Behavior Genetics, Vol. 26, No. 4, 1996
Manual Performance and Laterality in Twins of Known Chorion Type Mieh~le Carlier, ~,2 Elisabeth Spitz, t Marie C6eile V a c h e r - L a v e n u , 3 Pierre Vill6ger, 1,4 Benoit Martin, 1 and Francois MicheP
Received 10 Sept. 1995--Final 4 Dec. 1995
Manual performance, direction, and degree of laterality were tested in monozygotic (MZ) twins (8-12 years old) of known chorion type and dizygotic (DZ) twins. Three manual tasks rarely employed in twin studies were used: dot-filling, tapping, and peg-moving tasks. No chorion effect was observed: the monochorionic and dichorionic MZs differed neither for frequency of discordant pairs nor for handedness, laterality measurements, and manual performance. The pooled MZs and DZs were then compared in a classic twin design. The within-pair resemblance was not higher in MZs than in DZs for variables measuring level of manual performance. For laterality scores intraelass correlations were close to zero in MZ and DZ twin groups. KEY WORDS: Chorion type; laterality; motor skill; twins.
b y the same criteria, in the same study, by the same investigators" (p. 352). At least four studies met these requirements. In Japan, Shimizu and Endo (1983) concluded that the incidence o f left-handedness in twins was the same as that in singletons. This study presented drawbacks, however, due mainly to a large number o f twins who had been forced to convert to right-hand usage in childhood. Social pressure against left-handedness was strong, leading to very low percentages o f those using this hand for writing and eating (0.84 and 1.91%, respectively). In Norway, Tambs e t al. (1987) collected information on handedness from twins and their parents, spouses, and children. The difference in percentages o f left-handed writers (twins > nontwins) was not significant when twins and singletons in the same generation were compared. In North America, Coren (1994) observed a significant higher incidence o f left-handedness in twins compared to singletons (14.5 vs. 9.9%). This result was confirmed by Davis and Annett (1994) in the United K i n g d o m (11.7 vs. 7.1%, respectively, for left-hand writers).
INTRODUCTION The high incidence o f left-handedness among twins is still a subject o f debate. After reviewing 11 twin studies published from 1924 to 1950, Nagylaki and Levy (1973) concluded that the frequency o f sinistrality was significantly higher in twins than in singletons. Reviewing almost the same studies, and the more recent one by Carter-Saltzman e t al. (1976), Springer and Searleman (1980) reached the same conclusion. However, this conclusion was challenged by McManus (1980) on the relevant argument that almost all comparisons were based on twins and singletons who had not been "assessed URA 1294 CNRS G~nrtique Neurogrnrtique et Comportement, Universit6 Paris V Ren6 Descartes, UFR Biomrdicale, 45 rue des Saints P~res, 75270 Paris Cedex 06 France; email:
[email protected]. 2To whom correspondence should be addressed. 3 Service Central d'Anatomie et Cytologie Pathologiques, Groupe Hospitalier Cochin, 27 rue du Faubourg Saint Jacques, 75674 Paris Cedex 14 France. 4 Current address: Centre Bobillot, Centre Hospitalier Esquirol, 87000 Limoges, France. SCNRS and U 280 INSERM, 151 Cours Albert Thomas, 69300 Lyon, France. 409
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Several explanations have been put forth for the increased incidence of left-handers in twins (Carter-Saltzman et aL, 1976; Segal, 1989). Some refer to the specific prenatal and perinatal conditions of twins, compared to singletons (crowding in womb, higher incidence of prematurity, prenatal stress), or to a possible common genetic influence between twinning and symmetry development (Boklage, 1987). Other explanations focuse on the timing of the embryo splitting after fertilization, but this is of course relevant for monozygotic (MZ) twin pairs only. The effect of the timing of the egg splitting, hence producing MZ twins, has been known for a long time (Price, 1950). If the division occurs within 72 h after fertilization, each of the cotwins will develop its fetal membranes, i.e., chorion and amnion, and they will be classified as dichorionic diamniotic (DC-MZ). About 70% of the MZ twin pairs are DC-MZ. If the division occurs later (from day 4 to day 7), the two embryos will share the same chorion but different amnions (monochorionic diamniotic twins; MC-MZ). When division is even later, the two embryos share the same two membranes (monochorionic monoamniotic twins). Ever since Newman (1928) it has been speculated that delayed embryo splitting is associated with "mirror-imaging effects" if the division occurs after the establishment of an axis of bilateral symmetry (Springer and Searleman, 1980). In such a situation opposite handedness in the same pair of twins is expected and discordant pairs for handedness are expected to be more frequent in MC than in DC-MZ pairs. Breland (1974) and more recently Davis and Phelps (1995) did not hesitate to consider mirrored handedness as a marker of chorion status. However, before this special issue, very few data were available for testing this speculation due to the relatively low frequency of left-handedness, even in twin populations, and the paucity of information on chorion type in MZ twin pairs. Hay and Howie (1980) chose an indirect approach. These authors classified MZ pairs according to within-pair birth weight difference: large birth weight differences were observed in the transfusion syndrome, one complication o f monochorionic pregnancies. Thus, the MZ pairs with a birth weight difference over 450 g were classified as MC-MZ, and the others as DC-MZ. Results confirmed the hypothesis: first, left- or mixedhandedness was significantly more frequent in MZ
Carlier et al.
pairs with large birth weight differences; second, no difference was observed in DZ pairs classified using the same criteria. To our knowledge, Karras Sokol and co-workers' paper (1995) is the only one providing information on both chorion type and handedness before those published in this special issue. In a sample of 44 pairs of MZ twin children (23 MC-MZ and 21 DC-MZ) selected from hospital records, they observed 4 of 22 (18.2%) discordant pairs in the MC-MZs and 5 of 19 (26.3%) in the DC-MZs. It should be noted that this nonsignificant trend is not the expected tendency (discordant pairs are more numerous in the DC-MZ group). For classification purposes in laterality research, it is also possible to consider the degree of handedness, i.e., the absolute or relative difference between the performance with either hand, instead of the tendency to use a particular hand. The degree of laterality has been studied extensively in animal models since the work o f Collins (1985), who demonstrated that in mice it was under genetic control (see also Biddle et al., 1996). In a study on singleton siblings (Carlier et al., 1994), significant sibling resemblance for the performance of the preferred and nonpreferred hands in two manual tasks was observed. When the degree of laterality was considered, the sibling ressemblance was significant in the manual task closer to writing (a dot-filling task) only. In the GNC-EFTCJ project (G~n&ique Neurog~n~tique et Comportement--Etude Franqaise du Type de Chorion chez les Jumeaux; Genetics Neurogenetics and Behavior--French Chorion Type Twin Study), we assessed twins with three manual performance tests. The aim of this paper is twofold: to test a chorion effect on manual performance, direction, and degree of laterality and to compare MZ and dizygotic (DZ) twin resemblance in manual tasks rarely used in twin studies.
MATERIALS AND M E T H O D S Subjects The selection procedure of the sample will be fully presented by Spitz et aL (1996). All birth records (335) from three hospitals from 1980 to 1985 were analyzed. Those with two living like-sex twin pairs were reviewed (198). Ninety-six families still living in France were traced. Eight families refused
Manual P e r f o r m a n c e and Laterality in Twins
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Table I. Characteristics of the Groups of Twins"
MC-MZ Number of pairs Gender composition (number of pairs) Gestational age (weeks; mean 4- SE) Birthweight (g; mean 4- SE) Test age (years; mean 4- SE)
20 11 ~ - 9 d' 36.4 4- 0.5 2514 4- 87.4 10.9 __ 0.2
DC-MZ 12 9 36.6 2532 9.9
24 - 12 d _ 0.4 4- 79.2 4- 0.2
a NS NS NS
