No abstract

A family handedness study of 2632 families and 8605 offspring was conducted. Of the 2632 parental couples, there were 2123 in which both parents were right-handed (RR), 232 in which the mother was left-handed and the father right-handed (LR), 254 in which the mother was right-handed and the father was left-handed (RL), and 23 in which both parents were left-handed (LL). Results showed some important differences from the composite results of four earlier large scale studies that had also employed the same criterion of handedness (writing hand). These had collectively found that the incidence of left-handed offspring, of both sexes, was significantly lower for RR couples than for LR or RL couples, but not lower than for LL couples. Present results, however, suggest an X-linked pattern of genetic influence on handedness. The LR parents produced significantly more left-handed offspring than did RR couples, and this was particularly the case for sons; but while RL couples produced a higher incidence of left-handed daughters than did RR parents, they failed to produce a higher incidence of left-handed sons than did RR couples. Additionally, the present sample showed a significantly greater incidence of left-handed offspring of LL couples than of RR couples, a finding that, while predicted by genetic-influence theories of handedness, was not found in the composite of previous comparison samples. The finding of sex-linkage is potentially important, but will require replicative studies that take special care to preclude possible biasing factors such as selective volunteering of participants and inaccurate offspring ascriptions of parental (particularly paternal) handedness.

This experiment enquired: (1) whether right visual field (RVF) recognition superiority was greater in bilateral than in unilateral word presentation; (2) whether left field-favouring attentional or recall sets could be induced by presenting left visual field (LVF) words 20 msec prior to RVF words or by instructions to report LVF words prior to RVF words. Results showed: (1) all conditions studied yielded significant RVF superiority; (2) RVF superiority magnitude was significantly greater in bilateral than in unilateral presentation, suggesting the tenability of hypotheses that different mechanisms operate in these conditions; (3) neither earlier delivery nor earlier report of LVF words altered the pattern of RVF superiority in bilateral presentation, the later result demonstrating that differential receptive organization rather than differential recall of the two stimuli is responsible for RVF superiority in bilateral presentation.

Annett's (1985) 'right-shift' theory of language dominance and handedness posits three genotypes, rs++, rs(+)- and rs(-)-, and Annett has hypothesized that there are cognitive ability correlates of these genotypes. The rs++ genotype person is held to be 'at risk' for maldevelopment of spatial or other right hemisphere-based cognitive abilities, and the rs(-)- genotype individual is held to be at risk for maldevelopment of phonological abilities. Noting that there must be some adaptive advantage conferred by the heterozygous genotype for it to have survived over a presumably long period of evolution, Annett has hypothesized that heterozygotes are afforded an adaptive advantage over homozygotes because of their freedom from 'risks' to intelligence generally. Annett and colleagues have used two different indices, or markers, from which they have inferred differing concentrations of the three genotypes within groups of participants. One marker, based on responses to hand preference items of the Annett Handedness Inventory, was found by Annett (1992) to support her theory in that the least dextral of right-handed participants did best on spatial tests. The other marker Annett has used is based on the degree of right-hand advantage on a simple peg moving speed task. The present study utilized both methods and studied the performances of 259 dextral college men and women on two tests of mental rotation ability and two tests of verbal abilities. Results were not supportive of the heterozygote advantage hypothesis, and suggested that visuospatial ability was modestly related to greater dextrality of participants.

Whether left-handedness is due to genetic factors or to pregnancy and birth stress events is an important question for models that attempt to explain the origins and distribution of human handedness. Major genetic theories of handedness, such as those of Annett (1985) and McManus (1981), allow for a nondetermination of some left- or right-handedness by "chance", but they hold that "pathological left-handedness", specifically, is of minimal influence. On the other hand, theorists such as Bakan (1971) and Coren (1995) take the view that right-handedness is a universal human characteristic, presumably due to a polygenic influence, and that left-handedness is essentially always the product of pregnancy and birth risk factors. Research attempting to find associations of left-handedness and unfortunate pregnancy and birthing events has produced very inconsistent results. We report a large-scale study that employed handedness for writing as the criterion for handedness, mothers' reports regarding the occurrence or nonoccurrence of a large number of possible pregnancy and birth stress events, and mothers' reports regarding the handedness of the biological parents and of their offspring. Of the 25 potential stressors studied, only maternal age showed a significant association with left-handedness of offspring. Further, this relationship was quite weak and would account for no more than 1/84th of the approximate 11% incidence of left-hand-edness in the general population. Implications for theories of handedness are discussed.