A new family handedness sample with findings consistent with X-linked transmission.

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.

Developmental instability and right shift theory hypotheses concerning correlates of familial sinistrality: negative findings.

Both the right shift (Annett, 1985) and developmental instability (Yeo & Gangestad, 1993) theories of handedness predict that familial sinistrality (FS) should relate to hand skill asymmetries (HSA) on peg-moving tasks. Annett also hypothesises that both HSA and a procedure she derived for classifying individuals according to the pattern of their hand preferences for different manual activities can index the genotypes posited in her theory, i.e. she believes these variables are highly correlated. These hypotheses were tested in a sample of 280 dextral college students. Results failed to support the finding of Gangestad and Yeo (1994) that subjects showing greater hand skill asymmetry deviations from typical asymmetry had a greater probability of having a left-handed parent. There was also no support for Annett's finding that greater dextrality, as defined by her hand preference classification system, was negatively associated with familial sinistrality. Additionally, the relationship between HSA and Annett's hand preference classes was found to be exceedingly weak, contrary to Annett's theorising. These and other failures to find highly replicable differences between FS- and FS+ dextrals cast doubt on the validity of genetic or partial-genetic theories of handedness that posit a ''penetrance'' of a recessive non-dextrality-favouring gene that causes presumed dextral heterozygotic persons to differ from presumed homozygotic dominant persons in cognitive or manual skills.

Family size, miscarriage-proneness, and handedness: tests of hypotheses of the developmental instability theory of handedness.

The handedness theory of Yeo and Gangestad (1993) posits that moderate right-handedness is the modal manifestation of genetic handedness and that ''developmental instability'' produces deviations from modal handedness. It is also suggested (1) that sinistral parents may produce fewer offspring than do dextral parents; and (2) that sinistral mothers may be more prone to miscarriages than are dextral mothers. In line with these hypotheses, Gangestad et al. (1996) reported that a human leukocyte antigen (B8) was related to both left-handedness and to reduced offspring number in their study. They also found that left-handedness was related to the human leukocyte antigen DR3, and Yeo and Gangestad (1998) noted that this antigen has been found by Christiansen et al. (1996) to be associated with an increased risk of spontaneous abortion in women. We assessed the first hypothesis through a study of the family sizes of 2083 families with two right-handed parents and 502 families having one or more left-handed parents; we assessed the second hypothesis from miscarriage history data supplied by 300 dextral and 52 sinistral mothers. Results supported the developmental instability theory with respect to the hypothesis regarding family size, but not with respect to the hypothesis regarding miscarriage-proneness.

Failure to support the right-shift theory's hypothesis of a 'heterozygote advantage' for cognitive abilities.

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.

On language laterality in normal dextrals and sinistrals: results from the bilateral object naming latency task.

Rasmussen and Milner [N.Y.Acad. Sci. Vol. 299, pp. 355-379, 1977] published data on late-lesioned (after age 6) epileptic patients who had suffered left hemisphere lesions. They estimated that left hemisphere dominance occurred in 96% of dextrals and 70% of sinistrals. These figures have been regarded as valid estimates for normal dextrals and sinistrals. We administered the Bilateral Object Naming Latency Task, a verbal tachistoscopic task with very good psychometric properties, to 188 dextral and 72 sinistral normals. Results showed that 93.6% of the dextrals and 80.3% of the sinistrals were left hemisphere dominant. A consideration of results from a number of carefully conducted dichotic listening studies suggests, as do present results, that the 70% left-dominance estimate of Rasmussen and Milner for normal sinistrals may be too low by about 10%. It is suggested that 'bilateral dominance', present in 15% of the epileptic sinistrals of Rasmussen and Milner, may be much less common in normal sinistrals.

Maternal age and parity correlates of handedness: gender, but no parental handedness modulation of effects.

Mothers supplied information on the handedness of a total of 1079 children (238 left handed, 841 right handed), and also reported information on the handedness of the biological parents, parental ages when each child was born, parity, and birth complications. A MANOVA, employing offspring handedness and parental handedness (presence versus absence of at least one left handed) as the bases of classification, and parity, maternal age, paternal age, and a birth stress composite score as dependent measures, was applied to the data of female and male offspring separately. For females, there was a significant multivariate effect for handedness, but no effect of parental handedness nor any interaction of the handedness and parental handedness factors. Univariate ANOVAs, following the multivariate analysis, showed significant effects of handedness for the maternal age and parity measures, but not for paternal age or birth stress composite score. The analysis for males showed no significant multivariate or univariate effects. The data suggest, that in the absence of high risk parity and maternal age over 32, only about 7.8% of females are left handed. This implies that as many as 29% or so of female left handers may owe their sinistrality to factors associated with high risk parity and maternal ages over 32.

Cross-modal correlation of dichotic and tachistoscopic language laterality tasks: the importance of familial sinistrality.

Previous studies have found little if any correlation between dichotic and tachistoscopic language laterality task performance asymmetries. Problems with these studies have been that quite dissimilar auditory and visual tasks have often been used, and the reliability of the asymmetry measures has generally been unknown or, when known, relatively poor. We assessed the cross-modal correlation for two tasks, the Bilateral Object Naming Latency Task (BONLT) and the Dichotic Object Naming Latency Task (DONLT). These tasks are highly similar and have demonstrated high reliabilities. A significant, though rather small, cross-modal correlation was found (r = +.28). When cross-modal correlations were computed for FS- and FS+ subjects separately, no correlation was found for FS+ subjects (r = +.02), but the correlation for FS- subjects was highly significant (r = +.54, p less than .004). This led us to reexamine some previously collected data (P.L. Van Eys and W. F. McKeever, 1988, Brain and Cognition, 4, 413-429) which had administered two highly reliable language laterality tasks (the BONLT and the Dichotic Consonant Vowel Task), but had not assessed cross-modal correlation. A significant cross-modal correlation was found for FS- but not for FS+ subjects. The results are consistent with the hypothesis of H. Hecaen, M. De Agnostini, and A. Monzon-Montes (1981, Brain and Language, 12, 261-284) which suggests that one effect of FS+ is to induce a greater heterogeneity of localizations of different language processes.

Left handedness and immune disorders.

The Geschwind-Behan hypothesis that immune disorder (IMD) is more common among left than among right handed persons was tested in a sample of 3080 college students. Subjects indicated, for each of a list of IMDs, whether they: (1) had no reason to believe they had the disorder; (2) thought they might have the disorder; (3) had the disorder diagnosed by a physician; or (4) had had a disorder diagnosed and treated by a physician. Females reported significantly more IMDs than did males. Among females, the distribution of responses across the four response categories was not different for left and right handers, but IMD was significantly higher among left handed females when only the extreme categories were considered. Among males, no handedness effect was seen across the four categories, nor when only the extreme categories were considered. Laterality quotients, from the Edinburgh Handedness Inventory, were unrelated to IMD reports in both sexes. Results provide no real support for the Geschwind-Behan hypothesis. Even in the case of the extreme categories of IMD reports of females, the association of left handedness (for writing) and IMD accounts for only three-tenths of one percent of the variance.

An investigation of immune system disorder as a "marker" for anomalous dominance.

Geschwind and Galaburda (1987) proposed that immune disorder (ID) susceptibility, along with left handedness and familial sinistrality (FS), is a "marker" for anomalous dominance. The theory predicts lesser left lateralization for language processes, lessened left hemisphere abilities, and enhanced right hemisphere abilities. We assessed language laterality (dichotic consonant vowel task) and performances on spatial and verbal tasks. Subjects were 128 college students. The factors of handedness, sex, FS, and immune disorder history (negative or positive) were perfectly counterbalanced. Left-handers were significantly less lateralized for language and scored lower than right-handers on the spatial tasks. Females scored lower on mental rotation than males, but performed comparably to males on the spatial relations task. The only effect of ID was by way of interaction with FS on both spatial tasks--subjects who were either negative or positive on both FS and ID status factors scored significantly higher than subjects negative for one but positive for the other factor. A speculative explanatory model for this interaction was proposed. The model incorporates the notion that FS and ID factors are comparably correlated, but in opposite directions, with hormonal factors implicated by other research as relevant for spatial ability differences. Finally, no support for the "anomalous dominance" hypothesis predictions was found.

Inverted handwriting posture in left handers is related to familial sinistrality incidence.

McKeever (1979) reported that left handers who wrote with an inverted handwriting posture (IHP) had a higher incidence of left handedness in their families than did left handers who wrote with a non-inverted handwriting posture (NHP). Parlow and Kinsbourne (1981), however, failed to find a relationship between handwriting posture (HWP) and positive familial sinistrality (FS+). McKeever had classified his left handed subjects according to whether they had several, one, or no left handers among their first degree relatives and biologically-related second degree relatives. Parlow and Kinsbourne classified subjects as FS- or FS+ on the basis of first degree relatives and did not inquire about the handedness of second degree relatives. In the present study, handedness for writing of all first and second degree relatives was recorded for 216 NIP and 255 IHP left handers. Results showed a significant association of HWP and FS factors when second degree relatives were considered, but not when second degree relative handedness was ignored. The relationship was strongest within the maternal family line, where the proportion of left handers was more than twice as high for the IHP as for the NHP group. These findings suggest that HWP may well be of some importance for a complete understanding of the cerebral organization of left handers.

Language laterality in Navajo reservation children: dichotic test results depend on the language context of the testing.

Language lateralization in 40 Navajo and 20 Anglo fifth graders was assessed via the Dichotic Consonant-Vowel Task (DCVT). One group of Navajo children was tested by an experimenter who spoke only Navajo with them. The other Navajo group and the Anglo group were tested by an experimenter who spoke only English to them. Strong right ear advantages (REAs) were obtained for the Anglo group and for the Navajo group tested by the Navajo-speaking experimenter. The Navajo group tested by the English-speaking experimenter showed minimal, nonsignificant REAs. Previous findings of an absence of REAs in Native American children failed to consider the possibility that this might occur only when the experimenter does not speak the dominant language of the children. Our results are not consistent with the view that Native Americans are more right hemisphere dominant as a function of an "appositional" mode of language and thought.

Subject knowledge of the experimenter's interest in handedness and familial sinistrality variables and laterality test outcomes.

We investigated the possible influence of common "demand characteristics" on the results on behavioral language laterality tasks. The demand characteristics were (1) whether Ss were or were not recruited for study according to their handedness, and (2) whether a detailed familial sinistrality inquiry was conducted before or after the tasks. Tasks were the Dichotic Consonant Vowel Task (DCVT) and the Bilateral Object Naming Latency Task (BONLT). Results showed no effect of demand characteristics on the BONLT. On the DCVT, however, prior inquiry regarding familial sinistrality was associated with significantly reduced right ear advantages (REAs). Interactions showed that the greatest reduction in REAs occurred in Ss with left-handedness in their families, who were asked about familial sinistrality prior to the tasks, who took the DCVT after the BONLT, and who were also recruited by handedness. Results, though generally reassuring regarding the robustness of the tasks against these sources of bias, raise some cautions for the DCVT.

Absence of exposure time influence on lateralized face recognition and object naming latency tasks.

J. Sergent (1982, Perception & Psychophysics, 31, 451-461; 1983, Psychological Bulletin, 93, 481-512) postulates that the left cerebral hemisphere preferentially extracts higher spatial frequency information, while the right hemisphere preferentially extracts lower frequency spatial information, from the visual scene. According to this view, shorter exposure times favor better right than left hemisphere performance, while longer exposure times favor better left than right hemisphere performance on tachistoscopic laterality tasks. We studied the effects of a threefold variation (40 msec versus 120 msec) in exposure duration, with constant 3-mL luminance, on face recognition and on object naming latency task performances. These are the same stimulus parameters employed by J. Sergent (1983, Psychological Bulletin, 93, 481-512) to demonstrate exposure duration effects in a task requiring the judgment of the sex of models from face photographs. We found the expected LVF superiority on the face recognition task and RVF superiority on the object naming task. There was, however, no influence of exposure duration on the performances. It is concluded that these tasks, which tap established lateralized processing asymmetries, are quite robust in their resistance to exposure time influence.

The influences of handedness, sex, familial sinistrality and androgyny on language laterality, verbal ability, and spatial ability.

This study assessed possible influences of handedness, sex, familial sinistrality (FS), and self-rated androgyny on language laterality and on spatial and verbal test performances of 225 right-handers and 134 left-handers. Left-handers and right-handers performed differently on all tasks. Right-handers were significantly more lateralized on the language laterality task and were superior to left-handers on the spatial visualization task. Left-handers obtained slightly, but significantly, higher vocabulary scores. Androgyny had a significant main effect on spatial ability, the more androgynous subjects being superior. Androgyny also interacted with sex and FS factors on the spatial task. All in all, present result are consistent with the conclusion that each of the subject factors studied may prove to be of some relevance for a complete understanding of laterality and cognitive ability patterns.

Evidence that fixation digits can contribute to visual field asymmetries in lateralized tachistoscopic tasks.

Fixation point digits have been widely used in tachistoscopic laterality studies as a simple and convenient means of ensuring unihemispheric projection of stimulus materials to the hemispheres. Previous findings demonstrate that fixation digits do not influence asymmetries in recognition accuracy studies with adult Ss. Present results, comparing four conditions differing in their use of fixation digits, show that in the naming latency paradigm the requirement to remember and report fixation control digits significantly augments RVF superiority. Implications for other latency tasks are discussed.

The roles of stimulus repetition and hemispheric activation in visual half-field asymmetries.

Hardyck, Tzeng, and Wang (1978, Brain and Language, 5, 56-71) hypothesized that ample repetition of a small number of stimuli is required in order to obtain VHF differences in tachistoscopic tasks. Four experiments, with varied levels of repetition, were conducted to test this hypothesis. Three experiments utilized the general task of object-picture naming and one utilized a word-naming task. Naming latencies constituted the dependent measure. The results demonstrate that for the object-naming paradigm repetition is required for RVF superiority to emerge. Repetition was found to be unnecessary for RVF superiority in the word-naming paradigm, with repetition actually reducing RVF superiority. Experiment I suggested the possibility that RVF superiority developed for the second half of the trials as a function of practice or hemispheric activation, regardless of repetition level. Subsequent experiments, better designed to assess this possibility, clearly refuted it. It was concluded that the effect of repetition depends on the processing requirements of the task. We propose that, for tasks which can be processed efficiently by one hemisphere, the effect of repetition will be to reduce VHF asymmetries; but tasks requiring substantial processing by both hemispheres will show shifts to RVF superiority as a function of repetition.

Handedness and language laterality: discrimination of handedness groups on the dichotic consonant-vowel task.

Previous studies of the performances of left and right handed S groups on the DCVT have failed to discriminate the groups. The present study examined the performance of 102 right handers and 104 left handers on the DCVT, with a free report of only one syllable per trial procedure. Results showed that, as administered, the DCVT possessed good reliability and left and right handed groups were clearly discriminated during the second half of the dichotic trials. Neither sex nor familial sinistrality factors contributed to this group discrimination. Analysis of the relationship of manual preferences for various activities (Edinburgh Handedness Inventory) with DCVT REAs showed that right hand preferences were associated with REAs across groups for most Edinburgh items during the second half of the trials. This association was shown to be strongest in the left handed Ss. In general, the results offer support for both the reliability and validity of the DCVT.

Unilateral versus bilateral presentation methods in the reaction time paradigm.

The use of bilateral presentation in lateralized tachistoscopic reaction time (RT) tasks has been precluded by problems of how to instruct the subject to respond to the left visual field (LVF) or right visual field (RVF) stimulus and by the necessity of fixation control with bilateral presentation. J. Schmuller and R. Goodman (1979, Brain and Language, 8, 81-91; 1980, Brain and Language, 11, 12-18) offered a method, applied to date only in recognition accuracy studies, which can overcome these problems. They used an "arrowhead pointer" at fixation. This instructs the subject as to which stimulus to report first and also controls fixation. We adapted the method to the RT paradigm by having subjects report only the stimulus indicated and applied it in a bilateral presentation version of the Object Naming Latency Task (W. F. McKeever & T. L. Jackson, 1979, Brain and Language, 7, 175-190). Fifty subjects received the unilateral task and 50 received the bilateral task. The bilateral task yielded right visual field superiority in 98% of the subjects and the magnitude of the superiority was over three times as large as in the unilateral task. Different sex x familial sinistrality x VHF interactions obtained on the tasks. Combining the advantages of reaction time and bilateral presentation methods may substantially increase the resolving power of lateralized tachistoscopic tasks.

Failure to replicate the Scott et al. finding of reversed ear dominance in the Native American Navajo.

The apparent universality of left hemisphere language specialization in neurologically normal right-handed persons has been questioned by Scott et al. (Neuropsychologia 17, 89-92, 1979). They found that Native American Navajos had a significant left ear advantage on the dichotic C-V task, while Anglo Ss had a right ear advantage. We attempted to replicate this finding with essentially the same task, more Ss and four times as many trials. Results showed a very clear right ear advantage in Navajos. While we had previously questioned the Scott et al. conclusion on the basis of visual laterality test findings, present results directly contradict those of Scott et al.

Interacting sex and familial sinistrality characteristics influence both language lateralization and spatial ability in right handers.

In 1981 McKeever and Hoff found a pattern of sex-familial sinistrality (FS)-visual field interaction on an Object Naming Latency Task. The FS - females and FS + males performed more symmetrically. Also in 1981, Hécaen, DeAgostini, and Monzon-Montes found a substantially lower incidence of aphasia following left hemisphere lesions in FS - females and FS + males. Both findings suggest a less strict left hemisphere dominance in FS - females and FS + males. We report a replication of the McKeever and Hoff findings and also evidence of a pattern of sex-FS interaction in spatial visualization ability wherein FS - females and FS + males perform better than FS + females and FS - males.