The relationship between birth stress and writing hand posture

Family patterns in handedness: evidence for indirect inheritance mediated by birth stress

The most common alternative to a genetic explanation of left-handedness is that sinistrality arises because of birth stress factors. In a sample of 1398 subjects, the association between birth stress and left-handedness was confirmed. More importantly, it was found that left-handed mothers are more likely to have birth-stressed offspring and that the presence of any left-handed sibling increases the likelihood of a history of birth stress in the proband. This was interpreted as suggesting that a plausible alternative to the genetic explanation for the usual pattern of association observed in family studies of handedness (where a left-handed mother increases the probability of left-handed offspring) is that the mother-offspring association may actually be mediated by birth stress rather than representing heritable aspects of handedness.

Twinning is associated with an increased risk of left-handedness and inverted writing hand posture

Left-handedness and inverted handwriting postures are more frequent among birth-stressed individuals. Because twinning is also associated with increased birth risk, 298 twins were compared to 1192 age and sex matched singletons. Both left-handedness and inversion were significantly more common in twins, although other indexes of laterality were not affected.

The diminished number of older left-handers: differential mortality or social-historical trend?

Recent research suggests that left-handers have shorter life spans. This interpretation could be erroneous if there is a social-historical trend in handedness with older individuals more frequently switched to right-handedness. This would result in fewer older left-handers due to social factors. Two samples (N = 1960 and N = 8526), separated by a 12 year interval were tested. Social-historical hypotheses predict that the later sample should have higher proportions of left-handedness. Although both samples showed decreased sinistrality with age, there was no increase in the proportion of left-handers over the 12 year period, suggesting that the rarity of older left-handers is due to reduced longevity.

Are fingerprints a genetic marker for handedness?

Fingerprints and handedness were measured in 2169 subjects. Left-handers were more likely to have an increased number of arches and radial loops, while having fewer whorls and lower pattern intensity than right-handers. Analyzing by hand and digit, the differences were most marked on the left hand, which showed significant differences on four of the five digits which were related to handedness. On the right-hand, handedness was associated with fingerprint patterns only on digit IV (ring finger). This finding suggests a genetic mechanism in the development of handedness and may provide a means of separating pathological from natural left-handers.

Birth stress and lateral preferences

Conflicting evidence exists concerning the possible role of birth stress in the etiology of left-sided lateral preferences. In order to clarify this issue, associations among lateral preferences of hand, eye, and foot and eight indices of prenatal and perinatal stress were examined in the present study on a sample of 987 boys and girls who participated in the Philadelphia Collaborative Perinatal Project. Controls were instituted for some of the methodological and measurement problems encountered in past birth stress and laterality research. Results showed that subjects with different lateral preferences did not differ significantly in their distributions of all but one birth stress items. Hence, there was no substantial evidence for a link between birth stress and left-sided preferences. Alternative hypotheses for the etiology of left-sidedness should therefore be explored.

The role of familial sinistrality in cerebral organization

The literature on the relationship between familial sinistrality (FS) and laterality is conflicting. A large scale investigation employing multiple measures of laterality assessment and rigorous methods of handedness and FS determination was conducted with a normal population of left- and right-handers. The results failed to find a relationship between FS and hemispheric representation of speech despite the fact that a robust relationship was found between handedness and hemispheric speech specialization. Possible reasons for these null findings are discussed.

A review, analysis, and some new data on hand-posture distributions in left-handers

Different methods have been employed to assess handwriting posture in left-handers, and different dimensions of hand posture have been used for categorization. Within North American samples, there is a remarkable consistency between studies in the distributions when the same assessment method is used, but there are very large and highly significant differences for different methods of assessment. The various methods and dimensions of categorization evidently index different variables, and these are likely to have different relations with neuropsychological factors. It is possible, however, to gather questionnaire data on handwriting posture that closely resemble direct experimenter observation, as a brief report on left-handed males shows.

Writing hand posture and cerebral dominance for speech

Levy and Reid's claim (1976, 1978) that writing hand/posture is predictive of cerebral dominance was examined in patients who had undergone bilateral sodium Amytal speech testing. Contrary to their prediction, inverted writers showed the same pattern of speech representation as did straight writers.

Writing hand posture and four indexes of lateral preference

The relationship between writing hand posture and lateral preference for hand, foot, eye, and ear was examined in a sample of 3709 college undergraduates. A markedly different pattern of lateral preferences was observed in left-handed males and females as a function of hand posture. Left-handed male inverters displayed a tendency toward more leftward lateral preferences in all four indexes; while it was found that hand inversion during writing in left-handed females reflected, if anything, a tendency toward more rightward lateral preferences.