L A T E R A L IT Y , 1 9 9 7 , 2 (3 / 4 ), 3 1 7 ±3 3 6
O n th e G e n e tic s a n d M e a su r e m e n t o f
H u m a n H a n d e d n e ss
M .P . B ry d e n
²
E .A . R o y
U niv er sity o f W a te rlo o , O n ta rio , C a na d a
I.C . M c M a n u s
U n ive rsity C olleg e L on d o n, a nd Im p eria l C o lle ge S ch o o l o f M e d ic in e a t S t.
M a ry ’ s, L o nd o n , U K
M .B . B u lm a n -F le m in g
U niv er sity o f W a te rlo o , O n ta rio , C a na d a
L ate ra l p r ef er e nc e s in h u m a n s a nd o th e r p rim a te s m ay b e a k e y to m a ny im po r ta nt
is su es in b io log y a n d p sy c h o lo g y . T h e r e is st ro ng e v id e nc e th a t th e r e p r es e nta tion
of la n gu a g e f un c tio ns in the c er e b r a l c o rte x is diff e re nt in le ft-h an d e d ( L H ) p eo p le
th a n in r ig ht-h a nd e d ( R H ) p e op le , an d a n u n de rs ta n din g o f h a n d e dn e ss m a y le a d to
va lua b le c lu es a s to h o w th e b r ain be c o m es or g an is e d in th e w a y th a t it d oe s .
A ltho u gh th er e a re c le a rly cu ltu ra l e ff ec ts tha t in f lu e nc e m a n ua l a c tivities , th er e is
ne v e rth e le s s e v id e nc e tha t h u m an h a n d e d n e ss a n d oth er la te ra litie s a r e a t le as t in
pa r t ge n etic a lly d e te rm in e d : c o m pa r ed w ith tw o R H pa r en ts , o ne R H a n d on e L H
pa r en t a re 2 ±3 tim es m o re like ly to h a ve a n L H c h ild , a n d tw o L H p ar e nts ar e 3 ±4
tim e s m o re lik e ly to h a v e a n L H c hild . T h u s, o n e m ig h t w ish to inv e st ig a te th e
ge n e tics of la ter a lity w ith th e g o al o f u n d e rs ta n ding th e b io log ic a l m ec h an ism s th a t
le a d to th e p re fe re n tia l u se o f o n e h a nd ( or e y e o r fo ot). O n e m a y a ls o s ee lin k s
be tw e e n h u m a n h an d ed n es s a nd fo ote dn e ss a n d th e m o to r as y m m e trie s fo u nd in
m a ny n on h u m an p rim a te sp e cie s, an d pe r ha p s in lo w e r m a m m a ls a s w ell. F ro m
th is p e rs pe c tive , o n e m ig h t se e th e st ud y o f hu m a n la te ra lity as r ele v a n t to
ev o lu tio na r y b io log y . I nv e st ig a to rs o f b o th h um a n a n d n o n h u m a n as y m m e trie s
ha v e to g ra p ple w ith su ch d iffic u lt m e as ur e m en t is su e s a s th e re la tio ns b etw e en
R e q u e s ts fo r re p rin ts sh o u l d b e se n t to D r. E r ic R o y , D e p art m e n t o f K in es i o l o g y , U n iv ers i ty o f
W a t erl o o , W a t e rlo o , O n ta rio , C a n a d a , N 2 L 3 G 1 . E -m a i l: e ro y @ h e a lt h y .u w a te rl o o .c a
P re p a ra tio n o f t h i s m a n u s c rip t w as fu n d e d th ro u g h re s ea rc h g ra n ts fro m th e N a tu ra l S c ie n c e s a n d
E n g i n e e ri n g R e se a rc h C o u n ci l o f C a n ad a (B ry d en a n d R o y ), a n d fro m th e O n t a rio M e n ta l H e a lth
F o u n d at io n (R o y ).
²
P h i l B ry d e n d ie d s u d d en ly i n A u g u s t 1 9 9 6 , s h o rtl y a fte r th e jo in t m e e tin g s o f t h e In t e rn a t io n al
P ri m at o l o g ic al S o c ie t y a n d t h e A m e ric a n S o c ie ty o f P rim a t o l o g ists in M a d is o n , W is c o n s in , w h e re h e
m a d e a p re se n ta tio n o n w h ic h th is p a p e r is b a s e d .
Ó
1 9 9 7 P s y ch o lo g y P re s s L td
318
BRYDEN ET AL.
p r ef e re nc e an d p e rf o r m a n c e , a nd th e in f lu e nc e s o f po st u ra l a nd ta sk de m a n ds , an d
h a v e m uc h to o ff er e ac h o th e r in th e q u e st fo r th e n a tu r e o f la te r ality. O u r re c e nt
w o rk se e m s to in d ica te th at th e v a rio u s s en so r y a n d m o to r la te ra litie s m a y b e
r e la te d , in hu m a n s, b u t no t in a si m ple w ay . In fu tu r e w o rk , th e ch a llen g e w ill b e to
id en tify th e r ela tio n s b e tw e e n th e va rio u s la te r ality ``p r o f iles ’ ’ a nd p atte r ns of
f u nc tio na l c e re b r a l or ga n is atio n .
INTRODUCTION
T h is m an u sc rip t is b ase d o n th e B ryd e n a n d R o y p a pe r, w hic h P h il B ryd e n
pre se nte d a t th e jo in t m e e tin gs o f th e In te rn atio n a l P rim at olo gic al S o ciety a n d
th e A m eric an S o ciety of P rim ato lo gists in M a d iso n , W isc o ns in , in A u g u st o f
19 9 6 ve ry sh o rtly b efo re h is p re m a tu re d e ath . In it, w e re vie w so m e o f o u r re ce n t
co lla b o rativ e w ork inv o lvin g c u ltu ra l a nd g en e tic in flu en ce s o n h a nd e d n ess, a n d
disc u ss pe rform a nc e m ea su re s o f h an d ed n e ss as w e ll a s th e effe cts o f ta sk a n d
po stu ral de m a n d s o n h a n de d ne ss m e asu re m e n t. F in ally , w e disc uss th e rela tio n s
of late ra l pre fere nc e s to b ra in o rg an isatio n a n d su g ge st som e p o ssibly fe rtile
are a s fo r furth er inv e stig a tio n .
It is o b v io u s th a t so m e of o u r m an u a l a ctivitie s are he a vily in flu e n ce d b y th e
soc ie ty in w hic h w e liv e. F o r ex a m p le, so m e stud ies in w h ich ha n d ed n e ss h a s
be e n u sed a s a fa cto r ha v e d ete rm in ed ha n de d n ess b y th e re spo n se to tw o
qu e stio n s: ``W h ic h h an d d o y o u w rite w ith? ’ ’ , a n d ``In w h ic h ha n d d o yo u ho ld
a fo rk w he n e a tin g? ’ ’ . T he re sp o nse to th e first q u estio n is q u ite straig h tfo rw ard ,
w h e re as th a t to th e se c on d is in flu e nc ed b y th e re sp on d e nt’ s cu ltu ra l e xp e rien c e.
T ra ns fe rrin g th e fo rk to th e p refe rred h a nd is an A m eric an c u sto m , w he rea s th e
E u ro p ea n style is to k ee p th e fork in th e le ft h an d . T h is o b serv a tio n ref le cts th e
im po rta nc e o f co n side rin g c u ltu ral d iffe ren c e s in ha n d usa g e be fo re o n e ca n
offe r a g en e ral m e tho d o f asse ssin g h um a n h a nd e d n ess.
In rec en t ye a rs, w e ha v e h a d the op p o rtu n ity to ad m in ister h an d ed n ess
qu e stio n n aires in C a na d a, In d ia , Ja pa n , an d a m o ng th e T uc a no , a n a b orig ina l
pe o ple in th e A m a zo n w ate rsh ed o f C o lo m b ia (B ryd e n , A rd ila , & A rd ila, 19 9 3 ;
Id a & B ryd e n, 1 99 6 ; S in g h & B ry d en , 19 9 4 ; S te en h u is & B ry d en , 1 9 8 9b ).
C o m m o n lore h a s it th a t le fth a n d ed n e ss is disc o ura g ed in th e O rien t, an d so it
sho u ld n o t b e p a rtic ula rly su rp rising to n ote th at left-h an d ed n e ss is le ss co m m o n
in Ind ia a nd Ja p an th a n it is in C an a da (se e T ab le 1 ). O n e m ig h t a lso no te tha t
le ft-ha n d p refe re n ce is a ra rity a m o n g th e T u ca n o . A ltho u g h so m e T u ca n o c arry
ou t so m e ac tiv itie s w ith th e left h a nd , w e h av e y e t to fin d a ny o n e w ho m w e
w o u ld c lassify a s e v en m o d e rately L H . S im ila r da ta in o th e r a b orig ina l p e op les
ha v e b e en rep o rte d b y M a rch an t, M c G re w , a n d E ibl-E ib esf eld t (1 99 5 ). A se co n d
po in t is th at le ft-h a nd u sag e is m o re freq u e n t for rela tiv e ly un sk ille d a ctivities,
suc h a s d u stin g o r pic k in g u p sm a ll ob je cts, th a n it is fo r m o re sk illed a c tiv itie s
suc h as w ritin g o r th ro w ing .
T h e o b vio u s in terp re ta tio n of th ese da ta is th a t h a n d p refe ren c e is he a vily
in flu en c ed b y th e c u ltu re in w h ich on e liv e s, a n d ce rta in ly th e re a re stro n g
GENETICS AND HANDEDNESS
319
TABLE 1
Prevalence of Left-handedness for Selected Hand Preference Items (%)
C an ad a
Ite m
15 6 3
1
2
In d ia
Ja pa n
T uca no
2 10 4
65 5
65
W rit e
9 .8 1
5 .1 9
1 .3 7
T o o th b ru s h
8 .2 7
6 .0 4
7 .4 3
S tri k e m a tc h
0 .00
8 .7 7
6 .6 7
3 .9 2
1 .54
T h ro w d a rt /s p e a r
1 0 .5 5
7 .1 6
4 .5 8
0 .00
C o m b h a ir
1 4 .0 9
6 .7 9
5 .1 4
0 .00
P ic k u p o b je c t
9 .7 0
1 0 .7 9
3 .0 0
1 .54
C a rry s u i tc a s e
1 4 .2 6
8 .0 8
1 7 .17
7 .69
D u st
1 5 .8 1
9 .6 5
7 .2 9
M ean
1 1 .4 1
7 .5 5
6 .2 3
1 .81
9 .3 5
6 .2 7
4 .3 2
0 .55
1 3 .4 7
8 .8 3
8 .1 4
3 .07
M e a n s k i lle d
M e a n u n s k illed
1
2
N u m b e r in th e sa m p le .
P e rc e n t.
p ro p o n en ts of su ch a v ie w (e .g . H ar ris, 1 9 90 ; P ora c , R e es, & B u ller, 1 9 90 ). O n
the oth e r h a nd , if h an d ed n e ss is g e ne tica lly co n tro lled , it is e ntirely po ssib le tha t
the ge n e th at lea d s to left ha n d ed n ess is les s c o m m on in p a rtic u la r bio lo g ica l
su bg ro u ps. M o st ce rtain ly th is is true fo r the g e n es re sp o n sib le for su c h thin g s a s
A ±B ±O b loo d ty p e, as ha s b e en so eleg a n tly sh o w n by C a va lli-S fo rza , M e rroz z i,
an d P ia zz a (1 9 9 4) in th eir bo o k o n th e g e og ra p hy o f h u m a n g e ne s. L ike w ise , w e
(M c M an u s & B ry d e n, 1 99 3 ) h a v e arg u ed tha t th e lac k of leftha n d ed n ess a m o n g
the re side n ts o f th e S o u th A tla n tic islan d of T ristan d a C u n h a m ay ha v e a rise n
sim p ly b ec au se th e re le v an t g e ne is n ot p re sen t in th e p op u latio n, h a vin g
d isa p pe a red a s a re su lt o f a c om bin atio n of fo u n de r e ffe cts, ev o lu tio na ry
1
b ottle n e ck s, a n d ra nd o m drift .
ON THE GENETICS OF HANDEDNESS
A t pre sen t, th e re a re at le ast th re e rea so n ab le ge n etic m o d els o f h a nd e d ne ss:
T h o se o f A n n ett (1 9 85 a ); M cM a n us (1 98 5 c ; M cM a n us & B ry d en , 1 9 9 2); an d
Y eo a nd G an g e sta d (1 9 9 3) . T h e A n n e tt a nd M cM a n us m od e ls a re b o th sin g le 1
It s h o u ld b e n o t ed th a t t h e s am e m e c h a n i sm c an n o t p ro d u ce o c c a s io n a l p o p u la t io n s in w h ic h
e v e ry o n e i s le ft -h a n d e d , b e c a u s e t h e C C g e n o t y p e (s e e th e n e x t se c t io n f o r a n e x p la n a tio n o f th e
h a n d ed n es s m o d e l o n w h ic h w e b a s e th is a rg u m e n t) o n ly a llo w s flu c tu a tin g a sy m m e t ry (i. e . 5 0 %
le ft- h a n d e d n e ss a n d 5 0 % rig h t-h a n d e d n e s s). It is a ls o h ig h l y u n li k e ly t h a t th e C a l le le ra th er th a n th e
D a ll el e ev e r g o e s to fi x a ti o n in s m a ll i s o la t e d p o p u la tio n s , b e c a u s e a fa r lo w e r p ro p o r tio n o f th e
in itia l g en e p o o l c o n s is ts o f C al le le s in th e firs t p la c e .
320
BRYDEN ET AL.
g e ne m o de ls . E s se n tia lly , th e y pr op o se th a t o n e a lle le co d es fo r rig h tha n de d n ess (a n d le ft-h e m isp he re la n gu a ge d o m ina n c e) w h ile a se co n d allele
le a v es th e d ete rm ina tio n o f la te ralisa tio n to flu c tu a tin g a sym m etry. In c o ntrast,
Y e o a nd G a n ge stad h a ve p rop o se d a p o ly g e n ic m o d e l in w h ic h b oth leftha n de d n ess a n d stro ng rig h t-h an d ed n e ss a re se en a s de v ia tio n s fro m th e n o rm o f
m o d era te rig h t-h an d e d ne ss. A lth o ug h th e ge n etic s o f th e ir m o de l h a ve n o t b ee n
spe c ifie d in d e ta il, the y h av e sh ow n th at stro ng ly R H pe o ple , a s w e ll a s leftha n de rs, h a v e a n e lev a ted pro p o rtio n of p are n ts w h o are le ft-h a nd e rs.
W e ha v e c h o sen th e M c M an u s (19 8 5 c ) m o de l to fo c us o n h e re b e ca u se it is
re lativ ely ea sy to e x pla in. T h is m o d el in vo lv es tw o alle le s, D an d C . In div id ua ls
w ith th e D D g e n oty p e w ill b e R H a n d left-latera lise d fo r la n g u ag e . C C
in d iv id u a ls w ill b e la tera lised by th e c h an c e m ec h a nism s of flu ctu atin g
asy m m e try , an d th ere fore 5 0% w ill b e L H an d an in d ep e nd e n t 5 0% w ill b e
rig h t-la te ralise d fo r la ng u ag e . T h e m o d el is a n ad d itiv e o ne , an d the refo re in th e
he te ro z yg o te s o n e ca n im a gin e th e re sult as th e D allele be in g ex p resse d in 5 0 %
of the ge n o ty p e a nd the C allele b e in g e xp resse d in th e o th er 5 0% . T hu s, 2 5 % o f
th o se w ith th e D C g e no ty p e w ill b e L H . L est th e re is c o nfu sio n, it sho u ld b e
em p ha sised th a t the M c M a nu s (19 8 5 c) m o d el use s flu ctu a tin g a sy m m etry in a
sub tly d iffere nt w ay from th a t o f Y eo a nd G an ge stad (19 9 3 ), a lth ou g h bo th are
re fe rrin g to th e sam e p he n o m e no n . In b o th ca se s, sm a ll am ou n ts o f ra n do m
no ise m e a n th a t th e tw o sid es of a n o th e rw ise sy m m e tric org an ism are n o t th e
sam e , a n d d iffe re n ce s be tw ee n th e tw o sid es a re distrib u te d w ith a m e an o f ze ro
an d a n o n -ze ro stan d ard d e v iatio n . T h e M c M an u s (1 9 85 c ) m od e l tak e s this ca se
an d a d ds c an a lisatio n to it, w ith th e resu lt th a t h alf o f th e p op u lation be c om es
``d o m in a nt’ ’ o n on e sid e a n d th e o th e r ha lf ``do m in a nt’ ’ o n the o th e r side , w ith
th e sp e cific resu lt fo r th e C C g en o typ e th at 50 % b ec o m e ``R H ’ ’ an d 5 0 %
``L H ’ ’ .
In tere sted rea d ers m ay also ca re to co m pa re th e M cM a n us m o de l w ith th a t o f
A n n e tt (A nn e tt, 19 8 5 a,1 99 5 ), w hic h in its a pp lic a tio n to fam ily an d tw in da ta is
bro a d ly sim ila r to th at o f M c M an u s, a lth o ug h the re are d iffere n ce s in d eta ils,
pa rtic u larly w ith re g ard to ph e no ty p ics (A n n ett, 1 98 5 b ; M cM a n us, 1 9 8 5a ,b ).
D e taile d statistic a l c o m p ariso n of th e tw o m o de ls fin d s ev ide n ce in fa v ou r o f
on e (A n n e tt, 1 9 9 6) o r th e oth e r (M c M an u s, 1 9 85 c ).
In rec en t ye a rs, w e h a v e c arrie d ou t fa m ily stu die s in b o th In dia a n d C an a da .
In th e p a st, m o st suc h fam ily stu d ies h a ve relie d o n th e m e m b e rs o f a p a rtic u la r
ta rge t p o p ula tio n rep o rtin g o n th eir h a nd e d ne ss a n d th a t o f th e ir rela tiv es. O u r
fa m ily stu d ie s d iffere d fro m p rev io us stud ies in th ree im po rtan t w a y s. F irst, w e
em p lo y ed a m u lti-ite m q u e stio nn a ire, ra th e r tha n ba sing o u r sta te m e n ts so le ly o n
w r it in g h a n d ; se c o n d , e a c h p a r tic ip a n t in th e st u d y re s p o n d e d to t h e
qu e stio n n aire ra th e r th an sin g le targ et su b je cts in d ica ting th e ha n de d n ess o f
th e ir rela tiv e s; th ird, w e a d m in iste re d a co m m o n se rie s o f 1 0 q u e stio n s to
GENETICS AND HANDEDNESS
321
2
p artic ipa n ts in C an a da a nd In d ia . (S ee T a ble 1 fo r a su m m a ry o f th e pre v a le n ce
o f le fth a nd e d ne ss fo r se lec te d h an d pre fe re n ce ite m s.)
R e sp on d e n ts a sse ssed th e ir h a n d p re fe ren c e s o n a 5 -p o in t sca le, ran g in g from
``alw ay s u se le ft h a nd ’ ’ (±2), to ``alw ay s u se rig h t h a nd ’ ’ (+ 2 ), a n d re spo n se s
w ere sum m ed to o b ta in a h an d p re fe ren c e sc ore . B a sed o n th e to tal sco re , e ac h
su bje ct w a s c la ssifie d a s b ein g e ith er L H o r R H , a n d as stro n g ly o r w e ak ly
h an d ed , b a sed o n th e ab so lute va lue o f th e sc o re’ s d ev ia tio n fro m z e ro .
W e ob tain e d d a ta from 2 0 8 n u cle ar fa m ilie s w ith 4 5 7 c h ild ren in C a n ad a an d
4 75 fa m ilie s w ith 1 07 1 ch ildre n in In dia . T he inc rea se in left-h an d e dn e ss
resu ltin g fro m ha v in g a t lea st o n e L H pa ren t w a s d ete rm in ed b y c a lc u la tin g a n
o dd s ra tio .
F or tw o R H p are n ts, th e od d s o f ha v in g a n L H ch ild a re o b ta in ed b y sim p ly
d iv idin g th e to tal nu m b er o f L H c h ild ren p rod u c ed from su c h m a tin gs b y th e
n um b er o f R H c hildr en re su ltin g (e .g . if th e re a re ten rig h t-h an d ed a nd o n e lefth an d ed offsp ring th e o dd s are 1 :10 [o r 1 0 :1 ag a in st] o r .1). F o r tw o pa re nts a t lea st
o ne of w h o m is L H (this w o u ld inc lu de ca ses in w hic h o n e p are nt is L H an d on e is
R H as w e ll as ca se s in w h ich bo th p a re n ts a re L H ), the od d s of h av ing a lefth an d ed ch ild are c alc ula ted in a sim ilar m a nn e r (sa y 1 :5 o r 5:1 a g ain st, o r .2). If
o ne th en fo rm s th e ra tio o f the se o dd s (th e ``a t-le ast-o ne -L H -p a re n t’ ’ ca se div ide d
b y th e ``tw o-R H -p are nts’ ’ c ase ) on e o b ta ins th e o d d s ra tio . T h is, th en , is a
m e asu re of the inc rea sed like lih o o d o f L H ch ild re n re su lting from m a tin g s
inv o lv in g at lea st o ne L H p a ren t. (In this c ase , .2 div ide d b y .1 w hic h giv e s a v a lu e
o f 2 , o r 2 :1 , so th a t L H p are n ts a re tw ice as lik e ly to ha v e a n L H c h ild a s are R H
p are nts.) T his ratio is eq u al to 1 u n d er th e n ull h y p o th e sis th at th e tw o ty p e s o f
m a tin g s re su lt in eq u al nu m b ers o f L H c h ild re n. O n e o f th e nic e thin g s a bo u t o d d s
ratios is tha t, u nd e r th e nu ll h y p oth e sis, th e sam pling d istribu tio n o f the na tu ral
log a rith m o f th e o dd s ratio is no rm al, h as a m ea n o f ze ro , an d a k no w n a n d e a sily
o bta in e d va rian c e (p ro v id e d th e ce ll fre q ue n c ie s a re su fficien tly la rge ), so on e ca n
u se a sim p le z-sco re fo r sig n ific a nc e te stin g (M a ra scu ilo & S e rlin , 1 9 88 ).
T a b le 2 pre sen ts th e pa ren t±c hild o d d s ra tio s w h e n resp o n d en ts w ere
cla ssifie d as L H an d R H . A s ca n b e see n fro m T a b le 2 , in C a n ad a , L H p a ren ts
p ro d u c e L H o ffsp rin g a b ou t tw ic e as o ften a s d o R H p a ren ts. In In dia , L H
p are nts p rod u c e L H o ffsp rin g ab o u t th ree tim es m ore o fte n th an d o R H p are n ts.
W e a lso c la ssifie d o ur su b jec ts in tw o o th e r w ay s. First, th ey w e re div ide d
into stro n gly ha n d ed an d w ea k ly h a nd e d c ate go ries, disre ga rd in g d ire ctio n o f
p re fere n c e. S tro ng ly h a n de d p eo p le w ere th o se w h o cla im ed to a lw ay s u se th eir
p re ferre d ha n d for e ac h o f th e ac tiv itie s w e q u estio n ed , w h e rea s w ea k ly h a n de d
2
T h e te n it e m s w ere : W h ic h h a n d w o u ld y o u u s e to (1 ) w rit e (2 ) h o ld a to o th b ru s h (3 ) s trik e a
m a tc h (4 ) t h ro w a s p ea r (5 ) c o m b y o u r h a ir (6 ) p ic k u p a s m a ll o b je c t (7 ) c a rry a h e av y s u itc a s e (8 )
d u s t (9 ) fli p a c o i n (1 0 ) th ro w a b a l l?
322
BRYDEN ET AL.
TABLE 2
1
Odds-ratios for Parent± Child Relations
C an ad a
In d i a
D ire c tio n (L e ft/ R ig h t)
2 .0 9 *
3 .0 7 *
D eg re e (S tro n g / W e ak )
1 .7 5 *
5 .1 3 *
E x tre m e /M o d e ra te
1 .2 3
2 .7 8 *
* P < .0 1
1
F o r t h e d ire c ti o n m ea s u re , th is is th e ra tio o f th e o d d s o f
tw o p a re n ts , a t l e as t o n e o f w h o m is le ft -h a n d e d , p ro d u c in g
a le ft-h a n d e d ch il d t o t h e o d d s o f tw o rig h t -h a n d e d p a re n ts
p ro d u c in g s u c h a c h i ld . F o r th e d eg re e m e a s u re , it i s th e
ra tio o f th e o d d s o f tw o p a re n ts , a t le a st o n e o f w h o m is
st ro n g ly h a n d e d p ro d u c in g a s tro n g l y h a n d ed c h ild , to th e
o d d s o f tw o w e a k ly h a n d e d p ar en ts p r o d u c i n g a s tro n g ly
ha nd ed
c h i ld . F o r t h e
e x tr e m e / m o d e r a t e c la ss if ic a t io n
(b a s e d o n th e Y e o ±G a n g es ta d m o d e l , s e e t ex t), i t i s th e
ra tio o f t h e o d d s o f tw o p a re n ts , a t le a s t o n e o f w h o s e s c o re s
fa l ls i n
e i th e r o f th e
e x tr e m e
q u a rti l e s o f t h e h a n d
p re fe re n c e d is tri b u ti o n , p ro d u c in g a c h ild w it h a s im i la r
sc o re , to th e o d d s o f t w o p a re n ts w h o s e s c o r es fa ll in th e
in te rq u a rti le ra n g e p ro d u c in g a c h ild s c o rin g in o n e o f th e
e x tr em e q u a rti le s .
pe o ple w e re th o se w h o o fte n sa id th a t th e y u sed bo th h an d s ab o ut e q u ally o r tha t
th e y u sua lly use d th e p re ferre d h an d . Inte resting ly , th e re a re sig n ific a nt p a ren t±
ch ild a sso cia tio n s fo r d eg re e o f h an d pre fere n ce as w e ll a s fo r d irec tio n . In In d ia,
stro n g ly h an d ed p are n ts a re a bo u t fiv e tim e s m o re lik e ly to p ro du c e stron g ly
ha n de d c hild ren tha n are w ea k ly h a nd e d p are nts. In C an a d a, th e relatio n h o ld s
on ly fo r fa th e rs, a n d it is rath er w ea k er th a n it is in Ind ia (see T a ble 2 ). T h is
re su lt is rem in isc en t o f C o llin s’ m ou se stu die s, in w h ich , alth ou g h u n suc c essfu l
in h is attem p ts to b ree d fo r d ire c tio n o f p a w p re fe ren ce , h e e sta b lish e d tw o lin es
of m ice tha t d iffe red in the ir de g ree s o f p aw p refe re nc e Ð th e C o llin s H I an d L O
stra ins (se e C ollins, 1 9 85 , fo r a re v iew ). A rec en t larg e su rve y o f g en e tic ally
diffe re n t inb re d m o u se strain s b y B id d le a nd h is c o lle a gu e s (B id d le & E a les,
19 9 6 ) h as re ve ale d th a t stra in s fa ll in to thre e classe s b a sed o n d e g ree of p a w
pre fe re n ce . T h e se m o use stu d ie s m ay w e ll p ro v ide th e m o d el sy stem th ro u g h
w h ich th e b io lo g ic a l un d erp inn in g s of h u m a n la te ra l a sym m etrie s a re u n co v ere d
(C a rlie r, 1 99 5 ).
T h e e x tre m e/m o d era te (T a b le 2 ) cla ssifica tio n w as d o n e in a n attem pt to
co m pa re th e M c M an u s (1 98 5 c) a n d Y e o ±G a n g esta d m o d els. Y e o an d G an g esta d ’ s m o d el pre d ic ts tha t tho se pa ren ts w h o d iffe r fro m th e p op u lation no rm o f
m o d era te rig h th an d ed n e ss w ill b e m o re like ly to p ro du c e sim ila rly de v ian t
ch ild re n th a n th o se w h o are c lo se r to the p o pu latio n no rm . W e th e refo re d iv id e d
th e d istrib u tion o f h an d p refe ren c e sc o res in to q u a rtiles, an d c la ssifie d th o se
GENETICS AND HANDEDNESS
323
p eo p le in the m idd le tw o q u artile s a s ``m od e rate ’ ’ a nd tho se in th e o u te r tw o
q u a rtile s a s ``e x tre m e ’ ’ , a n d r e p e a te d o u r o d d s -ra tio (O R ) c a lc u la tio n s.
A lth o u gh th e e xtre m e /m o d era te c la ssific atio n le ad s to sig n ific an t e ffe c ts in
3
the Ind ian sa m p le , the v alu es fo r th e C a n ad ia n sa m p le fa il to re a ch sign ifica n ce .
In oth e r w o rd s, in o u r da ta th e Y eo ±G a ng e sta d m od e l is no t su pp o rted b e ca u se,
at le ast fo r th e C an a d ia n sa m p le , a c o u ple a t le ast o n e of w h o m de v iate s from
m o d era te rig h t-h a n de d n ess is n o m o re lik ely to p ro du c e a n ``e xtrem e ly ’ ’ h a n de d
ch ild th an is a c o u ple b o th o f w h o m a re ``m o d e ra tely ’ ’ h a nd e d . T h e se d ata
w ou ld su g g es t th at th e M c M a nu s m o d el is a b etter fit to th e d ata th a n is th e Y e o ±
G an g esta d m o de l (see T a b le 2 ).
In th e M cM an u s m o de l, a s the freq u e n cy o f the C allele d ec rea se s, th e O R fo r
the p a re n t±ch ild asso cia tio n in cre as es. T a b le 3 sho w s th e p rop o rtio n s o f L H
o ffsp rin g, in re latio n to th e ha n de d n ess of th e p are nts, in th ree situa tion s. C a se A
sh ow s th e m o de l d esc rib e d b y M c M an u s (1 98 5 c) in w h ic h the C allele ha s a
freq u en c y of 0 .15 5 , D D , D C , a nd C C g en o typ e s p rod u c e 0% , 25 % , a n d 50 %
left-h a n de rs, re sp ec tiv ely , a n d the ref ore th e fre q ue n cy o f le ft-h an d e dn e ss in th e
p op u latio n is 7 .7 5 % . C a se B sho w s th e situa tio n in w hic h th e freq u en c y o f th e C
allele ha s fallen to .0 7 7 5, b ut o th e rw ise th e m o de l sta y s th e sa m e; no w 3 .8 8 % o f
the p o pu la tio n a re L H . C a se C ke e ps th e sa m e C a lle le fre qu e n cy as in C ase A ,
b ut in ste a d th e e xp re ssion o f lef t-h an d e d ne ss is les s, b ein g 0 % , 1 2 .5 % , a nd 25 %
in g en o typ e s D D , D C , a nd C C . In e ffe c t the re is so cial p re ssu re a ga in st b ein g
L H . T h e resu lt is tha t ag a in 3.88 % o f the p o p ula tio n are L H , b u t no te th a t th e
p ro p o rtio n s o f left-ha n de rs in the d iffe ren t ty p es o f fa m ily a re d iffe ren t from
C ase B . T hu s, a lth o ug h m a nip u lation s for ca se s B an d C (se e T ab le 3 ) b o th h av e
the sa m e effe ct o n th e o ve ra ll p rev a le n ce o f left-h an d ed n e ss, th ey h av e d iffe ren t
effe cts o n th e p atte rn w ith in fa m ilie s, m ea n in g th a t su ch c ros s-cu ltu ra l d ata ca n
p ro v ide le v era g e o n th e qu e stio n o f w he the r th e in cid en c e o f left-h an d e dn e ss
d iffe rs a m o n g so cietie s b ec a use of differe nc e s in g e ne fre qu e n cy or be c au se o f
d iffe re n ce s in soc ial p ressu re s. T he inc rea se d O R in th e Ind ian p o p ula tio n s
o bse rv ed in co m pa rin g T a b le s 2 an d 3 p ro v ide s stro n g e vid e n ce fo r a differe nc e
in g en e fre q u en c y ra th e r th a n a sim ple differe nc e in so cia l p res sure , w h ic h
w ou ld resu lt in alm o st n o c h an g e in th e O R . T o sum m a rise, the d iffere nc e s
b etw ee n C a na d a an d In d ia are m ost p a rsim o nio u sly ex p lain ed b y differe nc e s in
the fre q u en c y of the C a lle le.
MEASUREMENT OF HAND PREFERENCE
In th e fam ily stu d ies, w e h av e m ad e u se of p ap e r-a n d -p e n cil p re fe re n ce
inv e nto ries, p a rtia lly be ca u se it is e asy to u se su c h m a teria ls to te st la rg e
n um b ers o f su b jec ts. B e c au se d iffere n t p eo p le m ay inte rp re t th e instru ctio ns o n
3
T h e st a tisti ca l s ig n i fic an c e o f th e O R s c a n b e d e te rm in e d b y e v a lu a tin g th e n a tu r al lo g a rith m o f
th e O R fo r s ig n ific a n c e (s e e B ry d e n , M c M a n u s , & B u lm a n -F le m in g , 1 9 9 4 , fo r an e x am p le ).
324
BRYDEN ET AL.
TABLE 3
Left-handedness in Relation to Handedness of Parents
C a se A
p (C )
C ase B
.1 5 5
p (L |D D )
0
C a se C
.0 7 7 5
0
.1 5 5
0
p (L |D C )
.2 5
.2 5
.1 2 5
p (L |C C )
.5
.5
.2 5
p (L )
.0 7 7 5
.0 3 8 8
.0 3 8 8
R)
.0 5 9 8
.0 2 9 5
.0 3 4 5
L)
.1 7 4 3
.1 4 9 4
.0 8 9 4
L)
.2 8 8 8
.2 6 9 4
p (L |R ´
p (L |R ´
p (L |L ´
O d d s ra ti o
1
(R
´
L v s. R ´
R)
L o g (o d d s ra t io )
.1 4 4 4
3 .3 2
5 .7 9
2 .7 5
1 .2 0 0
1 .7 5 6
1 .0 1 1
A b b r ev ia t io n s: p (C ), t h e fre q u e n c y o f th e C a ll e le in th e M cM a n u s ( 1 9 8 5 c ) m o d e l
o f h a n d e d n e ss ; p (L |D D ), p (L |D C ), p (L |C C ), th e p ro b ab il itie s o f b e in g l e ft-h an d e d ,
g iv e n D D , D C , a n d C C g e n o ty p es , re s p ec t iv ely ; p (L ), th e fre q u e n cy o f l e fth a n d e d n e s s i n th e p o p u l at io n ; p (L |R ´
R ), p (L |R ´
L ), p (L |L ´
L ), th e c o n d it io n a l
p ro b a b ilitie s o f a c h ild b ei n g le ft-h a n d e d g iv e n th e h a n d e d n es s o f t h e p a re n t s.
1
T h e rat io o f th e o d d s o f o n e l eft -h a n d ed a n d o n e rig h t -h a n d e d p a re n t p ro d u c in g a
l e ft-h an d e d c h ild , t o th e o d d s o f t w o ri g h t-h a n d ed p a re n t s p ro d u c in g a le ft-h a n d e d
c h ild .
suc h q u e stio nn a ires differe ntly , w e m igh t b e fa ulted fo r n o t e m p loy ing m o re
pe rfo rm a nc e -ba se d ta sks. O ne p rob lem is th a t a p re fere n ce in ve n to ry a llo w s o n e
to a sk a bo u t a va riety o f ac tiv itie s, w h e rea s p erfo rm a n ce m ea su res asse ss a
sin g le task . In g e n era l, pe rform an c e o n p a rtic ula r m an u a l task s co rrela tes w ell
w ith re sp o n ses to th e relev a n t q u estio n na ire ite m s (S te en h uis, 1 99 6 ). F o r
ex a m p le, p e op le w h o c la im th a t th ey ``a lw a y s’ ’ thro w w ith th eir rig ht ha n d
sho w a la rge r h a n d diffe re n ce in th ro w in g a cc u ra c y tha n th o se w h o c laim tha t
th e y ``u su ally’ ’ thro w w ith th e rig h t h an d . F u rthe rm o re , th e re is no q ue stio n tha t
p ra c t ic e o n a p a rti cu la r ta sk w ith t h e n o n p re f e rre d h a n d ca n im p ro v e
pe rfo rm a nc e w ith th at ha n d. T h u s, it w o u ld be d esirab le to d e v elo p a ge n era l
ob je ctive m e asu re o f h a nd p refe ren c e.
A nu m b er o f p erfo rm a n ce te sts of h a n de d n ess ha v e be e n de v elo p ed a n d
po p u larised o v er the y ea rs, su c h a s the T a p le y an d B ryd e n (1 9 8 5) d o t-fillin g
ta sk , D o ro th y B isho p ’ s (1 99 0 ) sq u are -tra cin g task , an d M aria n A n n e tt’ s (1 9 85 a )
pe g -m o v ing task . A lth o u g h th ese m e asu re s yie ld re lia ble d iffere nc e s be tw e e n
th e ha n ds, th e y e a ch m e asu re a sp ec ific a c tiv ity , a nd th at ac tiv ity m a y no t b e th e
m o st im p ortan t o ne to m ea su re. V a n H orn , in L o n d on , h a s ca rrie d o ut a fa cto ran a lytic stu d y o f an n u m b er o f v arian ts o f th e se p e rfo rm a n c e m ea su res, a n d
fo u n d th at ea c h test lo a d ed on a d istin ct fa c to r (V a n H o rn , 1 99 2 ). B a rn sle y a n d
R a b in o v itc h ( 19 7 0 ) h a v e al so sh o w n th a t h a n d diffe re n c e s on d iffe re n t
pe rfo rm a nc e tes ts do n o t c orre late v ery w e ll. O ne is th u s fa ce d w ith th e
qu e stio n o f w h ich p erfo rm a nc e te st to c ho o se .
GENETICS AND HANDEDNESS
325
It see m s p la u sible th at th e d iffe ren c e b etw ee n p re ferred an d n o n pre fe rre d
h an d s sho u ld b e larg e r fo r co m ple x tas ks tha n fo r sim p le on e s. T h ere is little
d iffe re n ce b e tw e e n the h an d s in o u r ab ility to p et a c at o r w a v e g o o d by e
alth ou g h m o st of u s w ill u se the p refe rre d h an d for su c h a ctiv ities m or e o fte n
tha n no t. In co n trast, th er e is a fa irly la rg e d iffe ren c e b etw ee n th e p re ferre d an d
n on p re fe rred h an d s in the a bility to th row a b a ll a cc u rately o r to w rite le gib ly.
T h e se d iffe ren c es are re fle c te d in the re sp o nse s to p re fe re nc e in ve n to ries. B y
an d larg e , p eo p le te nd to en d orse the e xtre m e ca teg o ries o n q ue stio n s inv o lvin g
so m e skill, suc h as w ritin g , th ro w in g , a nd u sing a to o th b ru sh, w h e rea s the y
en d o rse m o re m od e rate ca teg o ries o n q ue stio n s in vo lv in g less sk ill, su ch a s
p ic k in g u p sm a ll ob je cts or p e tting a n im als (S tee n hu is & B ryd e n , 1 9 89 a ).
T hus, the p referred han d adv antage is larger for h and w riting than for g rip
strength (P rovins & M aglia ro, 1 989 ), for a im in g as o ppo se d to ta pping (Flow ers,
19 75), an d for in se rting peg s as opp osed to rem ov in g th em (R oy, B ry den ,
B ulm an-Flem ing & R oy , 19 96). W e h ave plo tted the ratio of th e tim e taken by th e
no npreferred han d (N P) to th at take n b y the p referre d ha nd (P ) for a va riety of
different task s exa m ined in our labo ratory on the sam e su bje cts. In on e task, th e
G roov ed Pe gbo ard , su bjects w ere asked to insert the 2 5 peg s an d re m o ve them . In
ano th er task , th e A nn ett peg boa rd, v ariou s task dim e nsion s w ere m a nip ula ted .
O ne m anipulation invo lve d p erform ing the stand ard ta sk eith er se ated or stand ing .
A no th er m an ipu latio n inv olved v aryin g th e size o f the pe gs a s a m ean s of va ry ing
th e spa tial co m p lex ity (F itts, 1 954 ) of the p eg-pla cin g m ovem en ts (B ryde n, R oy ,
B ryden , A llard, W h itela w , B ulm an-Flem ing , & R oy, 19 97). T he third task w as th e
T apley±B ry den (1 985 ) d ot-aim ing ta sk . B eca use th e m easure in all o f these tasks
is the tim e taken to p erform the task w ith eac h han d, th e value of th is ratio w ill
no rm ally b e g re ate r th an 1.00. A n analysis of th is N P/P ra tio (see F ig. 1 ) revea ls
th at it is v ery sim ilar across the v arian ts of the se pla cin g ta sk s, but tha t it d oes
ra nge from very sm all for rem ov ing peg s to v ery large for the T ap ley ±B ryde n do ts
ta sk . T h is ana lysis do es raise the in teresting po ssibility of using the slope of th e
N P /P func tion over a rang e of task s as a m easure of d egree of ha nde dness.
W e ha v e also ca rrie d o ut a m o re d e tailed k ine m a tic a na ly sis o f th e in flu e n ce
o f ta sk d e m a nd s o n m a nu a l a sy m m etries (R oy , in pre ss). In th is ta sk sub jec ts ha d
to re a ch a n d p ic k u p a sm a ll m e ta l d o w e l. In o n e co n d itio n th e d o w el h a d to b e
p la ce d into a sm all re c ep ta cle a b ou t the sam e siz e a s th e d ow e l o r in to a la rg e
rec ep ta cle , w ith the m ov e m e n t p re cision re qu ire m e nts b ein g gre a te r in th e sm all
p la ce c on d itio n . T h e m ov e m e n t w a s e xa m in e d in tw o p h ase s, a rea c h (p ic k u p
the do w e l) a n d a pla ce ph a se. T h e e ffec t o f pre c isio n de m a n ds w a s see n on ly in
the se co n d p h ase o f th e ta sk (m o v e a nd p la ce th e o b jec t) as op p o se d to th e first
p ha se (re ac h a n d p ick u p th e ob je ct). T h a t is, the re w e re n o d iffe ren c e s in
m o v em e nt tim e, pe a k sp ee d , o r tim e in d ec e lera tio n b e tw e en th e sm a ll a n d la rg e
p la ce ta sk s in pic kin g u p th e d o w e l. In the sec o n d p h ase , h ow e ve r, w h en p la cin g
the d o w el in to th e sm a ll re c ep tac le sub je cts m o ve d m o re slo w ly a nd spe n t m ore
tim e in d e ce lera tio n .
326
FIG. 1.
P e rfo rm a n c e o f t h e n o n -p re fe rre d re l a tiv e to th e p re fe rre d h an d o n v a ri o u s m o to r ta s k s.
GENETICS AND HANDEDNESS
327
W e m ig h t in te rpre t d iffe ren c es b etw ee n th ese tw o c o n ditio n s as re fle c tin g
d iffe re n ce s in th e sk ill req u ire d to p erfo rm th e m o v e m e n t in th e tw o p h a ses o f
the ta sk. T ha t is, if a m o ve m en t re qu ires so m e d e gre e o f sk ill, it sh o uld b e
sen sitiv e to th e d iffe ren c es in th e spa tia l p rec isio n d em an d s o f the tw o
co n d itio n s. U sin g th is in te rp re tatio n, the ab se n ce o f a d iffe ren c e be tw ee n th e
co n d itio n s in th e first p ha se re fle cts th e rela tiv e ly lo w sk ill re q u ire d to p ic k u p
the d o w el. T h e larg e d iffe ren c e in th e sec o n d ph a se im p lie s th e hig h er le ve l o f
sk ill de m a n de d w h en m o v ing a n d p lac in g th e d o w e l in the rec e p ta cle .
A na lys es of the p e rform an c e w ith ea ch h an d re ve a le d a rig h t- or p refe rre d h an d a d v an tag e o n ly in th e se co n d p h ase . T h e rig h t h a nd m o v ed w ith gre a te r
sp ee d a n d sp e nt less tim e in d e ce lera tio n . T h e pre se nc e o f a p refe rre d -h an d
ad v a nta ge o n ly in the se co n d p ha se ap p e ars re la ted to th e d iffe re n ce s in sk ill
d em an d ed in th e tw o p h ase s, w ith this a d va n tag e a pp e arin g on ly in th e m o re
sk ilfu l se co n d ph a se . T h is in flue n c e o f th e spa tia l pre cision d e m a n ds o r sk ill
lev el on m a nu a l a sym m etrie s in p e rfo rm a n c e is co n siste n t w ith S tee n h uis an d
B ry d en ’ s (1 9 89 a ) o bse rv ation s o n ha n d pre fere n ce . T h ey fo un d th at sk ill w as a
fac tor in h an d p re fere n ce in tha t h a nd p refe re n c es w ere stro n ge r for sk illed tha n
for u n skilled a ctivitie s. T h is link b etw ee n p refe re n ce a nd p erfo rm a n ce a s it
rela te s to tas k d e m a nd s is ex a m in ed m o re clo sely in o u r w o rk o n p erfo rm a nc e b ase d m e asu re s o f p refe ren ce .
O ne o f th e first m e asu re s d ev e lo p ed to attem p t to in te gra te m ea sur es o f
p re fere n c e a n d pe rfo rm a nc e w as th a t o f B ry d en , S in g h , Ste en h u is, a nd C lark so n
(19 9 4 ). T h ey h a d sub jec ts se ate d at o n e en d o f a lo n g p eg b o ard , a sk ed th em to
m o v e pe g s, le ap fro g-sty le , fro m th e ne a r e n d to th e fa r en d , sta rtin g w ith th e
h an d c on tralate ral to th e pe g b oa rd an d sh ifting to the o the r h an d w he n e ve r the y
ele cte d to do so , a n d m ea su red the po in t a t w h ich this sh ift o cc u rred . R ig h th an d ers m ain tain ed th e u se o f th e rig h t h a n d w h e n m o v in g leftw ard o v er a
lon g er ra n g e th a n th e y u sed th e le ft h an d w h en m o vin g righ tw ard . L e ft-h a nd e rs
p erfo rm e d in th e op p o site m a n ne r.
T h e p rob lem w ith th is tas k is th a t it is q u ite tran sp a ren t to th e su b jec t. B ry de n
an d E d m on d s (1 99 5 ) d ev e lo p ed a so m e w h at d iffe ren t task . S u b je c ts w e re se a te d
b efo re a co m pu te r d isp la y te rm in al w ith c on tro l k n ob s b e lo w a nd o n e a ch sid e o f
the scre en . W he n a lig h t c am e o n a b ov e o ne of the co n tro l kn o b s, su bje cts w ere
to turn tha t c o ntro l to ad ju st a c u rsor o n th e sc ree n to a targ e t. A ltho u g h th is
stud y w as o sten sib ly a stud y co n ce rn in g th e re la tio n b etw ee n k no b loc a tio n an d
turn ing b eh av io u r, w e ob se rv ed w h ich h an d w a s u se d to tur n th e co n tro l k n o b.
L e ft-h a n de rs w ere m o re lik ely to turn the low e r kn o b (m id dle ) w ith th eir left
h an d s, w h e rea s rig h t-h a n de rs u sed th eir rig ht h an d s. F u rth erm ore , stro ng ly
h an d ed pe o ple , as d e fin e d b y a pre fere n ce in ve n tory , w ere fa r m o re lik ely to
rea ch a c ro ss th e m id lin e w ith th e ir pre ferre d ha n ds th a n w ere w ea k ly h a n de d
p eo p le . T h u s, this ta sk h as th e p o te n tia l fo r p ro v idin g a n o b je c tiv e m e a sure o f
h an d p refe ren ce , a s op p o se d to h an d sk ill. A sim ilar tas k, in v o lvin g p ic k in g u p
p la y in g ca rd s, h as b ee n d e v elo p ed b y B isho p , R os s, D an iels, a n d B rig h t (1 9 9 6).
328
BRYDEN ET AL.
R e ce n tly , w e h av e e xa m in e d the w a y in w h ich ta sk d em a nd s in flue n c e th e
ha n d th at is u se d to c a rry ou t a task th a t is ve ry sim ila r to th e B ry d en a n d
E d m on d s (19 9 5 ) ta sk (B ry d en , R o y , B ryd e n , B o uc h er, P ry d e , & F ra nk , in p ress).
S ev en d o w els w e re p lac ed at 3 0 ° inte rv als fro m 9 0 ° to th e left o f th e su b jec t to
90 ° to th e rig h t. S ub je cts w ere th e n ask e d to ca rry o u t so m e a ctio n o n the se
do w e ls. T h e va riou s ac tio n s in vo lv ed sim ply to u ch ing th e d o w el, k no c kin g it
ov e r w ith a sw e e pin g m o tio n , p ic k in g it u p an d to ssin g it o n the ta ble , a n d
pic k in g it u p an d p la c in g it in a sm all re c e pta cle. S u b je cts c ro sse d th e m id lin e
w ith th eir p re fe rred h an d s fa r m o re o ften th a n w ith th eir n o n pre fe rre d h a nd s.
F u rth erm or e, as ta sk co m p lex ity in c re a se d fro m to u ch in g to p la c ing , th e
pre fe rre d h an d en cro a ch e d fu rth er in to c o ntrala tera l sp ac e. A lth o u g h the se
fin d ing s sh o w the sam e ge n era l p attern a s re po rted b y B ryd e n an d E d m o n d s
(1 9 9 5) a nd by B ish op et a l. (1 9 96 ), the y a lso in d ica te th a t the re gio n o f sp ac e in
w h ich the p refe rre d ha n d is u sed is la rg er fo r c o m p lex ac tio n s th an fo r sim ple
on e s. T h us, p erfo rm an c e a n d pre fere nc e are in e xtrica b ly in te rw ov e n .
THE RELATION OF LATERAL PREFERENCES TO
BRAIN ORGANISATION
W h y sho u ld w e be so in te reste d in th e m e a sure m e n t o f ha n d ed n ess? W e h av e
lo n g k n o w n tha t th ere is a w ea k b u t co n sisten t re la tio n be tw e e n h an d e d ne ss a n d
ce re bra l spe e c h la tera lisa tio n (se e R o y , 19 9 6 , fo r a d iscu ssio n o f p ra x is
la tera lisa tio n a n d h a nd e d ne ss). In th e stu d ies th at sup p o rt th is co n clu sion ,
ha n de d n ess h as c on v e n tio na lly b ee n d e te rm in ed by w ritin g ha n d . R e c en tly , w e
ha v e also b ee n inv e stig atin g th e late ralisation o f th ro w ing , k ic k in g , an d sigh ting ,
an d h av e se e n th at the y are in terre late d in ra the r co m ple x w a y s.
F irst o f all, it is no tab le th at w ritin g h an d an d th ro w in g h a n d a re o ften no t th e
sam e . C u rio u sly , a rela tiv e ly hig h pro p ortion o f L H w riters th ro w w ith the ir
rig h t h a n ds, w h e rea s v ery few R H w rite rs th ro w L H (G ilbe rt & W y so c ki, 1 9 92 ).
T h is is a v e ry d iffe ren t p atter n fro m th e o n e w e w o u ld e x p ec t w er e th e
disso c ia tio n du e to c ultura l p re ssure to w rite w ith th e righ t ha n d, in w h ic h o n e
w o u ld ex p ec t to fin d a la rg e n u m b er o f ``n a tu ra l’ ’ left-h an d ers w h o ha d le arn e d
to w rite w ith th e rig h t ha n d b u t re tain ed th e ir le ft-h an d p re fere n ce for thro w ing .
F u rth erm or e, a c lu ster an a lysis o f ou r h a nd e d n ess d ata ind ica ted th a t leftha n de rs split in to tw o distin c t g ro u p s, w ith ab o u t 7 0 % th ro w in g w ith th e le ft
ha n d an d 30 % th ro w in g w ith th e rig h t h a nd (G rim sh a w & B ry d en , 1 9 9 5 ).
W e h a ve also ex a m in ed the asso c ia tio n be tw ee n h a n de d n ess a nd the p re ferre d
ey e for sig h tin g . In th e g en e ral p op u latio n, ab o u t 65 % o f R H in div idu a ls sig h t
w ith the ir rig h t e y es, w h ere as on ly 4 3% o f le ft-h a nd e rs d o (B o u ra ssa, M cM a nu s,
& B ry d en , 1 99 6 ). T h u s, th e re is a re lation b etw ee n h an d e d ne ss a n d e ye d ne ss.
W h en o n e sep a rate s w riting h a nd a n d thro w in g h a nd , a so m ew h a t c le are r p ic tu re
em e rg e s. O f th ose w ho thro w an d w rite R H , a bo u t 7 5 % a re a lso rig h t-e y ed .
S im ila rly , o f th o se w h o th row a n d w rite L H , ab o u t 7 5 % are left-ey e d . It is th o se
GENETICS AND HANDEDNESS
329
w ho th ro w w ith on e h a nd an d w rite w ith the o th er w h o do n o t fo llo w th is p attern :
ab o u t ha lf are le ft-ey ed a n d ha lf are rig h t-e y ed (M cM a n u s, P o rac , B ryd e n , &
B ou c he r, 1 9 97 ). T h ese d a ta sug g es t tha t w ritin g h a n d a nd th ro w ing h an d
co n trib u te a bo u t eq u ally a s p red icto rs of e ye d o m ina n c e.
T h e se e ffec ts sug g e sted to u s th a t ey e dn e ss a nd th row in g ha n d m ig h t p ro v e to
b e b etter pre d ic tors o f lan g u ag e late ralisatio n tha n w ritin g h a nd , a nd w e h av e
fou n d at lea st lim ited su p p ort fo r th ese ide a s. T o a ssess la n gu a g e late ralisatio n,
w e h av e u se d a no n in va sive tec h niq u e c alled d ic h otic liste n in g , w h ic h in v o lv e s
the pre sen ta tio n o f d iffe re n t w o rd s o r sy llab le s to th e tw o ea rs at th e sam e tim e.
In d ich o tic liste nin g, sub jec ts no rm ally sh o w su p e rio r p erfo rm an c e o n th e ea r
co n tra la te ral to th e h em isp h e re sp e cia lise d fo r la n g ua g e (Z a to rre , 1 9 8 9). T h us,
m o st p eo p le sh ow a rig h t-e ar a d va n tag e (R E A ) fo r id e n tify in g v erb al m a teria l,
b ec au se m o st h um an s a re left-h em isp h e ric fo r lan g u ag e .
B ry d en (19 8 8 ) re p orte d d ic h o tic -listen in g d ata fro m a C V -syllab le s tas k o n a
sam p le o f 3 0 2 u nd e rgra d ua tes w ho h a d b ee n asse sse d fo r h a n de d n ess, e ye d ne ss,
an d fa m ilia l sin istra lity . In th is stu d y h an d ed n e ss an d e ye d n ess w e re bo th relate d
to th e m a g n itu d e of th e d ich o tic R E A . A n aly ses o f th e d iffe re nc e in th e
p erc en ta ge of left- an d rig h t-h a n de rs an d in left- an d rig h t-e ye d su b je cts sh o w in g
an R E A rev e a le d tha t the ey e dn e ss e ffe c t is co n sid era bly sm aller th a n th e
h an d ed n e ss effec t, a lth o ug h it is cle arly p re se nt (see F ig. 2).
T h e F u se d D ich o tic W o rd s T e st (F D W T ), d ev elo p ed b y W ex ler a n d H alw e s
(19 8 3 ), is a so m ew h a t m ore v alid m ea su re o f lan g u ag e latera lisa tio n . B y
ca refu lly a lig n in g th e stim u li, th e tw o w o rd s fu se in to a sin gle p erc ep t, so tha t
the liste ne r is u n aw a re o f the fac t th a t tw o d iffe re nt stim u li a re b ein g pre sen te d,
y et a rig h t-e a r e ffec t is still o b serv ed . T h u s b iase s to atten d to on e e a r in
p re fere n c e to the oth er d o n o t a ffe c t p erfo rm a nc e . Z a to rre (1 9 8 9) h as sh o w n tha t
p e rfo rm a n ce o n th e F D W T is c lo se ly re la te d to la n g u a g e late ra lisa tio n
d e t e rm in e d b y u n il a te ra l so d iu m a m y ta l su p p re ss io n . B e c a u s e o f th e se
p ro p e rtie s, w e h a ve b ee n usin g th e F D W T in re c en t y e ar s.
G rim sh a w a nd B ry de n (1 99 5 ) a dm in istere d th e F D W T to 1 5 5 p artic ip an ts
w ho v a rie d in the ir w ritin g a nd th row in g h a n ds. A na ly ses o f th e p erc e nta ge o f
tho se w ith a n R E A a m o ng rig h t- a nd le ft-w rite rs a n d rig h t- a nd le ft-thr ow e rs
(see F ig . 3) sh o w ed th e w ritin g-h a nd effe ct to b e som ew ha t larg er th a n it w a s in
the ea rlie r stu d y w ith C V -s yllab les, reflec ting the be tte r m e asu re m e n t q u alitie s
o f the F D W T . In a d ditio n , the re w as an in d e pe n de n t e ffe c t o f thro w ing ha n d o f
ap p ro x im ate ly the sam e m ag n itu d e . In a m o re re ce n t stu dy , ag a in usin g th e
F D W T , E lia s (1 9 96 ) fo u n d thro w ing ha n d to be a b e tte r p red icto r o f lan g u ag e
latera lisa tio n th a n w ritin g h an d : th e co rre la tio n b etw ee n th ro w in g an d th e R E A
w as .4 5 (d f = 3 0 , P < .0 1 ), w he rea s th at be tw e e n w ritin g ha n d a nd th e R E A w a s
o nly .2 2 (P > .2 0 ).
B e c au se o f n u m e rou s sub tle h in ts in th e lite ratu re (e.g. S e a rle m a n, 1 9 8 0), w e
h av e re ce n tly co m p le te d a stu d y in w hic h w e lo ok e d a t h an d ed ne ss an d
foo ted n e ss a s p red icto rs o f th e dic h otic R E A in the F D W T (E lia s & B ry d en , in
330
BRYDEN ET AL.
FIG. 2.
T h e p e rc en ta g e o f p a rtic i p a n ts e x h i b it in g a ri g h t-e a r a d v an ta g e o n a d i ch o tic -C V s y ll ab le s
ta s k , cl a ss i fie d a c c o rd in g to h a n d e d n e s s an d e y e d n e s s (fro m B ry d e n , 1 9 8 8 ). B e c a u s e o f m is s in g d a ta
a n d t h e fa c t th a t s o m e p a rt ic ip a n t s d id n o t sh o w a n ea r a d v a n ta g e , t h e to ta l N s fo r h a n d e d n e ss a n d
e y e d n e s s d o n o t to ta l 3 0 2 , th e n u m b e r o f p a rti c ip a n ts in th e s t u d y .
pre ss). W e re cru ite d 32 p articip an ts h alf of w h om w ere R H a n d h a lf L H su c h
th a t e q u al n u m b ers of e ac h g rou p w ere left-foo ted a n d rig h t-fo o te d . T h ese d ata
(se e Fig . 4 an d T a b le 4 ) sh o w th a t th e h an d e d ne ss effe ct w e h a ve see n be fo re is
still p re sen t (a lth o u gh n o t sig nifica n t b e ca u se o f th e re la tiv e ly sm all sa m ple
siz e s), b u t the effec t o f fo o te d ne ss is v ery m u ch la rge r. S im ila r d ata , u sin g a
diffe re n t d ic h o tic tas k, h a v e be e n rep o rte d by D ay a n d M a cN e ila g e (19 9 6 ). W e
ha v e, ev e n m o re re ce n tly , fo un d e v id e n ce th at fo ote dn e ss a p p ea rs to a lso b e a
be tte r pre d icto r o f th e dic ho tic left-ea r ad v an tag e (L E A ) fo r th e d ete c tio n o f th e
em o tio n al co n ten t of w o rds (E lia s, B ryd e n , & B u lm a n-F lem in g, in p ress).
T h e d a ta fro m th ese th re e stud ies a re su m m a rised in T ab le 4 . H e re , w e h av e
sho w n th e re lativ e od d s (th e O R ) fo r a rig ht-sid ed p erso n sh o w in g a n R E A
ra the r th a n a n L E A a s o p p ose d to a le ft-side d p e rson sho w in g tha t p a ttern . F o r
ex a m p le, in B ry d en ’ s 1 9 88 stu d y , a righ t-ha n de r w a s 1 .7 7 tim e s m o re lik ely to
sho w an R E A tha n w a s a left-h an d e r. T h ese d a ta in d ic ate th at fo o ted n e ss is b y
fa r th e be st p red ic to r o f th e R E A , w ith th ro w in g an d w riting ha n d s b ein g
GENETICS AND HANDEDNESS
FIG. 3.
331
T h e p e rc e n ta g e o f p a rt ic ip a n t s e x h ib it in g a ri g h t-e a r a d v a n ta g e o n th e F u s e d D i ch o tic
W o rd s T a sk , cl a ss i fie d a c c o rd i n g to h a n d e d n es s a n d th ro w i n g h a n d (fro m G ri m s h a w & B ry d e n ,
1 9 9 5 ).
TABLE 4
1
Odds Ratios Relating Laterality Variables to the Dichotic Right Ear
Advantage (REA)
B ry d e n , 1 9 8 8
H and
1 .7 7 *
Eye
1 .3 2
G ri m s h a w & B ry d e n , 1 9 5 5
H and
3 .1 3 * *
T h ro w
2 .9 0 * *
E li as & B ry d en , in p r es s
H and
2 .6 0
Foot
1 5 .0 *
* P < .0 5
* * P < .0 1
1
T h e ra tio o f th e o d d s o f a ri g h t-s i d e d (h a n d e d , ey e d , th ro w i n g h a n d , fo o te d )
p e rs o n s h o w in g a n R E A (a s o p p o s e d to a n L E A ) to th o s e o f a le ft -s id e d p e rs o n
sh o w i n g an R E A .
rou g h ly e q u ally im p ortan t, an d e y ed n ess ha v in g b u t a sm a ll e ffe c t. T h e n o tio n
tha t fo o te d n ess, ra the r be tte r th a n p refe rred ha n d fo r w ritin g , rela tes to c ere b ral
o rg a n isa tio n p ro v id es so m e su p po rt for th e n o tio n th a t la ng u a ge late ra lisation is
an ev o lu tion a ry c on se qu e n ce o f a sy m m etrie s in p o stura l c on trol (M a cN e ila g e,
1 99 1 ; P re v ic , 1 9 9 1), alth ou g h th e sp ec ific asp e cts o f ``fo o te d ne ss’ ’ th a t w e h av e
332
BRYDEN ET AL.
FIG. 4.
T h e p e rc e n t ag e o f p a rti c ip an ts ex h ib itin g a rig h t-e a r a d v a n ta g e o n th e F u s e d D ic h o t ic
W o rd s T a s k , c la s s ifie d a c c o rd i n g to h a n d e d n e s s a n d fo o te d n e s s (f ro m E l ia s & B ry d e n , in p re s s ).
fo u n d to b e p re dic tiv e (the skilled or b allistic c om p on e nt) are n o t th os e tha t
w o u ld su pp o rt th e M a cN e ila g e (19 9 1 ) m o d e l. T h e e ffe c t o f th ro w ing h a n d see n
in the G rim sha w an d E lias stu d ie s is re m in isc e nt o f C a lv in’ s (1 9 83 ) p rop o sitio n
th a t h um a n h an d e d ne ss ev o lv e d fro m th ro w in g sto n e s a t m o v in g targ ets.
In te resting ly , H o p k in s a n d h is c o lle a gu e s o b serv e d th a t a sa m ple o f ca p tiv e
ch im p a nz e es p re fe ren tially th rew righ t-h an d ed w h en th ey w ere in a bip e da l
po stu re (H o pk in s, B a rd , Jo n es , & B ale s, 1 9 93 ).
T h e th re e stu d ie s w e h av e ju st d esc rib e d w e re c arried o ut at qu ite d iffe ren t
tim e s, w he n so m e w h at d iffe re nt issu es w ere fo re m o st. O u r p ro ble m no w is to
de te rm ine h o w th e fo u r v a riab les o f ey e d ne ss, fo o ted n ess, th ro w ing h an d , a n d
w r iting h an d in tera c t to pre d ic t la tera lisa tio n of fu nc tion .
CONCLUSIONS
W h at d o es a ll th is h a v e to do w ith p rim a te ha n d ed n e ss? O u r d ata in d ic ate tha t
th e re is a sig n ific a nt ge n etic c on tribu tio n to h um an h an d ed n e ss, an d th e re is a t
le a st so m e ev ide n ce fo r th is in ou r clo sest liv in g relativ e, th e ch im p a n ze e
(H op k ins, B a le s, & B en n e tt, 1 99 3 ). T h e se find in gs su g ge st th a t h an d ed n e ss h a s
GENETICS AND HANDEDNESS
333
its o rig in in n o nh u m an la te ralitie s, a n d th ere is n o w in c re a sing e v id e n ce fo r
co n sisten t la tera l p re fere n ce s n o t on ly in p rim a te s, b ut a lso in ro de n ts an d o the r
m a m m a ls (se e B rad sh a w & R og e rs , 1 9 93 , fo r a re vie w ). T hu s, w e w ou ld join
w ith m an y o th ers in spe c ula tin g th at h u m a n la te ral pre fere n ce s p ro ba b ly aro se
from la te ra l b ia se s in n o n hu m a n sp ec ies, altho u g h th ere a re m a n y c o m p lica tin g
issue s, suc h a s th e so m e tim e s-c o n flic tin g resu lts of stu d ie s in vo lv ing c ap tiv e- a s
o pp o se d to w ild- no n h um an -p rim a te p o p u la tio n s (se e M a rch a n t & M c G re w ,
1 99 6 ).
A s disc u ssed e arlier w ith re spe c t to so m e o f ou r o w n w o rk , th e o b ser va tio n tha t
task d e m a n ds influ en c e th e m ag n itu d e an d dire ctio n of m a nu a l a sym m e trie s h a s
also b e en o f in te rest in w o rk w ith n o n hu m an p rim a tes. F ag o t an d V a u cla ir (1 9 9 1),
for e xa m p le , h a v e m a d e th e d istin ctio n b e tw e en lo w - a n d h igh -lev e l task s an d
d isc u sse d ho w th e m a g n itu de a n d d ire ctio n o f m a n u al a sy m m etries are a ffec te d
b y the se tw o ty p e s o f ta sk. H ig h-lev e l ta sks, w h ich re q uire fine ly tun e d m oto r
ac tio n s, pro d uc e stro n g bia ses to w ard s u sing o n e h a nd a n d so a re a sso cia te d w ith
asy m m e trie s of g re a te r m a gn itu d e, alth ou g h th e d ire c tio n d o e s n o t a pp e ar to b e a s
co n sisten tly to w ard rig ht-h an d u se as in h u m a ns. L ow -le ve l ta sk s, w hic h in v o lv e
sim p le r o r m o re fa m iliar o r w ell-pra ctise d m o ve m en ts, a re a sso cia te d w ith m ore
sy m m e trica l d istrib u tion s of b ia s in ha n d u se. T h e a sym m etrie s ob se rv e d in h ig h lev el ta sk s a re th ou g h t to a rise fro m h em isp h e ric a sy m m etries in th e co n tro l
p ro c esse s n ee de d to p e rform th e h ig h -le v el task . T h e h a nd co n siste n tly use d to
p erfo rm the task is c on trala te ral to an d co n trolled b y th is h em isp h e re . A ltho ug h
the re a re po p u latio n -le v e l a sy m m etries in ha n d u sag e, the se va ry ac ro ss spe c ie s o f
p rim ate s, th u s th is m a y su g g es t u nd e rly in g sp e cies d iffe re n ce s in h em isp h e ric
asy m m e trie s for c on trolling the h igh -lev e l m ov e m en ts. S o m e clu es to th is
q ue stio n o f d iffe ren c es in u n d erlyin g he m isp h eric a sy m m etries h av e be en so ug h t
b y c o m p a rin g spe c ie s o n ev ide n ce o f oth e r n e u ro stru ctu ral (e.g . larg er b ra in a rea s
in o n e he m isp h ere ) or n e uro fu nc tion a l (e .g . e a r or v isua l-h e m ifield a dv a nta ge s fo r
p artic u lar task s) m e asu res. A n u m b e r o f stu d ie s h av e sh o w n su b sta n tial
co n v erg e n ce b e tw e en m a n ua l a n d h em isph e ric asy m m etrie s o f struc tu re an d
fun c tio n . T h is c o nv e rg en c e of d iffe ren t asy m m e trie s p rov id es su p p ort fo r th e
n otion o f sh ar ed m e c ha n ism s a nd fo r th e p ossib ility of h o m o lo gy b etw e en h u m a n
an d n o nh u m a n la tera lity .
O ne p rob lem in th e stu d y of m an u al a sym m etrie s in n o n hu m a n p rim a tes is
tha t it is d ifficu lt to d iffe ren tiate be tw e e n asy m m e trie s in p re fe ren c e an d
p erfo rm a n ce . T yp ic ally, asy m m e trie s in p erfo rm a nc e are in fe rred fro m th e
m a g nitu d e a n d direc tio n o f asy m m e trie s in p re fe ren c e (i.e . th e h an d th e an im a l
ch o o se s to u se in p erfo rm in g th e task ). S o m e in v estiga tors h av e o b se rve d
co n sisten c y b etw ee n th e se tw o m e a su re s, w h ere a s o the rs ha v e n o t (F ag o t &
V au c la ir, 1 9 9 1). T h e a bility to m a k e th is d istin c tio n op e n s o p po rtun itie s to
co m pa re th e d etails of h an d p e rfo rm a n c e, a n a n aly sis th at m a y b e m o re
rev e aling ab o u t u n de rlyin g h e m isp h eric asy m m e trie s th an on e th a t fo cu se s o n
p attern s o f h a nd p refe ren c e .
334
BRYDEN ET AL.
O u r o w n da ta a lso su g g est th at the m e a sur em en t of la te ralisa tio n is a co m p le x
iss ue , an d th a t o n e m u st k n o w w h a t to m e a su re an d h o w to m ea su re it. A ltho u g h
w e ha v e n o fin a l a nsw e r, th e d a ta w e h a ve p re sen te d ind ica te tha t o ne m u st
asse ss a n u m b e r o f differe n t a sp ec ts of h an d ed n e ss, a nd tha t su c h va ria b le s a s
th ro w in g h a nd , e ye d ne ss, a nd m o st p artic u la rly foo ted n e ss, m a y u ltim a tely
pro v e to b e v ery critic al. O ur find ing tha t fo ote d n ess is an im p o rta nt p re d ic tor o f
la n g ua g e la te ra lisa tio n (cf S e arlem a n, 1 9 80 ) p rov id es so m e su p p or t fo r rela tio n s
b e tw e e n p o s tu ra l c o n tro l, a n i n te g ra l p a rt o f fo o te d n e ss , a n d la n g u a g e
la tera lisa tio n in hu m an s, a lth o ug h th e d e ta ils o f th e n atu re o f th os e rela tio n s
ha v e y e t to be sp ec ifie d .
REFERENCES
A n n e tt, M . (1 9 8 5 a ). L ef t, r ig h t, h a n d a n d b r a in : T h e r i g h t s h ift t h e o r y. H o v e , U K : L a w re n c e
E rlb a u m A s s o c ia t e s L t d .
A n n e tt, M . (1 9 8 5 b ). W h ic h t h e o ry fa i ls ? A r ep ly to M cM a n u s . B r it is h J o u r n a l o f P s y c h o lo g y , 7 6 ,
1 7 ±2 9 .
A n n e tt, M . (1 9 9 5 ). T h e ri g h t s h ift th e o ry o f a g e n e t ic b a l an c e d p o ly m o rp h is m fo r c e re b ra l
d o m in an c e an d co g n i tiv e p ro c e ss in g . C a h ie r s d e P s y c h o lo g ie C o g n i tiv e , 1 4 , 4 2 7 ±4 8 0 .
A n n e tt, M . (1 9 9 6 ). I n d e fe n c e o f th e rig h t sh ift t h e o ry . P e rc e p tu a l a n d M o to r S k ills, 8 2 , 1 1 5 ±1 3 7 .
B a rn s le y , R .H ., &
R a b i n o v itc h , M .S . (1 9 7 0 ). H a n d e d n e s s , p ro f ic ie n cy v s . st a te d p re fe re n c e.
P e r ce p tu a l & M o to r S k ills, 3 0 , 3 4 3 ±3 6 2 .
B id d l e , F .G ., & E a l es , B .A . (1 9 9 6 ). T h e d e g re e o f la te ra li z a tio n o f p a w u s a g e (h a n d e d n e s s ) i n th e
m o u se i s d e fin e d b y t h re e m a jo r p h e n o ty p e s . B e h a v i o r G e n e ti c s, 2 6 , 3 9 1 ±4 0 6 .
B is h o p , D .V .M . (1 9 9 0 ). H a n d e d n e s s a n d d e v e lo p m e n ta l d is o r d e r. O x fo rd : B l a ck w e ll S c i e n ti fic
P u b li c at io n s .
B is h o p , D .V .M ., R o s s, V .A ., D a n i el s , M .S ., & B rig h t, P . (1 9 9 6 ). T h e m e a s u re m e n t o f h a n d
p re fe re n ce : A v a l id a tio n s tu d y c o m p a ri n g t h re e g ro u p s o f r ig h t- h a n d e rs. B r itish J o u r n a l o f
P s y ch o lo g y , 8 7 , 2 6 9 ±2 8 5 .
B o u ra s s a, D .C ., M c M a n u s, I. C ., & B ry d e n , M .P . (1 9 9 6 ). H a n d e d n e s s a n d e y e -d o m i n a n c e: A m e ta a n a ly s is o f th e ir re l a tio n s h i p . L a te r a li ty , 1 , 5 ±3 4 .
B ra d s h a w , J.L ., & R o g e rs, L .J. (1 9 9 3 ). T h e e v o l u t io n o f l a t er a l a s ym m e tr i es , l a n g u a g e , t o o l u s e a n d
i n t e lle c t. S a n D ie g o , C A : A c ad e m ic P re s s .
B ry d e n , M .P . (1 9 8 8 ). C o rre l at e s o f th e d i ch o tic rig h t-e a r e ffe ct . C o rt e x, 2 4 , 3 1 3 ±3 1 9 .
B ry d e n , M .P ., A rd ila , A ., & A rd i la , O . (1 9 9 3 ). H a n d e d n e s s in n a tiv e A m a z o n ia n s , N e u r o p s y c h o l o g ic a , 3 1 , 3 0 1 ±3 0 8 .
B ry d e n , M .P ., & E d m o n d s , A . (1 9 9 5 ) . A n o b j ec t iv e m e a s u re o f h an d p r efe re n ce . J o u r n a l o f th e
I n te r n a t io n a l N e u r o p s y c h o lo g i ca l S o c i et y , 1 , 3 2 2 .
B ry d e n , M .P ., M c M a n u s , I.C ., & B u l m a n -F le m in g , M .B . (1 9 9 4 ). G B G , B M B , R + L , X + Y . . . R e p ly
t o C o m m e n ta ri e s. B r a i n a n d C o g n it io n , 2 6 , 3 1 2 ±3 2 6 .
B ry d e n , M .P ., S in g h , M ., S te e n h u is , R .E ., & C la rk s o n , K .L . (1 9 9 4 ). A b e h a v io u ra l m e as u re o f h a n d
p re fe re n ce a s o p p o s e d to h a n d s k il l. N e u r o p s y ch o lo g ic a , 3 2 , 9 9 1 ±9 9 9 .
B ry d e n , P .J., R o y , E .A ., B ry d e n , M .P ., A lla rd , F .A ., W h i te la w , S ., B u l m an -F le m in g , M .B ., &
R o y , P . ( 1 9 9 7 ). T a sk d i fficu lt y a n d m a n u a l a s y m m e t ri e s. P o st e r p res e n ta tio n at t h e N o rth
A m e ri ca n A n n u a l M e e t in g o f t h e In t ern a ti o n a l N e u ro p sy ch o lo g i ca l S o c ie ty , O rla n d o , F lo rid a ,
F e b ru a ry .
B r y d e n , P .J ., R o y , E .A . , B r y d e n , M .P . , B o u c h e r , R ., P r y d e , K .M ., &
F r a n k , J .S . ( i n
p re s s ). H a n d e d n e s s a n d ta s k c o m p l e x i ty : T h e re la t io n sh ip o f p re fe re n c e an d p e rfo rm a n c e.
S o c ie ty fo r N e u r o sc i en ce A b s t ra c ts .
GENETICS AND HANDEDNESS
335
C a lv in , W .H . (1 9 8 3 ). A s to n e ’ s th ro w a n d i ts la u n c h w in d o w : T im in g p re c is io n an d i ts i m p li c a tio n s
fo r l a n g u a g e a n d h o m in id b ra in s . J o u r n a l o f T h e o r e ti ca l B i o l o g y , 1 0 4 , 1 2 1 ±1 3 5 .
C a rli er , M . (1 9 9 5 ). G e n e ti c s a n d d e g re e o r d i re c tio n o f la te ra li ty : W h at ca n w e le a rn fro m th e
m o u s e m o d e l ? C a h ie r s d e P s y c h o lo g i e C o g n i tiv e, 1 4 , 5 1 6 ±5 1 9 .
C a v a l li-S fo rz a , L .L ., M e n o z z i, P ., & P ia z za , A . (1 9 9 4 ). T h e h is to r y a n d g e o g r a p h y o f h u m a n g e n e s .
P rin c e to n , N J : P rin c et o n U n iv e rs ity P re s s .
C o llin s , R .L . (1 9 8 5 ). O n th e in h e ri tan c e o f d ire c ti o n an d d e g re e o f a s y m m e try . In S .D . G li c k (E d .),
C e re b r a l l a t er a li za ti o n i n n o n h u m a n s p e ci e s. O rl a n d o C A : A c a d e m i c P re s s.
D a y , L .B ., & M a c N e i la g e , P .F . (1 9 9 6 ). P o s tu r al a s y m m e tri es a n d la n g u ag e l a te ra liz a tio n i n h u m a n s
(H o m o s a p i e n s ). J o u r n a l o f C o m p a r a tiv e P s y c h o lo g y , 1 1 0 , 8 8 ±9 6 .
E l ia s , L .J. (1 9 9 6 ) . [P ers o n a l c o m m u n ic a tio n .]
E l ia s , L .J., & B ry d e n , M .P . (in p r es s ). F o o t e d n e ss is a b e tt er p re d ic to r th an h an d e d n e ss o f la n g u a g e
la te ra li z at io n . L a te r a lity.
E l ia s , L .J., B ry d e n , M .P ., & B u lm a n -F l e m in g , M .B . (in p re s s ). F o o t ed n es s is a b e tt er p re d ic to r t h a n
is h a n d e d n es s o f e m o ti o n a l la te ra li za t io n . N e u ro p sy c h o lo g i a .
F a g o t , J ., & V a u c l a ir, J . (1 9 9 1 ) . M an u a l la te ra l ity in n o n h u m a n p ri m a te s : A d is ti n c t io n b e tw e e n
h a n d e d n e s s an d m a n u a l sp e c ia li za t io n . P s y c h o l o g ic a l B u l le ti n , 1 0 9 , 7 6 ±8 9 .
F i tts, P .M . (1 9 5 4 ). T h e in fo rm a ti o n c a p a c it y o f th e h u m a n m o to r s y s te m in c o n t ro l lin g am p li tu d e o f
m o v e m e n t. J o u r n a l o f E x p e r im e n ta l P s y c h o lo g y , 4 7 , 3 8 1 ±3 9 4 .
F l o w e rs , K . (1 9 7 5 ). H a n d e d n es s a n d c o n tro lle d m o v e m e n t. B r it is h Jo u rn a l o f P sy c h o l o g y , 6 6 , 3 9 ±5 2 .
G i l b e r t , A .N ., &
W y s o c k i , C .J . ( 1 9 9 2 ) . H a n d
p re f e r e n c e
and
age
in
th e
U n ited
S tates.
N e u r o p s y c h o lo g ia , 3 0 , 6 0 1 ±6 0 8 .
G rim s h a w , G ., & B ry d e n , M .P . (1 9 9 5 ). A re th er e m e a n in g fu l h a n d ed n e ss s u b -ty p es ? J o u r n a l o f th e
In t e rn a tio n a l N e u ro p sy c h o lo g i c a l S o ci e ty, 1 , 3 6 7 .
H a rris , L .J. (1 9 9 0 ). C u l tu r al in flu e n c e s o n h an d e d n e ss : H i st o ri c a l an d c o n te m p o ra ry th e o ry a n d
e v i d e n ce . In S . C o ren ( E d .), L e ft-h a n d e d n e s s : B e h a v io ra l i m p li c a t io n s a n d a n o m a l ie s, (p p .2 9 2 ±
3 2 0 ) . A m st e rd a m : E l s ev ie r S c i en c e P u b li s h e rs .
H o p k in s , W .D ., B a le s , S .A ., & B e n n e tt , A .J. ( 1 9 9 3 ). H e ri ta b ility o f h a n d p re fe re n c e in c h i m p a n z e es
(P a n ). In t e rn a tio n a l J o u r n a l o f N e u ro s c ie n c e , 7 4 , 1 7 ±2 6 .
H o p k in s , W .D ., B ard , K .A ., Jo n e s, A ., & B a le s , S .L . (1 9 9 3 ). C h im p a n z e e h a n d p re fe re n c e in
th ro w i n g an d in fa n t cr ad li n g : Im p li ca t io n s fo r th e o ri g i n o f h u m a n h a n d e d n e s s . C u r re n t
A n th ro p o lo g y , 3 4 , 7 8 6 ±7 9 0 .
Id a, Y ., & B ry d e n , M .P . (1 9 9 6 ). A c o m p a ris o n o f h a n d p re fe re n c e in J ap a n a n d C a n a d a . C a n a d ia n
J o u r n a l o f E x p e ri m e n ta l P s y c h o lo g y, 5 0 , 2 3 4 ±2 3 9 .
M a c N e ila g e , P .F . (1 9 9 1 ). T h e ``p o s tu ra l o rig in s ’ ’ t h e o ry o f p rim a te n e u ro b i o lo g ic a l a s y m m e t ries . In
N .A . K ra s n e g o r, D .M . R u m b a u g h , R .L . S h i e fel b u s c h
&
M .G . S t u d d e rt -K en n e d y (E d s .),
B io lo g ic a l a n d b e h a v io u r a l d e t er m in a n ts o f la n g u a g e d e v e lo p m e n t (p p .1 6 5 ±1 8 7 ). H i llsid e , N J:
L a w re n c e E rl b a u m A s s o c i at e s In c .
M a ra s c u i lo , L .A ., & S e rlin , R .C . (1 9 8 8 ). S ta ti s tic a l m et h o d s f o r th e s o c ia l a n d b eh a v io r a l s c ie n c e s .
N ew Y o rk : F re e m an .
M a rc h a n t, L .F ., & M c G re w , W .C . (1 9 9 6 ) . L at e ra lity o f lim b fu n c t io n in w i ld c h i m p a n ze e s o f
G o m b e N a tio n al P a rk : C o m p re h en s iv e s tu d y o f s p o n ta n e o u s a c tiv it ie s . J o u r n a l o f H u m a n
E v o lu tio n , 3 0 , 4 2 7 ±4 3 3 .
M a rc h a n t, L .F ., M c G re w , W .C ., & E ib l-E ib e s fe ld t, I. (1 9 9 5 ) . Is h u m a n h a n d e d n e s s u n i v e rs a l?
E th o lo g i ca l a n a ly se s fro m t h re e tra d it io n a l c u ltu re s . E th o lo g y , 1 0 1 , 2 3 9 ±2 5 8 .
M c M an u s, I. C . (1 9 8 5 a ). R ig h t- a n d l eft -h an d s k ill: F a il u re o f th e ri g h t s h i ft m o d e l. B ri tish J o u r n a l
o f P s y ch o lo g y , 7 6 , 1 ±1 6 .
M c M an u s, I .C . (1 9 8 5 b ). O n te s ti n g th e rig h t s h i ft th e o ry : A re p ly to A n n e tt . B r it is h J o u rn a l o f
P s y c h o lo g y , 7 6 , 3 1 ±3 4 .
M c M a n u s , I.C . ( 1 9 8 5 c ). H a n d e d n e s s, l a n g u a g e d o m i n a n c e
P s y c h o lo g ic a l M e d i ci n e , M o n o g ra p h S u p p le m e n t N o . 8 .
and
a p h a s ia : A
g e n e ti c
m o de l.
336
BRYDEN ET AL.
M c M a n u s , I.C ., & B r y d e n , M .P . (1 9 9 2 ). T h e g e n e t ic s o f h a n d e d n e ss a n d c ere b ra l la te ra li z a tio n , In I.
R a p in &
S .J . S e g a l o w it z ( E d s .) , H a n d b o o k
o f n e u r o p s y c h o lo g y , V o l . 6 , (p p .1 1 5 ±1 4 4 ).
A m s te rd a m : E l se v ie r.
M c M a n u s , I .C ., &
B ry d e n , M .P . ( 1 9 9 3 ). H a n d e d n e s s o n
T ri s t a n
da
Cunha: The
g en etic
c o n se q u e n c e s o f so c ia l is o l at io n . In te r n a ti o n a l Jo u rn a l o f P s y c h o lo g y , 2 8 , 8 3 1 ±8 4 3 .
M c M a n u s , I.C ., P o ra c , C ., B ry d e n , M .P ., & B o u c h er , R . (1 9 9 7 ). E y e d o m i n a n c e , w rit in g h a n d , a n d
t h ro w in g h a n d . M an u sc rip t s u b m i tte d f o r p u b li ca t io n .
P o ra c, C ., R ee s , L ., & B u l le r, T . (1 9 9 0 ). S w it ch in g h a n d s : A p la c e fo r l ef t h a n d u s e in a rig h t h a n d
w o rld . In S . C o re n (E d .), L e ft-h a n d e d n es s : B e h a v io r a l im p lic a tio n s a n d a n o m a l ie s, (p p .2 5 9 ±
2 9 0 ). A m s t erd a m : E l se v ie r S c ie n c e P u b li sh e rs .
P re v ic , F . (1 9 9 1 ). A g e n e ra l t h e o ry c o n c e rn in g th e p re n a ta l o rig in s o f c e re b ra l l a te ra li za t io n in
h u m a n s . P s y c h o l o g i ca l R e v ie w , 9 8 , 2 9 9 ±3 3 4 .
P ro v i n s , K .A ., & M a g lia ro , J. (1 9 8 9 ). S k i ll, s tre n g th , h a n d e d n e s s , a n d fat ig u e . J o u r n a l o f M o t o r
B e h a v io r, 2 1 , 1 1 3 ±1 2 1 .
R o y , E .A . ( 1 9 9 6 ). H a n d p re fe re n c e , m a n u a l a s y m m e t rie s a n d li m b a p ra x i a . In D . E llio t t & E .A . R o y
( E d s.), M a n u a l a sy m m e tr ie s in m o to r p e r fo r m a n c e . B o c a R a to n , F L : C R C P re s s .
R o y , E .A . ( in p re s s ). T as k d e m a n d s in re ac h in g a n d th e m a g n itu d e o f m a n u a l a s y m m et rie s in
p e rfo rm a n c e, B r a in a n d C o g n it io n .
R o y , E .A ., B ry d e n , M .P ., B u l m an -F l e m in g , M .B ., & R o y , P . (1 9 9 6 ). M a n u a l a sy m m e tr ie s in
p e g b o a r d p e rf o r m a n c e: T h e e ffe c t o f ta s k d e m a n d s. P o s te r p re s e n t ati o n at t h e T h e o re tic a l a n d
E x p e rim e n ta l N e u ro p sy c h o lo g y m e et in g , M o n tre a l, M a y .
R o y , E .A ., B ry d e n , M .P ., M c M a n u s , I.C ., & B o u c h e r, R . (i n p re p ara t io n ). T a s k d e m a n d s a n d a
p e rfo rm a n c e m e a s u re o f h an d p re fe re n ce .
S e a rle m a n , A . (1 9 8 0 ). S u b je c t v a ri a b l es a n d c e re b ra l o rg a n iz a ti o n f o r la n g u a g e. C o r te x , 1 6 , 2 3 9 ±
2 5 4.
S in g h , M ., & B ry d e n , M .P . (1 9 9 4 ). T h e fa c to r st ru c t u re o f h an d e d n e ss in In d i a . In te r n a t io n a l
J o u rn a l o f N e u r o sc i e n ce , 7 4 , 3 3 ±4 3 .
S te e n h u is , R .E . (1 9 9 6 ). H a n d p ref ere n c e a n d p e rfo rm a n c e in s k illed an d u n sk il le d a c tiv it ie s. In D .
E ll io tt & E .A . R o y (E d s .), M a n u a l a s ym m e tr ie s in m o to r p e r fo r m a n c e . B o c a R a to n , F L : C R C
P re s s.
S te e n h u is , R .E ., & B ry d e n , M .P . (1 9 8 9 a ). D if fer en t d im e n si o n s o f h a n d p ref ere n c e t h a t rel a te to
s k il le d a n d u n s k il le d a c tiv it ie s . C o rt e x, 2 5 , 2 8 9 ±3 0 4 .
S te e n h u is , R .E ., & B ry d e n , M .P . (1 9 8 9 b ). H a n d p re fe re n c e a n d p e rfo rm an ce : R ig h t-h an d e rs , l eft h a n d e rs , a n d l eft -h an d er s w h o a re n o t co n si s te n t ly l e ft-h a n d e d . J o u r n a l o f C lin ic a l a n d
E x p e r im e n ta l N e u r o p s y c h o lo g y , 1 1 , 3 7 .
T a p le y , S .M ., & B ry d e n , M .P . (1 9 8 5 ). A g ro u p t es t fo r t h e a ss e s s m e n t o f p erf o rm a n c e b e tw e e n th e
h a n d s . N e u r o p s yc h o lo g ia , 2 3 , 2 1 5 -2 2 2 .
V an H o rn , J.D . (1 9 9 2 ). B r a i n s tr u c tu ra l a b n o rm a li ty a n d l a t e ra li ty in s ch iz o p h r e n ia . U n p u b lish e d
P h D t h e s is , U n iv ers i ty C o ll e g e , L o n d o n .
W e x le r, B ., & H a lw e s, T . (1 9 8 3 ). I n c re a s in g t h e p o w e r o f d i c h o ti c m e th o d s : T h e fu se d rh y m ed
w o rd te s t. N e u ro p sy c h o lo g ia , 2 1 , 5 9 ±6 6 .
Y eo , R ., & G a n g e s t ad , S .W . (1 9 9 3 ). D e v el o p m e n ta l o ri g in s o f v a ri at io n in h u m a n h a n d p re fe re n c e.
G e n e tic a , 8 9 , 2 8 1 ±2 9 6 .
Z a to rre , R .J. (1 9 8 9 ) . P e rc e p tu a l a sy m m e try o n th e d ic h o t ic fu se d w o rd s t e s t a n d c e re b ra l s p ee c h
l a te ra li za t io n d e te rm i n e d b y th e c a ro ti d s o d iu m a m y t al t e st . N e u ro p sy c h o lo g i a , 2 7 , 1 2 0 7 ±1 2 1 9 .