A philosophical approach to the concept of handedness: The phenomenology of lived experience in left- and right-handers

A comprehensive scoping review and meta-analysis of upper limb strength asymmetry

The "10% rule" of handedness asserts the dominant hand is 10% stronger than the non-dominant hand. Primarily derived from handgrip data, it is unclear if a generalized asymmetry exists across the upper limb. Understanding how strength asymmetry may be affected by handedness, sex, and exertion type has important implications for ergonomics design, sports performance, and clinical rehabilitation. The purpose of this study was to systematically synthesize currently available evidence examining upper limb strength asymmetry. 10,061 results were retrieved, and 174 studies remained after title/abstract screening. 87 studies were synthesized. Results are compiled by exertion type and manner of asymmetry comparison (i.e. right/left, dominant/non-dominant). Asymmetry ratios were calculated to examine the effects of handedness, exertion side, arm region, and sex. Strength differences were most frequently reported for grip exertions (n = 49). 25 studies reported other joint strength asymmetries. Overall, the right limb was 6.7% stronger than the left limb (n = 9342) and the dominant limb was 11.6% stronger than the non-dominant limb (n = 9327), though strength asymmetry varied across joints and movements (2.1% to 19.5%). This research demonstrates that the 10% rule is a good approximation for upper limb strength asymmetry. However, several factors, including joint, movement type, and sex, can affect this relationship.

The role of sex and handedness in the performance of the smartphone-based Finger-Tapping Test

The Finger Tapping Test (FTT) is a classical neuropsychological test that assesses motor functioning, and recently it has been employed using smartphones. For classical protocols, it has been observed that sex and handedness influence the performance during the test. By assessing the influence of sex and handedness on the test, it is possible to adjust the performance measurements to ensure the validity of test results and avoid sex- and handedness-related bias. The present study aimed to evaluate the influence of sex and handedness on smartphone-based FTT performance. We developed an Android application for the FTT and recruited 40 males and 40 females to carry out three spatial designs on it (protocols I, II, and III). Participants' performance was measured using the global, temporal, and spatial parameters of the FTT. We observed that for the performance in protocol I, handedness had a significant influence on global and temporal variables, while the interaction between handedness and sex had a greater influence on spatial variables. For protocols II and III, we observed that handedness had a significant influence on global, temporal, and spatial variables compared to the other factors. We concluded that the smartphone-based test is partly influenced by handedness and sex, and in clinical implications, these factors should be considered during the evaluation of the smartphone-based FTT.

The sinister story of a gauche deliverer and his corrie-fisted tribesmen: Ehud and the left-handed artillery

Bias against left-handers is well-documented and seen in the etymology of "left" and "right" in most languages. The subject of this study, Ehud, lived between the exodus of the Hebrew slaves from Egypt and the establishment of the Israelite kingdom (c1200-1000 BC), at the transition between the Late Bronze Age and Iron Age 1. His left-handedness was crucial to his deliverance of the proto-nation from tyranny, recorded in Judges in the Hebrew Bible. The description of Ehud as left-handed ('iṭṭēr yaḏ-yεmînô) is used once more in the Hebrew Bible, also in Judges, to describe the artillery of Ehud's tribe. The words apparently mean "bound/restricted in the right hand", sometimes interpreted as "ambidextrous". This is unlikely: ambidexterity is uncommon. The artillery used the sling with either hand, but Ehud used his left (śεm'ōl) hand to draw his sword. śεm'ōl, used throughout the Hebrew Bible, means left, without bias or derogatory implications. We suggest that 'iṭṭēr yaḏ-yεmînô was a right-handed bias towards left-handed people, but Ehud's left-handed victory was recognized as significant. Significant enough that (a) the language changed and the biased description was dropped in favour of simple description, and (b) the army changed with the development of left-handed slingers (artillery).

Left-Handers Are Less Lateralized Than Right-Handers for Both Left and Right Hemispheric Functions

Many neuroscientific techniques have revealed that more left- than right-handers will have unusual cerebral asymmetries for language. After the original emphasis on frequency in the aphasia and epilepsy literatures, most neuropsychology, and neuroimaging efforts rely on estimates of central tendency to compare these two handedness groups on any given measure of asymmetry. The inevitable reduction in mean lateralization in the left-handed group is often postulated as being due to reversed asymmetry in a small subset of them, but it could also be due to a reduced asymmetry in many of the left-handers. These two possibilities have hugely different theoretical interpretations. Using functional magnetic resonance imaging localizer paradigms, we matched left- and right-handers for hemispheric dominance across four functions (verbal fluency, face perception, body perception, and scene perception). We then compared the degree of dominance between the two handedness groups for each of these four measures, conducting t-tests on the mean laterality indices. The results demonstrate that left-handers with typical cerebral asymmetries are less lateralized for language, faces, and bodies than their right-handed counterparts. These results are difficult to reconcile with current theories of language asymmetry or of handedness.

Half a century of handedness research: Myths, truths; fictions, facts; backwards, but mostly forwards

Although most people are right-handed and have language in their left cerebral hemisphere, why that is so, and in particular why about ten per cent of people are left-handed, is far from clear. Multiple theories have been proposed, often with little in the way of empirical support, and sometimes indeed with strong evidence against them, and yet despite that have become modern urban myths, probably due to the symbolic power of right and left. One thinks in particular of ideas of being right-brained or left-brained, of suggestions that left-handedness is due to perinatal brain damage, of claims that left-handers die seven years earlier than right-handers, and of the unfalsifiable ramifications of the byzantine Geschwind-Behan-Galaburda theory. This article looks back over the past fifty years of research on brain asymmetries, exploring the different themes and approaches, sometimes in relation to the author's own work. Taking all of the work together it is probable that cerebral asymmetries are under genetic control, probably with multiple genetic loci, only a few of which are now beginning to be found thanks to very large databases that are becoming available. Other progress is also seen in proper meta-analyses, the use of fMRI for studying multiple functional lateralisations in large number of individuals, fetal ultra-sound for assessing handedness before birth, and fascinating studies of lateralisation in an ever widening range of animal species. With luck the next fifty years will make more progress and show fewer false directions than had much of the work in the previous fifty years.

Profiles of Motor Laterality in Young Athletes' Performance of Complex Movements: Merging the MOTORLAT and PATHoops Tools

Laterality is a key aspect of the analysis of basic and specific motor skills. It is relevant to sports because it involves motor laterality profiles beyond left-right preference and spatial orientation of the body. The aim of this study was to obtain the laterality profiles of young athletes, taking into account the synergies between the support and precision functions of limbs and body parts in the performance of complex motor skills. We applied two instruments: (a) MOTORLAT, a motor laterality inventory comprising 30 items of basic, specific, and combined motor skills, and (b) the Precision and Agility Tapping over Hoops (PATHoops) task, in which participants had to perform a path by stepping in each of 14 hoops arranged on the floor, allowing the observation of their feet, left-right preference and spatial orientation. A total of 96 young athletes performed the PATHoops task and the 30 MOTORLAT items, allowing us to obtain data about limb dominance and spatial orientation of the body in the performance of complex motor skills. Laterality profiles were obtained by means of a cluster analysis and a correlational analysis and a contingency analysis were applied between the motor skills and spatial orientation actions performed. The results obtained using MOTORLAT show that the combined motor skills criterion (for example, turning while jumping) differentiates athletes' uses of laterality, showing a clear tendency toward mixed laterality profiles in the performance of complex movements. In the PATHoops task, the best spatial orientation strategy was “same way” (same foot and spatial wing) followed by “opposite way” (opposite foot and spatial wing), in keeping with the research assumption that actions unfolding in a horizontal direction in front of an observer's eyes are common in a variety of sports.

Faking handedness: Individual differences in ability to fake handedness, social cognitions of the handedness of others, and a forensic application using Bayes' theorem

People usually describe their handedness honestly, but that need not necessarily be the case. A legal case is described of a murder said by the pathologist to be committed by a left-hander but the defendant claimed to be right-handed, and the first author assessed the defendant's handedness as an expert witness. We know of no previous work on faking handedness, and so we tested 30 right-handers and 25 left-handers on various handedness tasks, and then asked the participants to repeat the tasks while faking being of opposite handedness. Social cognitions of handedness were assessed from participants' knowledge of how other right- and left-handers would answer handedness questionnaires. Fake handedness was best differentiated using cursive lower-case sentence writing, upper-case written letters being less good at distinguishing, as also were simple motor tasks. Participants differed in social cognitions of handedness, and those with more accurate social cognitions were better able to fake. Personality measures did not predict faking ability. For forensic purposes a Bayesian analysis was carried out to evaluate the likelihood of right and left hand performance being true rather than faked, and the cursive lower-case writing provided strong posterior odds that, as claimed, the particular defendant was a true right-hander.