e s o f g en e f r e q u en c i e s fo r E u r o p e a n p o p u l a t i o n s t h a t c o n t r i b u t e d t o

t h e U S b l a c k p o p u l a t i o n p o s e s i m i l a r p r o b l e m s , b u t t h e r e i s l es s h e t e r o g e n e i t y in

g e n e fr e q ue n c ie s a m o n g t h e N o r t h e r n E u r o p e a n g r o u p s t h a t a re t h o u g h t t o h a v e

c o n t r i b u t e d t h e m a j o r i t y o f g en e s t o t h e h y b r i d b l a c k s . T h i s p u t a t iv e p a r e n t

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

G i v e n t h e s e v e re r e s e rv a t i o n s t h a t a n y r e a s o n a b l e p e r s o n w o u l d h a v e a b o u t

o u r a b i li ty t o e s t im a t e ' p a r e n t ' p o p u l a t i o n s f o r c o n t e m p o r a r y U S b l a c k s , w e

a t t e m p t e d t o d e v e l o p c o n s t r u c t v a l i d i t y f o r t w o o d d s c o e ff ic ie n ts : o n e b a s e d o ne s t i m a t e d a n c e s t r a l fr e q u e n c ie s a n d o n e b a s e d o n c o n t e m p o r a r y s a m p l e s o f

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

m e a s u r e s o f i n t e ll e c t u a l s k il ls .

M a t e r i a l a nd M e t h o d s

Subjects. The subjects of this rep ort are same-sex twins who were sampled fro m black and whitepopulations in Philadelphia, Pennsy lvania, fo r the study o f genetic variability in physical,

mental, and personality development fro m 10 to 16 years of age. The fact that the subjects aretwins is no t directly relevant to the stud y of ancestry or adm ixture. Th e black twins were drawnfrom 181 different families, each of who m is represented by two offspring. A bo ut 59% of theblack pairs are dizygotic (DZ), and the remaining 41% monozygotic (MZ).



7 4

T a b l e 1. T w i n p a i r s b y r a c e , s e x , a n d z y g o s i ty

Z y g o s i t y B l a c k W h i t e T o t a l p a i r s

M a l e F e m a l e M a l e F e m a l e

M o n o z y g o t i c 3 4 4 3 6 7 6 3 2 0 9

D i z y g o t i c 4 4 6 0" 5 2 4 2 1 96

T o t a l 7 8 1 0 3 a 1 1 9 1 0 5 4 0 5

a i n c l u d e s 2 s e t s o f t r i p l e t s

S . S c a r r e t a l .

T h e m a j o r e f f e c t o f u s i n g t w i n p a i r s i n t h e s t u d y o f a n c e s t r y i s t o c o n f u s e th e i s su e o f h o w

m a n y d e g r e e s o f f re e d o m o u g h t t o b e a l l o w e d i n t h e s ta t i s ti c a l a n a l y s e s ( E l s t o n , p e r s o n a l

c o m m u n i c a t i o n 1 9 74 ). I n t h e c a s e o f m o n o z y g o t i c p a i r s , th e y h a v e t h e s a m e a n c e s t ry , a se s t i m a t e d b y g e n e t i c m a r k e r s , b u t t h e y s e l d o m h a v e e x a c t l y t h e s a m e m e n t a l t e s t s c o re s .

D i z y g o t i c t w i n s h a v e n e i t h e r t h e s a m e a n c e s t r y n o r t h e s a m e t e s t s c o re s . W e w o u l d h a v e u s e d

o n l y o n e t w i n f r o m e a c h f a m i l y a n d t h e r e b y e l i m i n a t e d th e c o n f u s io n a b o u t d e g r ee s o f f r e e d o m ,

b u t t h e a n a l ys e s w o u l d h a v e l o s t so m e i n f o r m a t i o n i n th e r e d u c e d s a m p l e . W e c o u l d h a v e

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

p r o c e d u r e f o r b o t h M Z a n d D Z p a i r s . T h e r e f o r e , a f t e r d i s c u s s io n s w i t h s e v e r a l s t a t is t i c i a n s , w e

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

b e t w e e n t h e n u m b e r o f i n d e p e n d e n t l y s a m p l e d f a m i l ie s a n d t h e n u m b e r o f i n d iv i d u al s . I n t h e

t a b le s , h o w e v e r , t h e n u m b e r o f fa m i l ie s a n d t w i n p a i rs a r e b o t h g i v e n.

T h e s a m p l e s o f b l a c k a n d w h i t e t w i n s a r e d e s c r i b e d i n T a b l e 1 . O f t h e b l a c k t w i n p a i r s , 1 5 7

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

e c o n o m i c c h a r a c t e ri s ti c s o f t h e i r n e i g h b o r h o o d s w e r e t a k e n f r o m c e n s us tr a c ts . T h e m e d i a ni n c o m e o f t h e t r a c t s i n w h i c h b l a c k t w i n s r e si d e is $ 7 9 1 0, a n d t h e m e d i a n a d u l t e d u c a t i o n a l l e ve l

i s 1 0 .2 y e a r s . B o t h f i g u r e s a r e v e r y c l o s e t o t h e a v e r a g e 1 9 7 0 c e n s u s f i g u r e s f o r u r b a n b l a c k

f a m i l i e s . T h e s u b j e c t s r a n g e d i n a g e f r o m 1 0 y e a r s t o 1 5 y e a r s , 1 1 m o n t h s .

T h e a c t u a l s a m p l e s i z e s a v a i l a b l e f o r t h e s e v e r a l a n a l y s e s to b e r e p o r t e d i n th i s p a p e r v a r i e d

f r o m a b o u t 3 0 0 i n d i v i d u a l s f ro m 1 60 t w i n p a i r s t o 2 88 i n d i v i d u a l s i n 1 4 4 p a i r s o n w h o m w e h a d

c o m p l e t e b l o o d g r o u p d a t a o n t h e 12 sy s te m s u s e d f o r e s t i m a t i o n o f a n c e s t r y a n d n e a r l y

c o m p l e t e m e n t a l t e s t d a t a . T h e l a r g e s t r e d u c t i o n i n s a m p l e s iz e o c c u r r e d f o r t h e p a i r e d - a s s o c i a t e

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

c i t y b l a c k c h i l d r e n , r e s u l t i n g i n t h e e l i m i n a t i o n o f m a n y o f t h e i r r e s u lt s . T h e o t h e r m e n t a l t e s t

r e s u l t s h a v e m o r e v a l i d p a i r s , a s i n d i c a t e d i n t h e t a b l e s .

Pro c edu r e s . T h e c h i l d r e n w e r e e a c h p a i d $ 1 0 t o p a r t i c i p a t e , a n d t h e y r e c ei v e d a f re e d e n t a lc h e c k u p , p h y s i ca l g r o w t h a s s e s sm e n t , a n d r e f re s h m e n t s . T h e y w e re b r o u g h t a f t e r l u n c h b y

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

a p p r o x i m a t e l y 5 h a t t h e D e n t a l S c h o o l , U n i v e r s i ty o f P e n n s y l v an i a .

C o - t w i n s w e r e s e p a r a t e d i n t o d i f f e r e n t s m a l l g r o u p s , e a c h w i t h a n a d u l t le a d e r w h o

e x p l a i n e d t h e p r o c e d u r e s , a n s w e r e d q u e s t i o n s , a n d g a v e a s s is t a n c e . A n a v e r a g e o f 2 8 c h i l d r e n ,

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

A u g u s t , 1 9 7 2 .

F o r t h e p s y c h o l o g i c a s s e s s m e n t s t h e s m a l l g r o u p s w e r e a s s e m b l e d in a la r g e a u d i t o r i u m .

S e a t i n g w a s a r r a n g e d i n a l t e r n a t e s e a t s a n d r o w s . T e s t m a t e r i a l s w e r e p r e s e n t e d b y 3 5 m m s l i d e s

o n a l a r g e s c r e e n. I n s t r u c t i o n s a n d t e s t i t e m s w e r e p r e s e n t e d b y a u d i o t a p e a n d c o o r d i n a t e d

a u t o m a t i c a l l y w i t h t h e s l id e p r e s e n t a t i o n s . N o r e a d i n g s k i ll s w e r e r e q u i r e d . A l l o f th e m a t e r i a l s

h a d b e e n p r e t e s t e d w i t h 3 0 b l a c k , i n n e r - c i t y c h i l d r e n w h o w e r e p a i d a c o n s u l t a n t fe e t o c ri t ic i z et h e p r o c e d u r e s a n d t e st s . B a s e d o n t h e p re t e s t , a l l t e s t i n s t r u c t i o n s w e r e m a d e m o r e r e d u n d a n t

t h a n s t a n d a r d i n s t ru c t i o n s t o h e l p t h e d i s a d v a n t a g e d b l a c k c h i l d re n to u n d e r s t a n d t h e n a t u r e o f

t a s k s. G r o u p l e a d e r s m o n i t o r e d t h e c h i l d r e n ' s u s e o f t h e s i m p l if i e d a n s w e r s h e e ts fo r th e t e s ts .



R a c i a l M i x t u r e a n d C o g n i t i o n 7 5

B l o o d s a m p l e s w e r e d r a w n a t t h e e n d o f t h e d a y , j u s t b e f o r e t h e p a y m e n t s w e r e g i v e n o u t .

A l t h o u g h s o m e c h i l d re n w e r e r e l u c t a n t to h a v e b l o o d d r a w n , p e e r p re s s u re a t t h e p r o m i s e o f te n

d o l l a r s p r o d u c e d e x c e l l en t c o o p e r a t i o n a n d m i n i m a l d is t re s s .

In te l l ectual Ski l l s . F i v e m e a s u r e s o f i n t e l l e c t u a l s k i ll s w e r e a d m i n i s t e r e d a s p a r t s o f t w o 11/4h

p s y c h o l o g i c a s s e s s m e n t s t h a t a l s o i n c l u d e d p e r s o n a l i t y a n d s e l f- e s te e m m e a s u r e s . T h e t w os e s s io n s w e r e s e p a r a t e d b y a p p r o x i m a t e l y 1 h , i n w h i c h d e n t a l , t a s te , d e r m a t o g l y p h i c , r a d i o -

l o gi c , p h y s ic a l g r o w t h , a n d o t h e r a s s e s s m e n t s w e re m a d e . R e f r e s h m e n t s w e re s e r v e d d u r i n g a

b r e a k b e t w e e n s e s s i o n s .

T h e R a v e n S t a n d a r d P r o g r e s s i v e M a t r i c e s , S e ts A , B , C , a n d D w e r e i n c l u d e d to m e a s u r e

a b s t r a c t r e a s o n i n g s k i ll s. S e v e n t y i t e m s f r o m t h e P e a b o d y P i c t u r e V o c a b u l a r y T e s t w e re u s e d to

m e a s u r e k n o w l e d g e o f s t a n d a r d E n g l is h vo c a b u l a ry . T h i r t y it e m s fr o m t h e C o l u m b i a T e s t o f

M e n t a l M a t u r i t y w e r e u s e d t o a s s es s c o n c e p t u a l s k i ll s. T h e R e v i s e d F i g u r a l M e m o r y T e s t w a s

i n c l u d e d t o t e s t c o n c e p t u a l m e m o r y f o r d e s i g n s. F i n a l l y , a p a i re d - a s s o c i a t e t a s k w a s in c l u d e d t o

t e s t r o t e , a s s o c i a t i v e l e a r n i n g s k i l l s .4

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

c o n s i s te n c i e s , r a n g i n g f r o m 0 . 82 t o 0 . 95 i n th e b l a c k s a m p l e ( K u d e r - R i c h a r d s o n , F o r m u l a 2 0 ).

T h e f i g u r a l M e m o r y T e s t a n d t h e p a i r e d - a s s o c i a t e t a s k a r e n o t s u i t e d t o c o n s i s t e n c y a n a l y s i s ,b u t t h e i r e x p e c t e d c o r r e l a t i o n s w i t h t h e o t h e r c o g n i t i v e m e a s u r e s ( - 0 . 5 a n d 0 . 3 r e s p e c ti v e ly )

w e r e o b s e r v e d . F a c t o r a n a l y s i s o f t h e f i rs t f o u r c o g n i t i v e te s t s s h o w e d s i m i l a r h i g h l o a d i n g s

( 0. 75 t o 0 .7 9 ) o n a f i r s t p r i n c i p a l c o m p o n e n t , a c c o u n t i n g f o r h a l f o f t h e v a r i a n c e in s e p a r a t e

a n a l y s e s o n t h e b l a c k a n d w h i t e s a m p l e s . T h e r e i s e ve r y re a s o n t o b e l i e v e t h a t t h e f i rs t f o u r t e s t s

a r e v a l i d , r e l i a b l e m e a s u r e s o f i n t e l l e c t u a l s k il l s i n a b l a c k s a m p l e . T h e p a i r e d - a s s o c i a t e t e s t i s

l es s re l a t e d t o t h e o t h e r s , b o t h t h e o r e t i c a l ly a n d e m p i r i ca l l y . T h e s c o r e s u s e d i n t h i s p a p e r w e r e

s t a n d a r d i z e d b y 1 - y e ar a g e in t e r v a l s to e l i m i n a t e a g e v a r i a n c e .

S o c i o eco n o mi c S t a t u s . T w o m e a s u r e s o f s o c i o e c o n o m i c s ta t u s w er e o b ta i n e d . T h e H o m e I n d e x

( G o u g h , 1 9 49 ), a 2 4 - i t e m m e a s u r e o f f a m i l y S E S , w a s a d m i n i s t e r e d a s p a r t o f t h e fi r st t e s t

b a t t e r y . I t w a s f o u n d t o b e u n r e l i a b l e f o r b l a c k c h i l d r e n b e c a u s e c o - t w i n s o f t e n d is a g r e e d a b o u t

i n f o r m a t i o n o n t h e i r fa m i l i e s ( C a r t e r - S a l t z m a n e t a l ., 1 97 5). A r e v i se d s ca l e o f t h e t e n m o s tr e l ia b l e i t e m s w a s i n c l u d e d i n t h is s t u d y . C e n s u s t r a c t m e d i a n v a l u e s f o r e d u c a t i o n a l le v e l a n d

i n c o m e w e r e o b t a i n e d o n a l l c e n s u s t r a c t s i n w h i c h b l a c k t w i n s li v ed . T h e c e n s u s t r a c t i n a n

u r b a n a r e a i s f a i rl y h o m o g e n e o u s w i t h r e s p e c t t o s o c i o e c o n o m i c c h a r a c t e r i s ti c s , b u t i t is a n

i m p e r f e ct m e a s u r e o f i n d iv i d u a l S E S. I t i s a g o o d m e a s u r e o f s o m e n e i g h b o r h o o d a n d s c h o o l

c h a r a c t e r i s t i c s t h a t a r e r e l a t e d t o c h i l d r e n ' s in t e l l e c tu a l d e v e l o p m e n t .

S k i n C o l o r Re f l ec t a n ce . B o t h b l a c k a n d w h i t e t w i n s w e r e m e a s u r e d o n s k i n r e f l e c t a n c e . T h r e e

f il t e rs ( r e d , b l u e , g r e e n ) a n d t h r e e l o c a t i o n s ( fo r e h e a d , m e d i a l a s p e c t o f th e l o w e r a r m , a n d

i n s id e o f t h e u p p e r a r m ) c o m b i n e d t o p r o d u c e n i n e m e a s u r e s o f s k in c o l o r r e f le c t a n c e . T h e

r e f l e c t a n c e v a l u e s w e r e s o h i g h l y i n t e r c o r r e l a t e d ( r > 0 .8 ) t h a t o n l y o n e , r e d f i l t e r - f o r e h e a d , w i l l

b e r e p o r t e d h e r e . T h e r e l i a b il i ty o f t h e s k i n c o l o r m e a s u r e s i s r e f le c t e d i n t h e v e r y h i g h h e r i t a b i l -

i t i e s , b e t w e e n 0 . 8 5 a n d 0 . 9 8 .

Bl o o d Gr o u p Ma r ker s . T w o 1 0 -c m 3 b l o o d s a m p l e s w e r e o b t a i n e d f r o m e a c h c h i l d , o n e i n E D C

s o l u t i o n , o n e i n a c l o t t u b e . B l o o d s a m p l e s w e r e s h i p p e d d a i ly b y a i r i n r e f r ig e r a t e d c a r t o n s t o

t h e M i n n e a p o l i s W a r M e m o r i a l B l o o d B a n k f o r ty p i ng . T h e f o l lo w i n g m a r k e r l o ci w e r e

a s s e s s e d : A B 0 ( A 1 , A 2 , B , 0 ) M N S s , K i d d ( J k ~, J k b ) , K e l l ( K , k ) , R h e s u s ( r , r' , R °, R ~, RE ),

C e r u l o p l a s m i n ( C p ~ , C p b , C p ° ), G r o u p S p e c i f ic ( G c 1, G c 2 ) , T r a n s f e r r i n ( T f c , TfD ), D u f f y ( F ya,

F y b ), H e m o g l o b i n ( H b A , H b s , H b C ) , H a p t o g l o b i n ( H p l, H p 2) , A d e n y l a t e k i n a s e ( A K 1, A K 2)

G m ( a , x , b, c ) , a n d I n v ( 1 ). T h e d i s t r i b u t i o n o f t h e b l o o d g r o u p p h e n o t y p e s a n d t h e i n t e l le c t u a l

t e s t sc o re s a r e a v a i l a b le f r o m t h e A m e r i c a n D o c u m e n t a t i o n S e rv ic e .

4 J . C . R a v e n , S t a n d a r d Pr o g r es s ive M a t r ices : S e t s A , B , C , D , a n d E ( H . K . L e w i s a n d C o . ,

L o n d o n , 1 9 58 ); L . M . D u n n , P e a b o d y p i c t u r e v o c a b u l a r y t e st (A m e r i c a n G u i d a n c e S e r v ic e , I n c . ,

C i r c le P i n e s , M i n n e s o t a , 1 9 59 ); C o l u m b i a T e s t o f M e n t a l M a t u r i t y ( H a r c o u r t , B r a c e , a n d

W i n s t o n , N e w Y o r k , 1 9 59 ); A . L . B e n t o n , T h e r e v i s e d v is u a l r e t e n t i o n t e s t, F o r m C ( W i l l i a m

C . B r o w n C o . , I n c ., D u b u q u e , I o w a , 1 96 3); H . W . S t e v e n s o n , G . A . H a l e , R . E . K l e in , a n d

L . K . M i l l e r , M o n o g r a p h s o f t h e s o c i e ty f o r r e s e a r c h i n c h i l d d e v e l o p m e n t , 3 3 , W h o l e N o . 1 23

( 1 9 6 8 )



7 6 S . Sc a r r e t a l .

T w i n z y g o s i t y w a s e s t a b l i s h e d b y c o m p a r i n g c o - t w i n ' s b l o o d g r o u p s a t e a c h o f t h e lo c i . I f

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

r e d o n e t o a f f i r m t h e d i a g n o s i s .

Ancestral Phenotype Frequencies. T o c a l c u l a t e t h e a n c e s t r a l A f r i c a n f r e q u e n c i e s d i f f e r e n t

w e i g h t i n g s c h e m e s w e r e a p p l i e d t o t h e a v a i l a b l e d a t a f r o m d i f f e r e n t re g i o n s o f s u b - S a h a r a nA f r i c a . C u r t i n ' s ( 1 96 9 ) s p e c u l a t i v e e s t im a t e s o f th e p r o p o r t i o n o f sl a v e s o r i g i n a t i n g f r o m e i g h t

a r b i t r a r y A f r i c a n r e g i o n s i s r e p r o d u c e d i n r o w a o f T a b l e 2 . C u r t i n b a s e d h i s c a l c u l a t i o n s o n

r e c o r d s f r o m c o l o n i a l V i r g i n i a a n d S o u t h C a r o l i n a a s w e ll a s th e t o t a l B r it i sh s l a v e t r a d e . T w o

o t h e r w e i g h t i n g s c h e m e s w e r e u s e d i n t h i s s t u d y . T h a t s h o w n i n r o w b o f T a b l e 2 g i v e s e q u a l

w e i g h t i n g t o e a c h r e g i o n w h i l e r o w c is a m o d i f i c a t i o n o f C u r t i n ' s e s t i m a t e s t o g iv e g r e a t e r

w e i g h t t o r e g i o n s V I , V I I , a n d V I I I .

G e n e f r e q u e n c y e s t i m a t es f o r e a c h r e g i o n w e r e o b t a i n e d f r o m a n e x t e n s iv e r e v i ew o f t h e

p u b l i s h e d l i t e r a tu r e o n A f r i c a n g e n e f r e q u e n c i e s . U n l i k e e a r l i e r e s t im a t e s , g r e a t e r w e i g h t w a s

g i v e n w h e r e p o s s i b l e t o g r o u p s w i t h i n 2 0 0 m i l e s o f t h e c o a s t t h a n t o i n l a n d g r o u p s , w h o

p r o b a b l y c o n t r i b u t e d l e ss to t h e s l a v e t r a d e . T h e p h e n o t y p e f r e q u e n c y e s t im a t e s f o r th e e i g h t

r e g i o n s a r e g i v e n i n T a b l e 3 . 5 W h i l e t h e se d a t a r e p r e s e n t s i n f o r m a t i o n o n m a n y t h o u s a n d s o f

i n d iv i d u a ls , t h e m a n y e m p t y c e ll s e m p h a s i z e t h e f r a g m e n t a r y n a t u r e o f o u r k n o w l e d g e o fm o d e r n A f r i c a n p o p u l a t i o n s , e s p e c i a ll y f o r t h o s e g e n e t i c l o c i o f g r e a te s t v a l u e f o r t h e p r e s e n t

s t u d y .

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

p r o d u c e t h r e e p o s s i b l e a n c e s t r a l p o p u l a t i o n s .

5 T h e a n c e s t r a l C a u c a s i a n a n d A f r i c a n g e n e f r e q u e n ci e s u s e d a r e f o u n d i n t h e f o l lo w i n g

s o u r c e s . T h e s a m e r e f e r e n c e s w e r e u s e d a s a r e f o u n d i n f o o t n o t e 1 8 o f J . A d a m s a n d R . H .

W a r d , S c i e n c e , 1 80 , 1 1 37 (1 97 3) p l u s t h e s e a d d i t i o n a l s o u r c e s w h e r e t h e y d o n o t o v e r l a p : R . E .

G . A r m a t t o e , A m . J . P h y s . A n t h r o p . , n . s. 9 , 3 71 (1 9 5 0) ; R . E . G . A r m a t t o e , E . W . I k i n , a n d A .

E . M o u r a n t , W . A f r . M e d . J . 2 , 8 9 ( 19 53 ); S . H . B o y e r a n d E . J . W a t s o n - W i l l ia m s , N a t u r e

( L o n d . ) 1 9 0 , 4 5 6 ( 19 6 1) ; J . B u e t t n e r - J a n u s c h , R . R e i s m a n , D . C o p p e n h a v e r , G . A . M a s o n , a n d

V . B u e t t n e r - J a n u s c h , A m . J . P h y s . A n t h r o p . 3 8 , 66 1 (1 97 3) ; L . L . C a v a l l i - S f o r z a a n d W . F .

B o d m e r , T h e g e n e t i cs o f h u m a n p o p u l a t i o n s ( F r e e m a n a n d C o . , S a n F r a n c i sc o , 1 97 1), p p .

2 6 7 - - 2 6 8 ; H . C l e v e a n d A . G . B e a m , i n P r o g r. M e d . G e n e t . , V o l . 2 , A . G . S t e i n b e r g a n d A . G .

B e a m , e d s. ( 19 62 ); G . M . E d i n g t o n , A . A f t . M e d . J . 5 , 7 1 ( 1 95 6 ); A . E y q u e m , L . P o d l i a c h o u k ,

an d J . P re s l e s , V o x San g . (B as e l ) 6 , 1 20 (19 6 1) ; I . Fay e , H . R u s c h e r , M . P . T s a l a , an d G . B l o c ,

B u l l . So c . M ed . A f r . N o i r e l an g . f r an 9 . 1 6 , 5 5 1 (1 9 7 1 ) ; E . R . G i b l e t t , i n P ro g r . M ed . G en e t . , V o l .

2 , A . G . S t e i n b e r g a n d A . G . B e a m , e d s . (1 96 2) ; E . R . G i b l e t t , G e n e t i c m a r k e r s i n h u m a n b l o o d

( F . A . D a v i s C o . , P h i la d e l p h i a , 1 9 69 ); G . H o l m g r e n a n d K . G . G o t e s t a m , H u m . H e r e d . 2 0 ,4 3 3

( 1 97 0 ); T . J e n k i n s , A . Z o u t e n d y k , a n d A . G . S t e i n b e r g , A m . J . P h y s. A n t r o p . 3 2 , 19 7 ( 1 97 0 ); G .

K e l l e r m a n n a n d H . W a l t e r , H u m a n g e n e t i k 1 5 , 8 4 (1 97 2); F . D . K i t c h in a n d A . G . B e a m ,

N a t u r e ( L o n d . ) 2 0 2 , 82 7 (1 96 4) ; J . L a m b o t t e - L e g r a n d a n d C . L a m b o t t e - L e g r a n d , A n n . S o c . b e l g .

m 6 d . t r o p . 3 0 , 5 4 7 ( 1 9 5 0 ) ; V . T . M a t z n e t t e r a n d W . S p i e l m a n n , Z . M o r p h . A n t h r o p . 6 1 , 5 7

(1 9 6 9 ); J . M o u l l ec , J . M . F i n e , C . H en r y , an d C . S i lv e r i e , P ro c . 7 t h C o n g . In t e rn a t l . So c . B 1.T r a n s f . ( R o m e , S e p t e m b e r 3 - - 6 , 1 95 8) , p p . 8 8 1 - - 8 8 3 , P . M o u r e a u a n d J . B r o c t e u r , B u l l. A c a d .

r o y . M 6 d . B e l g . 7 ( N o . 2 ) , 1 47 ( 1 96 2 ; W . C . P a r k e r a n d A . G . B e a m , A n n . h u m . G e n e t . 2 5 , 2 2 7

( 19 61 ); R . R . R a c e a n d R . S a n g e r , B l o o d G r o u p s i n M a n , 5 t h e d i t i o n ( B l a c k w e l l, O x f o r d , 1 9 68 );

L . R e y s , C . M a n s o , G . S t a m a t o y a n n o p o u l e s , a n d E . G i b l e t t , H u m a n g e n e t i k 1 6 , 2 2 7 ( 19 72 ); L .

R i v a t , M . B l a n c , C . R i v a t , C . R o p a r t z , a n d J . R u f f l e , H u m a n g e n e t i k 1 3 , 10 8 ( 19 71 ); H . S a g n e t ,

J . T h o m a s , L . V o v a n , C . J e s s e r a n d , A . M a r i e - N e l l y , a n d A . O r s i n i , P e d i a t r i e 2 6 , 6 11 (1 97 1) ; M .

H . K . S h o k e i r a n d D . C . S h r e f f l e r , B i o c h e m . G e n e t . 4 , 5 17 ( 19 70 ); A . G . A . S i m b e y e , H u m .

H e r e d . 2 2 , 2 86 ( 1 9 72 ) ; W . S p i e l m a n n , H . R u p p i n , L . S c h i ll in g , a n d D . T e i x i d o r , D t s c h . Z . ge s .

g e r i ch t l . M ed . 6 4 , 1 8 6 (1 9 68 ); A . G . S t e i n b e rg , A m . J . h u m . G e n e t . 1 8 , (1) , 1 0 9 (19 6 6) ; D . T i l l s ,

J . L . V a n d e n B r a n d e n , V . R . C l e m e n t s , a n d A . E . M o u r a n t , H u m . H e r e d . 2 0 , 5 17 ( 19 70 ) a n d

H u m . H e r e d . 2 1 , 3 0 2 ( 1 97 1 ); P . V . T o b i a s , i n T h e b i o l o g y o f h u m a n a d a p t a b i l i ty , P . T . B a k e r

an d J . S . Wei n e r , ed s . ( 1 9 6 6 ); R . M . W i n s t o n , W . A f t . M ed . J . 3 , 17 (19 5 4) . A l t h o u g h a s m a l la m o u n t o f A m e r i c a n I n d i a n a d m i x t u r e h a s b e e n f o u n d i n s o m e l o ca l b l a c k p o p u l a ti o n s ( B .

G l a s s , A m . J . h u m . G e n e t . 7 , 3 68 ( 19 55 )', t h e c o n t r i b u t i o n i s s m a l l e n o u g h t o b e s a f e ly ig n o r e d

w h e n s o m a n y o t h e r s o u r ce s o f e rr o r a r e m o r e o b v i o u s



Racial Mixture and Cognition

T a b l e 2. Weighting schemes used to obtain he ancestral African frequencies

77

I I I I I I I V V V I V I I V I I I

a 0.13 0.06 0.11 0.16 0.04 0.23 0.25 0.02

b 0.125 0 .125 0.125 0.125 0.125 0.125 0.125 0.125

c 0.05 0.05 0.05 0.10 0.15 0.30 0.20 0.10

Caucasian phenotype frequencies for England, Ireland, Scotland, and Wales were used for

the Caucasian ancestral population. While we recognize that other European groups also con-

tributed to the contemporary US black population, no weighting scheme exists for the white

ancestral populations comparable to the one Curtin provides for African groups. Besides, most

of the US black population resided in the southeastern states during the time that hybrid group

was forming and in that region the white populat ion derived predominantly from British Isles'

settlers. Table 3 also gives the Caucasian phenotype frequencies used as the second ancestralpopulation.

Phenotype frequencies from the three estimated African ancestral populations and the

Caucasian ancestral populations were inserted in Formula (1) to calculate the three ancestralodds coefficients. High values indicate closer resemblance to African phenotype frequencies.

Sample Frequencies . Since we were not concerned with an individual admixture estimate butwith a rank order coefficient, the phenotype frequencies actually obtained in the black and

white samples in Philadelphia could be used to rank order socially classified blacks according to

their degree of resemblance to the white sample. Those individual phenotypes that closely

resemble the black sample values, especially at those loci with large differences in phenotype

frequencies between the black and white samples, will receive higher rank order values than

those phenotypes that closely resemble the white sample frequencies. Using Formula (1), wecalculated a sample odds coefficient.

Construct Validation of the Odds Coefficient. If these odds coefficients are valid measures of

racial genetic variability, then they should meet two criteria. First, they should correlate with

skin color, which also reflects racial genetic variability. Second, the correlation for the odds

coefficients between dizygotic twins should be around 0.5 or a little higher if there is assortativemating for characteristics such as skin color that are related to admixture. DZ twins share halfof their genes on the average.

R e s u l t s

A n c e s t r a l O d d s . The three putative African ancestral populations produced

indis tingu isha ble ancestral odds coefficients. Alt hou gh the phenot ype frequencies

varied somewha t, the r ank orders of black children were essentially the same.

Thus, we chose to use Curtin's (1969) weighted values as the final measure.

V a l i d i ty o f t h e O d d s C o e f f i c ie n t s . The DZ twin correlations for ancestral and

sampl e odds were 0.55 a nd 0.61 respectively (SE = 0.11). These in tracl ass cor-

relations are in the expected range for a valid coefficient based on genetic

variability. Plots of the co-twin values for the odds indicated that variability

between co-twins was equally distributed across the range of the sample oddscoefficient. Variabilit y in ancestral odds was greater for low values that represent

less Afric an ancestry. Th us, a greater n um be r of individu als with higher degrees

of Caucasian ancestry are discriminated than there are in the range of the



7 8 S . S c a r r e t a l .

0

oe~

o

. . =

0a

o

0am

= .. =

o ~ ~

I

,. o



Racial Mixture and Co gnit ion 79

I I 1 + 1 1 +

~ ~ i~ i ~

~ i ~ i~ i ~

~ . ~

g i

No~

~ ¢ ~ ~ o

° " ~ , ~ o ~ o ~

g .~ .~2 " ~ = ~



80

Table 4. Correlations am on g skin color, ancestral and sample odds coefficients,based on three and nine marker loci

S. Scarr et al.

S C A O (3 ) A O (9 ) S O ( 3 ) S O ( 9 )

Skin colo r reflectance XAn cestral odds (3) 0.16 X

An cestral odds (9) 0.11 0.11 X

Sam ple odds (3) 0.22 (0.58) 0.11

Sam ple odds (9) 0.18 0.09 (0.76)

X0.10 X

n ~ 300 individuals f r o m - 160 twin pairs; If n = 160 an d r > 0.15, P < 0.05

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

c o e f f i c i e n t i s u s e d .

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

c o r r e l a t e d w i t h t h e s k i n c o l o r m e a s u r e , 0 .2 1 a n d 0 .2 7 re s p e c t i v e l y ( P < 0 .0 1 ).

A l t h o u g h w e w e r e u n a b l e t o m a k e a p o i n t p r e d i c t i o n f o r t h e c o rr e l a ti o n s ( b e c a u s e

l it tl e is k n o w n a b o u t t h e d i s t ri b u t i o n o f a d m i x t u r e i n t h e b l a c k p o p u l a t i o n ) w e

a n t i c i p a t e d a l o w p o s i t i v e re l a t i o n s h i p t h a t is c o n s o n a n t w i t h th e p o w e r o f t h e

g e n e t ic m a r k e r l o ci u s e d t o i n d e x d e g r e e o f a n c e s tr y .

T h e r e is a n o t h e r h y p o t h e s i s , h o w e v e r , t o e x p l a i n th e c o r r e l a ti o n b e t w e e n s k i n

c o l o r a n d b l o o d g r o u p m a r k e r s : t h a t o n e s k i n c o l o r l o c us is c l os e ly l in k e d t o G m

a n d / o r F y ( G e r s h o w i t z a n d R e e d , 1 97 2; C a v a l li -S f o rz a , p e r s o n a l c o m m u n i c a t i o n

1 97 4). I n d e e d , s k in c o l o r w a s f o u n d t o b e m o r e h ig h l y c o r r e l a t e d w i t h G m t h a n

a n y o t h e r s in g le l o c u s ( r = 0 .2 0 ), f o l l o w e d b y D u f f y , T r a n s f e r r i n ( r = 0 .1 3 f o r

e a c h ) a n d A B 0 ( r = 0 .1 1 ). T h e s e l o c i a r e g o o d m a r k e r s f o r a n c e s tr y , h o w e v e r , a n d

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

S k in c o l o r v a r ia t i o n p r o b a b l y d e p e n d s u p o n a f ew g o o d m a r k e r s o f a n ce s tr y .

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

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

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

s a m e a n c e s t r a l p o p u l a t i o n . I f , i n s te a d , t h e r e l a t io n s h i p b e t w e e n s k i n c o lo r a n d

a n c e s t r y d e p e n d s u p o n t h e cl o se l in k a g e b e t w e e n a sk i n c o l o r l o c us a n d F y / G m ,t h e n t h r e e b l o o d g r o u p m a r k e r s s h o u l d n o t c o r r e l a te p o s it i v el y w i t h t h e re s t, a n d

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

m a r k e r .

T o t e st t h e c o m p e t i n g h y p o t h e s e s w e s e le c t ed t h r e e g o o d b l o o d g r o u p m a r k e r s

( g m , F y , A B 0 ) t o c o r r e l a t e w i t h t h e r e m a i n i n g n i n e a n d t o c o m p a r e w i t h th e

c o r r e l a t i o n o f s k i n c o l o r t o o d d s c o e f f i c ie n t s c a l c u l a t e d o n t h e s e t o f t h r e e a n d t h e

s e t o f n in e . T h e c o r r e l a t i o n s a r e g i v e n i n T a b l e 4.

A l l o f t h e c o r r e l a t i o n s a m o n g s k i n c o l o r a n d t h e o d d s c o e f f i c ie n t s w e r e in a

p o s i t i v e d i r e c t i o n . T h r e e o f t h e e i g h t w e r e s t a ti s t ic a l l y s i g n i fi c a n t , in c l u d i n g t h e

c o r r e l a t io n s b e t w e e n s k i n c o l o r a n d t h e s a m p l e o d d s , w i th a n d w i t h o u t G m a n dD u f f y ( r = 0 .2 2 a n d 0 .1 8 r e s p e c t i v e l y ). T h e c o r r e l a t i o n s b e t w e e n t h e s e ts o f t h r e e

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

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




Table 5. The distributions of ancestraland sample odds coefficientscalculatedseparately for co-twins 1 and 2

Co-twin 1 Co-twin 2

Ancestral odds

Mean 4.13 4.22

SD 3.00 2.96SE 0.26 0.24

Skewness -0.93 -0.94

Kurtosis 1.67 1.46

Range --8.3 to 10.1 -8.3 to9.9

Sample odds

Mean 2.96 2.98

SD 2.15 2.07

SE 0.19 0.17

Skewness 0.09 0.14Kurtosis 0.18 -0.07

Range -3.0 to 8.7 -3.0 to 8.4

group markers does not depend solely upon a hypothesized linkage with Gm. The

data are consistent with a hypothesis of partial nondispersion of ancestry.

As a furthe r test o f the validity of the odds coefficients, the ancestral and

sample frequencies were used to calculate 'a dmix ture ' for the white sample. Since

African populations are not significant progenitors of the contemporary USwhite group, we did not expect the odds coefficients to correlate with skin color

within the white sample. Although both skin color reflectance and the odds

coefficients were sufficiently variable and reliable to produce the expected DZ

twin coefficients o f abo ut 0.5 (red filter-forehead rDz =0.51; ancestral odds

rDz = 0.48; sample odds rDz = 0.54), the genetic variability in skin color and

blood group markers were unrelated to African ancestry within the white sample

(skin color, ancestral odds r = 0.04; sample odds r = 0.05).

In the black sample the distributions of the ancestral and sample odds coeffi-

cients were calculated separa tely for co-twins, random ly designated 1 and 2. The

statistical characteristics of the four odds coefficients are given in Table 5.

Co-twins, separated into two samples, do not constitute a traditional

replication study, but they do provide two related samples on which to test the

distributional qualities of the proposed statistic. The odds coefficients for co-

twins 1 and 2 are very similarly distributed. As shown in Figure 2, the ancestral

and sample odds coefficients differed in the shapes of their distributions. The

sample odds coefficient produ ced more individuals with low degrees of estimated

African ancestry, and the ancestral odds produced a greater number of individ-

uals in the high ranges of estimated African ancestry.

Relat ionships o f Odds Coef f icients to Social Variables . There were negligible cor-relations between the two measures of socioeconomic status and the odds coeffi-

cients. The census tract data correlate negatively with increasing resemblance to

the black or Afri can groups. (The higher SES, the less the resemblance to b lack or



82

SES Ances t ra l Sample oddsodds

Census -0 .1 0 -0 .1 2

Indiv idu al +0 .09 +0.07

S. Sc arr e t al.

Table 6 . Corre la t ions of the odds coeff i -cients and SES

Table 7 . Corre la t ions o f ancestra l an d sam ple odds coeffic ien ts w ithintellectual skills

Ancestra l Sampleodds odds

Raven s tanda rd p rog ress ive ma t r ices -0 .08 -0 .13

Peabo d p ic tu re vocabu la ry t es t -0 .0 6 0 .00

Co lum bia t e s t o f men ta l ma tu r i ty 0 .02 -0 .04Rev ised te s t o f f igu ral me mo ry -0 .12 -0 .10

Paire d-ass ociate test 0.15 0.12

Fi r s t p r inc ipa l com pone n t -0 .03 -0 .05

n ~ 144 pairs; r _<-0.14, one-ta iled test, P < 0.05; SE = 0.083

A f r i c a n g r o u p s . ) T h e i n d i v i d u a l S E S m e a s u r e c o r r e l a t es p o s i ti v e l y w i t h t h e o d d s

c o e f f i c i e n t s . N o n e o f t h e c o e f f i c i e n t s is st a t i s ti c a l l y d i f f e r e n t f r o m z e r o , b u t t h e y

a r e g i v e n i n T a b l e 6 .S k i n c o l o r is o n l y s l i g h tl y r e la t e d t o S E S c h a r a c t e ri s t ic s , i n t h e s a m e d i r e c t i o n s

a s t h e o d d s c o e f fi c i en t s . T h e d a r k e r c h i l d r e n t e n d t o l iv e i n l o w e r S E S n e i g h b o r -

h o o d s ( r = 0 .1 5 ) b u t d o n o t t e n d t o h a v e l o w e r S E S f a m i l i e s (r = 0. 03 ). T h e r e a r e

n o s i g n i f i ca n t c o r r e l a t i o n s b e t w e e n s k i n c o l o r a n d S E S .

The Odds C oe f f i c i en t s and In te l l e c tua l Sk i l ls . N o n e o f t h e c o rr e l a ti o n s b e t w e e n t h e

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

f r o m z e ro . T h e r e w a s n o a s s o c i a t i o n b e t w e e n o u r e s t i m a t es o f a n c e s t ry a n d

i n t e ll e c t u a l p e r f o r m a n c e w i t h i n t h e s a m p l e o f b l a c k t w i n s. T h e f i rs t p r i n c i p a l

c o m p o n e n t f r o m t h e f o u r c o g n i ti v e te s t s, w h i c h m o s t p s y c h o l o g i s ts w o u l d c al l g ,

i s t h e s e t o f i n t e l l e c t u a l s k il ls t h a t i s g e n e r a l t o i n t e l l e c t u a l m e a s u r e s . T h e f ir s t

p r i n c i p a l c o m p o n e n t w a s s ig n i f i c a n t ly re l a t e d to s o c i o e c o n o m i c st a t u s ( r = - 0 . 20 ;

P < 0 . 05 ) a n d t e n d e d t o b e r e l a t e d t o sk i n c o l o r ( r - - 0 . 1 5 5 ) , b u t g e n e r a l i n te l le c -

t u a l s k il ls w e r e n o t c o r r e l a t e d w i t h a n c e s t r y . T a b l e 7 g i v e s t h e r e s u l ts b e f o r e s o c i a l

v a r i a b l e s w e r e p a r t i a l l e d o u t o f th e c o r r e l a t i o n s . A s c a t t e r p l o t o f o n e o f t h e

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

F i g u r e 3 . I t i s c l e a r t h a t n o s t a t i s t i c a l l y s i g n i f i c a n t r e l a t i o n s h i p e x i s t s .

A l l t h o u g h t h e s o c i a l c o r r e l a t e s o f t h e o d d s c o e f fi c i en t s a c c o u n t f o r v e r y li tt le

o f t he v a r i a n c e i n i n te l le c tu a l p e r f o r m a n c e , w e c o m p u t e d t h e c o r r e l a ti o n s

b e t w e e n t h e o d d s c o e f f ic i e n t s a n d t h e i n t el l e c tu a l s ki ll s h o l d i n g s k i n c o l o r a n dS E S ( c e n s u s t r a c t s) c o n s t a n t . S i n c e s o c i al d i s c r i m i n a t i o n c a n b e b a s e d o n vi si b le

m a r k e r s o f a n c e s t r y , i t s e e m e d a d v i s a b l e t o p a r t i a l o u t t h e s o c i al e f fe c ts . T a b l e 8

g i v e s t h e r e s u l t s .



R a c i a l M i x t u r e a n d C o g n i t i o n 8 3

F..-Z 1IJJ

z20

-1

Z

ee,

D . - - 3

I- -

- 5

I I I I I I I I I

• • I M o O o •

. o ° ; o " % •• o o o • • •

• _ , c 6 e l o o • •6 e e o - - o o - - o - - o ,

• . ~ 1 . . . . ~ . ~ -~ . . . . . .

= - 2 " : I . . - : ' ."• , , o O S e o S %

• • o 0 •

: . a

- - 6 I I I I I I I I--6 --4 --2 2.0 4.0 6.0 8.0 10 .0 12 .0 14.0

ODDS COEFFICIENT

Fig . 3 . A sca t t e rp lo t o f sco res on t he f i r s t p r i nc ipa l com pone n t o f fou r cogn i t i ve t e s ts and t hesam p le odds coe f f i c ien t o f ances t ry w i th in a sam p le o f Ph i l ade lph i a b l acks

Tab le 8 . Co r re l a t i ons o f ances t ra l and sam p le odds w i th i n t e l lec tua lsk i ll s : SES (census t rac t s ) an d sk in co lo r pa r t i a l l ed ou t

A n c e s t r a l S a m p l e

o d d s o d d s

R a v e n s t a n d a r d p r o g r e s s iv e m a t r i ce s - 0 . 0 9 - 0 . 1 0

P e a b o d y p i c t u r e v o c a b u l a r y t e s t - 0 . 0 3 0 .0 4

C o l u m b i a t e s t o f m e n t a l m a t u r i t y 0 .0 3 - 0 . 0 2

R e v i s e d t e s t o f f i g u r a l m e m o r y - 0 . 1 0 - 0 . 0 6

Pa i red- ass ocia te tes t 0 .14 0 .10

F i r s t p r i nc i p a l c o m p o n e n t - 0 . 0 ! - 0 . 0 2

n ~ 1 44 p a i r s; r < - 0 . 1 4 , o n e - t a i le d t e s t, P < 0 . 0 5 ; S E = 0 . 0 83

T o t e s t f u r t h e r t h e r e l a t i o n s h i p s b e t w e e n t h e o d d s c o e f f ic i e n ts a n d i n t e l l e c t u a l

p e r f o r m a n c e , e x t r e m e g r o u p s w e r e c o n t r a s te d . T h e d i s t r i b u t i o n s o f a n c e s t ra l a n d

s a m p l e o d d s c o e f f i c ie n t s w e r e d i v i d e d i n t o t h i r d s . T h e t e st s c o r es o f t h e g r o u p

w i t h t h e h i g h e s t o d d s f o r A f r i c a n a n c e s t r y w e re c o m p a r e d t o t ho s e o f t h e g r o u p

w i t h t h e l o w e s t . S k i n c o l o r a n d s o c i a l c l as s d i f fe r e n c e s a r e c o n f o u n d e d i n t h e s e

c o n t r a s t s , a n d t h e s a m p l e si ze s a r e o v e r e s t i m a t e d . D e s p i t e th e c o n f o u n d i n g o f

s o c i a l v a r i a b l e s , i n o n l y o n e c a s e o u t o f 1 2 d i d e x t r e m e g r o u p c o n t r a s t s a c h i e v e

s t a t i s t i c a l s i g n i f ic a n c e w i t h a s a m p l e s i ze in f l a t e d t o t h e n u m b e r o f i n d i v i d u a l s

i n s t e a d o f th e n u m b e r o f f a m i l i e s i n d e p e n d e n t l y s a m p l e d . T h e s e r e s u l t s ~ire g i v e n

i n T a b l e 9 .O n t h e R e v i s e d T e s t o f F i g u r a l M e m o r y , t h e t h i rd o f t h e s a m p l e w i th t h e l e a s t

A f r i c a n a n c e s t r y h a d h i g h e r s c o r e s t h a n t h e th i r d w i t h th e m o s t , b u t t h is r e s u lt

w a s n o t r e p l i c a t e d w i t h t h e s a m p l e o d d s c o e f f i c ie n t . M o s t i m p o r t a n t l y , th e



84 S. Scarr et al.

Table 9. Contrasts between extreme thirds of the ancestral and sample odds distributions on intellectualtest scores (least black minus most black in standard deviation units)

Sample odds Ancestral odds

m diff. SEa t P mdiff. SEa t P

Raven standard progressive matrices 0.21 0.13

Columbia test of mental matur ity 0.09 0.13

Peabody picture vocabulary test -0.08 0.13

Revised test of figural memory 0.07 0.12

Paired-associate test -0.06 0.15

First principal component 0.11 0.21

1.60 0.11 0.16 0.13

0.06 0.13

0.08 0.13

0.31 0.12

-0.23 0.15

0.13 0.22

2.31 0.02

1.63 0.11

a Assumes that individuals are independently sampled and n~ 280. In fact, n is best consideredbetween 280 individuals and 140 independent families

B L O O D G R O U PM A R K E R S ~B Y A N C E S T R Y .0 2

o,, , y / !

I IFig. 4. Path coefficients of the effects of ancestry and socioeconomic status on the intellectualskills of a sample of Philadelphia blacks

general factor of the intellectual tests was not related to ancestry. Altho ugh

intellect ual skills were not consistent ly related to estimated ancestry in the

extreme thirds, skin color was. The group with the highest estimated African

ancestr y was significa ntly dark er t han the group with the lowest (ancestral odds,

t = 2.05, P< 0.05; sa mpl e od ds, t = 3.25, P< 0.001).

To summ ari ze the results, Figure 4 presents a path analysis of the model

unde rly ing this research, p resente d earlier as Figure 1. The path coefficients of

ancestry (skin color an d b lood group markers) a nd l ife chances (skin color and

soci oecon omic status) with genera l intellectual skills suppo rt the stronger effects

of life chances th an ancestry on int ellectual perfor mance.

D i s c u s s i o n

The test of a relationship between degree of African ancestry, estimated with the

odds coefficients, and intellect ual skills failed to provid e evidence for genetic

racial differences in intelligence. Jens en (1973) predicted the co rrelat ion betwee n

the degree of Cauca sian admi xture a nd intellectual abil i ty as aroun d 0.50, under



Racia l Mixture and Co gnit ion 85

t h e a s s u m p t i o n s o f h is p r o p o s a l t h a t h a l f t o t h r e e - q u a r t e r s o f t h e I Q d i f fe r e n c e

b e t w e e n r a c e s i s d u e t o g e n e t i c d i f f e re n c e s ( J e n s e n , 1 9 73 ). I n t h e c o r r e l a t i o n a l

a n a l y s e s p r e s e n t e d i n t h is p a p e r , n o e v i d e n c e a t al l c a n b e f o u n d f o r a c o r r e l a t i o n

o f t h a t m a g n i t u d e ( T a b l e s 7 a n d 8 ). I n fa c t , n o n e o f th e c o r r e l a t i o n s w a s r e l ia b l y

d i f f e r e n t f r o m z e r o i n t h e e x p e c t e d d i r e c t io n , g i v e n t h e s a m p l e s iz e i n v o l v e d ( ac o r r e l a t i o n o f a b o u t - 0 . 1 4 w o u l d b e d i f f e r e n t f r o m z e r o a t P _-< 0 .0 5 , o n e - ta i l e d

t e st ). E v e n i f t h e c o r r e l a t i o n s b e t w e e n a n c e s t r y a n d i n t e l le c t u a l s k il ls c o u l d n o t

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

r e li a b il i ti e s , t h e y w o u l d h a v e b e e n d e t e c t e d b y th i s s t u d y .

N o w , i f w e l o o k a s w e ll a t t h e d i f f er e n ce s b e t w e e n t h e a v e r a g e s o f t h e u p p e r

a n d l o w e r t h i r d s o f t h e b l a c k g r o u p o n t h e v a r i o u s i n t e ll e c tu a l m e a s u r e s r a t h e r

t h a n r e l y s o l e ly o n a c o r r e l a t i o n a l a n a l y s i s , n o c o n s i s te n t s u p p o r t f o r J e n s e n ' s

h y p o t h e s i s e m e r g e s e i t he r . T h e a v e r a g e d i f fe r e n c e b e t w e e n b l a c k s a n d w h i t es i n

t h is s t u d y o n i n te l l ec t u a l m e a s u r e s i s a r o u n d 0 . 9 s t a n d a r d d e v i a t i o n s ( S D ) . I f w e

a s s u m e t h a t t h e m o s t e x t r e m e t h i rd o f t h e b l a c k g r o u p a v e ra g e s 35 % C a u c a s i a n

a n c e s t r y , w h i l e t h e l ea s t a d m i x e d t h i r d a v e r a g e s 1 5 % ( b as e d o n d a t a o f M a c L e a n

e t a l. , 1 9 74 ), th e a v e r a g e d i f f e r en c e b e t w e e n e x t r e m e t h i r d s s h o u l d b e a b o u t o n e -

f o u r t h o f a s t a n d a r d d e v i a t i o n o n t h e i n te l le c t u a l d i m e n s i o n . 6

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

i n t el l e ct u a l s ki ll s o f 0 .2 6 b e t w e e n t h e e x t r e m e t h i r d s o f t h e d i s t r i b u t i o n a r r a y e d b y

e s t i m a t e d d e g r e e o f a n c e s t r y , w i t h a s t a n d a r d e r r o r o f 0. 13 , w h e n P = 0 .0 5 . I f w e

l e t P = 0 . 1 0 , a n a v e r a g e d i f f e r e n c e o f 0 . 2 2 a c h i e v e s s t a t i s ti c a l s i g n i f i c a n c e. W i t h a

r e l a x e d a l p h a v a l u e , o n e o f t h e t w e l ve m e a n d i f f e re n c e s b e t w e e n e x t r e m e t h i r d s

w a s s t a t is t ic a l ly s i g n if i ca n t , a n d t w o o t h e r s a p p r o a c h e d s t a ti s ti c a l s i g n i f i c a n c e - -

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

d i r e c t i o n o f l o w t o h i g h d e g r e e o f b l a c k a n c e s t r y (s ee T a b l e 9) . A n e x t r a p o l a t i o n

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

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

o b s e r v e d d i f f er e n c e b e t w e e n t h e r a c e s c o u l d b e d u e t o g e n e t ic d i f fe r e n ce s . I n v i e w

o f t h e n e g l i g i b l e c o r r e l a t i o n s b e t w e e n e s t i m a t e d a n c e s t r y a n d i n te l l e c t u a l sk i ll s ,

e v e n t h i s s e e m s u n l i k e l y .

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

c a n b e p r o v i d e d b y i n c r e a s e d s a m p l e s iz es , i m p r o v e d e s t i m a t e s o f t h e a n c e st r a l

p o p u l a t i o n g e n e fr e q u en c i es , a n d a la r g er n u m b e r o f p o l y m o r p h i s m s t h a t d is -

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

t r a n s r a c i a l a d o p t i o n , h a v e s h o w n t h a t b l a c k a n d i n t e r ra c i a l c h i ld r e n r e a re d b y

m i d d l e - c l a s s w h i t e f a m i l i es a c h i e v e I Q s c o r es w e l l a b o v e t h e a v e r a g e o f w h i te

c h i l d r e n i n t h e U S ( S c a r r a n d W e i n b e r g , 1 97 6) . N e i t h e r t h e s t u d y o f t r a n s r a c i a l

a d o p t i o n n o r t h e p re s e n t s t u d y p r o v i d e a n y s u p p o r t f o r a s t r o n g h y p o t h e s i s o f

g e n e t i c r a c i a l d i f f e r e n c e s i n i n t e l l i g e n c e .

6 Th e roug h ca lcula t ion for the es t imate o f the difference between uppe r and lower th irds o f theblack g rou p pro ceed s as follows. If the resultan t difference in stan dar d deviations is 0.9 betweenthe races w hen the m ean difference in degree of Cau casian ances try is ab ou t (0.99 - 0.22 = 0.77,then the d ifference between up per an d lower th irds of the b lack group a lone shou ld be about

0 .23 S D when the d if ference in C aucasian ancestry is abo ut (0 .35 - 0 .15) = 0 .20 . Furtherm ore , i fthree -fourth s o f tha t m ean difference is due to racial genetic differences alon e the smallest ex-pected d ifference is (0 .75 x 0 .23) = 0.18. So , ab out one-f i f th to one-fo urth o f a SD wo uld be theexpected mea n d ifference between up per a n lower th irds of the b lack gro up



86 S. Scarr et al.

Acknowledgements. Our deepest appreciation to the many people who assisted in this research:

to Dr. Herbert Polesky, Director of the Minneapolis War Memorial Blood Bank, for all of theblood analyses; to William Thompson, who helped to fomulate the odds coefficients, wrote the

computer programs, and analyzed the data; to Louise Carter-Saltzman, who edited the psycho-logic test protocols; to Valerie Lindstrom, who supervised much of the data collection; to

Professors Jul ian Adams, V. Elving Anderson, Luigi Cavalli-Sforza, T.E. Reed, and PeterWorkman for consul tation on the design and analysis of the study. None of them is responsible

in any way for the conclusions drawn from the research by the authors. The research was

supported by the Grant Foundation and by NICHHD (HD-08016) and in part by Mental

Health Training Grant (MH 10679).

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Received November 11, 1976/April 12, 1977

Documents

Sandra Scarr, Andrew J Pakstis, Solomon H Katz and William B Barker - Absence of a relationship between degree of white ancestry and intellectual skills within a black population (1977)