Right Shift Theory Of Handedness Research Articles

Laterality and Types of Dyslexia

ANNETT, M. Laterality and types of dyslexia. NEUROSCI BIOBEHAV REV 20(4) 631–636, 1996.—The right shift theory of handedness and cerebral specialization suggests that there is an underlying substrate of random lateral asymmetries in all higher animals and a specific factor in humans which increases the probability of left hemisphere advantage. The specific factor displaces the random distribution along a continuum of asymmetry in favour of the left hemisphere and the right hand. The distribution of handedness in families can be explained if the shift to dextrality depends on a single gene, rs +, when the frequency of the gene is estimated from the proportion of dysphasics with unilateral lesions of the left versus the right hemisphere. The genotype proportions suggested a genetic balanced polymorphism with heterozygote advantage for laterality and ability. Application of the model to developmental dyslexia led to the hypothesis that both homozygotes (rs −− and rs ++ genotypes) are at risk for reading. The critical question was whether the type of cognitive problem differs as expected, speech based processing for the rs −− and non-speech based for the rs ++. The former should include more left and mixed handers, while the latter should be more strongly dextral than controls. Evidence for this double dissociation between type of cognitive disability and type of handedness has been found in poor readers and in other samples. Copyright © 1996 Elsevier Science Ltd.

Handedness as a Function of Twinning, Age and Sex

Questions about twin birth, sex, age and handedness for writing were asked as part of a survey of hearing disability (Davis, 1989) in a large sample of the adult population. The findings show unequivocally that the prevalence of left handedness is higher in twins than in the singleborn, in males than in females and in younger than in older adults. There was a marked and regular decline in the percentage of left writers with increasing age, but the effects for twinning and sex were evident in all of 7 age bands from 18 years to 80+. The findings are consistent with the assumption of the right shift theory of handedness (Annett, 1972, 1985) that the rs + gene is expressed more strongly in females than in males, and in the singleborn than in twins.

Handedness as a continuous variable with dextral shift: sex, generation, and family handedness in subgroups of left- and right-handers.

Percentages of left-handed relatives were compared between subgroups of left-handers and of right-handers to test the prediction that the subgroups are ordered for probability of carrying the rs +/gene (hypothesized by the right-shift theory of handedness). Data included families of undergraduates and Open University students who described their children as well as other relatives. Linear relationships were found between subgroup order and percentage of left-handed relatives. In agreement with previous evidence that some right-writers with weak sinistral preferences (class 2) are in fact more dextral than consistent right-handers (class 1), class 2 tended to have fewer left-handed relatives than class 1 in both samples. Left writers with weak dextral tendencies (class 7) tended to have more left-handed relatives that consistent left-handers (class 8). Comparisons of the distribution of subgroup handedness in undergraduates and their parents revealed strong effects for sex and for generation. In both cases, the main contrasts were not between left-handers and right-handers but between left-handers plus right-handers with weak dextrality and right-handers with strong dextrality. The findings are consistent with the theory that the relevant factor is not handedness as such but, rather, the absence or presence of the rs +/gene.

Handedness and educational success: The hypothesis of a genetic balanced polymorphism with heterozygote advantage for laterality and ability

A sample of children assessed for hand preference and hand skill at 14–15 years could be classified on two independent measures of educational success. One was whether they had been selected or not selected for grammar school two years earlier; the other was the performance of unselected children in the examinations of the General Certificate of Secondary Education (GCSE) some 18 months later. There were no differences for hand preference, but the successful children were significantly more often in the centre of the distribution of differences between the hands in skill than at the extremes, in both comparisons. The findings support the hypothesis prompted by the right shift theory of handedness (Annett, 1972, 1985) that there is natural variation for laterality and ability as expected for a genetic balanced polymorphism with heterozygote advantage. The findings cannot be attributed to slowness of the non‐preferred hand at the extremes of the distribution, as might be predicted by an explanation in terms of developmental pathology.

Inverse relationship between the size of pattern reversal visual evoked potentials from the left brain and the degree of left-hand preference in left-handed normal subjects: importance of the left brain.

The relation of the degree of left-hand preference to pattern reversal visual evoked potentials (VEPs) from right and left brain was studied in male left-handers. The degree of the left-hand preference was assessed by the Waterloo Handedness Questionnaire. Visual stimuli consisted of black and white checkerboard patterns generated on a TV screen. VEPs were simultaneously recorded from occipital leads of the right and left hemispheres. The degree of left-hand preference was found to be inversely and significantly related to size of VEPs only from left brain, not from right brain. That is, the conduction time, amplitude, duration, and area of N1-P1 waves linearly decreased as the degree of left-hand preference increased. These results were in accord with the testosterone hypothesis of cerebral lateralization, but not compatible with the right shift theory of handedness. It was concluded that visuomotor control by the left brain would be the main biological correlate of left-hand preference with regard to sensorimotor and cognitive functions.

Speech Lateralisation Phonological Skill

A phoneme segmentation task requiring Ss to produce deliberate spoonerisms was performed by 78 undergraduates, along with a dichotic monitoring task and tests of hand preference and hand skill. The hypothesis that those performing badly on the speech task would include a higher proportion of individuals with left ear advantage on the dichotic task was supported. The distribution of cases was consistent with other findings for relationships between cerebral speech lateralisation and side of ear advantage and also consistent with predictions of the right shift theory of handedness and cerebral speech (Annett, 1972, 1985).

Mathematical ability and the right-shift theory of handedness

A genetic theory of handedness, the right-shift theory, associates differential patterns of cerebral functioning with contrasting handedness groups and suggests that individuals with an rs++ genotype will be disadvantaged in mathematical performance. This hypothesis is investigated with contrasting handedness groups drawn from a national sample of over 11 000 children from the National Child Development Study. Some differentiation in cognitive performance between handedness groups is found in the direction predicted by the right-shift theory but the level of the findings is not statistically significant. The rs++/mathematical deficit hypothesis is not confirmed.

Comments on Lindesay: Laterality shift in homosexual men

The raised incidences of strong left-handedness and of mixed-handedness in homosexual men [9], as in dyslexics [5], are mutually consistent under the normal distribution function, as expected by the right shift theory of handedness [1]. It is argued that atypical laterality in these groups is better described as a “reduction of right shift” than as a “left shift” [8].

Comments on Tambs, Magnus, and Berg: Twinning and the Right-Shift Theory of Handedness

The evidence of Tambs, et al. (1986) that left-handedness is caused by environmental and not by genetic factors is shown to be consistent with Annett's right-shift theory of handedness. Maternal effects need to be evaluated in the light of possible reporting biases.

Lateral preference and skill in dyslexics: implications of the right shift theory.

The lateral preferences and L-R skill of 109 male and 20 female dyslexics were as expected if the distribution of lateral asymmetry is shifted less far to the right in dyslexics than in controls. Several aspects of the data were consistent with Annett's hypothesis that some dyslexics lack the left hemisphere speech-organising factor postulated by the right shift theory of handedness and that this would be sufficient to account for the proportion of affected relatives. Some dyslexics were strongly dextral and these differed from the less dextral cases in several ways which resembled the distinction between backward and retarded readers.

Right- and left-hand skill II: Estimating the parameters of the distribution of L-R differences in males and females.

The distribution of differences between the hands (L-R) in skill, as measured by a peg-moving task, was examined for several samples in which volunteer bias was absent or minimal. After comparing the main samples for hand preference and L-R times, they were combined to give 617 males and 863 females, aged 12-63 years. There was also a smaller sample of 122 males and 156 females, aged 6-15 years. The L-R distributions were negatively skewed and leptokurtotic. They were not compatible with the sum of two normal distributions, one of right-handers and one of left-handers. They were compatible with the sum of two or three normal distributions when one was unbiased to either side and the others were shifted to the right, as expected for the dominant or additive versions of the single gene interpretation of the right shift theory of handedness (Annett, 1978b, 1979). Sex differences in L-R scores confirmed that females tend to be more dextral than males. This was true of right-handers but not of left-handers, as expected if the sex difference is due to stronger expression of the rs+ gene in females than males and if this gene is absent in the majority of left-handers. There were trends suggesting that L-R asymmetry differs with educational status, undergraduates being less shifted toward dextrality than the general population.

The right shift theory of handedness and developmental language problems

The right shift theory of handedness and developmental language problems

Inheritance of Left-Handedness

Introdclction.-The fact that left-handedness runss in families" has probably attracted the attention of mnany observers, yet the method of inheritance has not been fully studied. Many people imagine the condition to depend entirely upon training or imitation. There is thus much of guesswork concerning the true nature of the condition. Literature.-A considerable bibliography of lefthandedness has recently been cited by Professor H. E. Jordan.2 Most of his references are, however, to articles of little value, especially since nearly all of them were written previous to the modern period of genetic study. Professor Jordan puts forth the tentative opinion that left-handedness is a recessive character. Unfortunately the data which lie presents consist chiefly of a few selected pedigrees from which the reader can obtain very little information. He suggests more than once that some of his cases are examples of the spontaneous appearance of left-handedness in a family. If such spontaneous development were so frequent the whole population would, in a few generations, be left-handed. The appearance of a left-handed child in a family without lefthanded ancestors for three or four generations is not to be considered remarkable, for this is the way in which recessive characters frequently behave. Method of Obtacining Data. At the beginning of a course of lectures on heredity in the University of Colorado in 1911 I distributed papers calling for informa-