Handedness and earnings

The effect of handedness on upper extremity isometric strength symmetry

BACKGROUND

Understanding upper extremity strength symmetry can have important implications for ergonomics assessment and design. Few studies have examined isometric joint strength symmetry of left-handed individuals, or examined how handedness can influence strength. As such, the purpose of this study was to investigate the influence of handedness on dominant/non-dominant (D/ND) strength ratio in several moment directions about the shoulder, elbow, wrist, and hand. It was hypothesized that the isometric strength symmetry of left-handed individuals would be significantly different from that of right-handed individuals.

METHODS

The study recruited 28 participants to perform a series of maximal voluntary isometric contractions (MVCs) with both arms for various efforts about the shoulder, elbow, and wrist, as well as handgrip for a total of 68 MVCs. Strength symmetry ratios were computed, and a two-way mixed-model ANOVA evaluated the effects of handedness and MVC test on strength symmetry.

INTERPRETATION

Significant differences in D/ND ratios between right and left-handed individuals were found for 11 of the 17 explored exertion directions. Left-handed individuals tended to possess greater strength in their non-dominant limb, while right-handed individuals tended to have greater strength in their dominant limb. Left- and right-handed individuals often significantly differed in D/ND ratio, suggesting that separate normative values should be created to account for handedness when considering return-to-work or strength-based ergonomics design criterion.

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).

Context-Dependent Gestural Laterality: A Multifactorial Analysis in Captive Red-Capped Mangabeys

Catarrhine primates gesture preferentially with their right hands, which led to the hypothesis of a gestural origin of human left-hemispheric specialization for language. However, the factors influencing this gestural laterality remain understudied in non-hominoid species, particularly in intraspecific contexts, although it may bring valuable insights into the proximate and ultimate causes of language lateralization. We present here a preliminary investigation of intraspecific gestural laterality in catarrhine monkeys, red-capped mangabeys (Cercocebus torquatus). We described the spontaneous production of brachio-manual intentional gestures in twenty-five captive subjects. Although we did not evidence any significant gestural lateralization neither at the individual- nor population-level, we found that mangabeys preferentially use their right hands to gesture in negative social contexts, such as aggressions, suggesting an effect of emotional lateralization, and that they adapt to the position of their receiver by preferentially using their ipsilateral hand to communicate. These results corroborate previous findings from ape studies. By contrast, factors related to gesture form and socio-demographic characteristics of signaler and receiver did not affect gestural laterality. To understand better the relationships between gestural laterality and brain lateralization from an evolutionary perspective, we suggest that the gestural communication of other monkey species should be examined with a multifactorial approach.

Competitiveness, gender and handedness

We conduct an intercultural experiment in three locations on three different continents to elicit competitiveness and study whether individual differences in competitiveness are related to handedness. Being a "lefty" (i.e., having either a dominant left hand or a dominant left foot) is associated with neurological differences which are determined prenatally, and can therefore be seen as a proxy for innate differences. In large-scale data with incentivized choices from 3664 participants from India, Norway and Tanzania, we find a significant gender gap in competitiveness in all cultures. However, we find inconsistent results when comparing the competitiveness of lefties and righties. In north-east India we find that lefties of both genders are significantly more competitive than righties. In Norway we find that lefty men are more competitive than any other group, but women's competitiveness is not related to handedness. In Tanzania, we find no relationship between handedness and the competitiveness of either gender. The merged data show weak evidence of a positive correlation between being a lefty and competitiveness for men, but no such evidence for women. Thus, our data provide suggestive but not robust evidence that individual and gender differences in competitiveness are partially determined by innate factors, where innate factors are proxied by the complex, prenatally shaped trait of handedness.

Prevalence of Left-handedness in China 2011: Small-area Estimates

Nationally representative survey data and small-area estimation techniques are used to assess geographic prevalence of left-handedness in China 2011. Measures of individuals' handedness are self-reported dominant hand and hand grip strength, yielding four estimates of left-handed prevalence at the provincial level. These estimates concord with one another. There are several geographic clusters of high-prevalence rates of left-handers located in ethnic minority-designated autonomous areas or historically revolutionary base areas, which may reflect a deep-rooted sense of defiance to authorities and promote such cultural values as individual autonomy and equality among local people.

The androgen model of suicide completion

Suicide is a devastating public health issue that imposes severe psychological, social, and economic burdens not only for the individuals but also for their relatives, friends, clinicians, and the general public. Among the different suicidal behaviors, suicide completion is the worst and the most relevant outcome. The knowledge of biological etiopathological mechanisms involved in suicide completion is limited. Hitherto, no objective markers, either alone or in combination, can reliably predict who will complete a suicide. However, such parameters are strongly needed to establish and optimize prediction and prevention. We introduce here a novel ideation-to-completion framework in suicide research and discuss the problems of studies aiming at identifying and validating clinically useful markers. The male gender is a specific risk factor for suicide, which suggests that androgen effects are implicated in the transition from suicidal ideation to suicide completion. We present multiple lines of direct and indirect evidence showing that both an increased prenatal androgen load (with subsequent permanent neuroadaptations) and increased adult androgen activity are involved in suicide completion. We also review data arguing that modifiable maternal behavioral traits during pregnancy contribute to the offspring's prenatal androgen load and increase the risk for suicide completion later in life. We conclude that in utero androgen exposure and adult androgen levels facilitate suicide completion in an synergistic manner. The androgen model of suicide completion provides the basis for the development of novel predictive and preventive strategies in the future.

Foot-preference underlies bite-scar asymmetry in the gecko Ptyodactylus guttatus

Scar-asymmetry may reflect brain laterality because scar location may reflect behaviour when being attacked. This has been studied in a few organisms, but never in lizards. Wild geckos (Ptyodactylus guttatus) from Israel were examined for bite-scar numbers and their lateral asymmetry. Social status was documented in the field before capture. Foot-preference, for either the right (R-footed) or left (L-footed) hind leg was determined in six trials for each gecko on adults captured in Jerusalem. I studied 48 geckos: 15 R-footed, 6 ambidextrous and 9 L-footed females; 6 R-footed, 3 ambidextrous and 9 L-footed males. Adults showed significantly more bite-scars than juveniles. The proportion of L-footed males mirrored that of R-footed females. Ambidextrous and L-footed geckos had a higher social status. In males, R-footed individuals had more bite-scars on the right side of the body, while L-footed individuals had more on the left side. R-footed females had more bite-scars on the right side, while L-footed females had bite-scars equally on both sides. Bite-scar asymmetry correlated with hind-leg preference, clearly reflecting brain laterality. Since all ambidextrous males had high social status, that may be the driving factor behind the females' different bite-scar pattern.

Handedness, Earnings, Ability and Personality. Evidence from the Lab

Evidence showing that on average left-handed (L), who are 10% in a population, tend to earn less than others is solely based on survey data. This paper is the first to test the relationship between handedness and earnings experimentally and also to assess whether the mechanism underlying it is predominantly cognitive or psychological. Data on 432 undergraduate students show that L do not obtain significantly different payoffs, a proxy for earnings, in a stylised labour market with multiple principals and agents. Similarly, scores in the Cognitive Reflection Test are not significantly different. Data on personality, measured using the Big Five test, show, instead, that L are significantly more agreeable and L females more extroverted. In addition, earnings significantly vary with personality only for L, increasing with extraversion and decreasing with neuroticism. Overall, our results fail to reject the null hypothesis that earnings do not differ by handedness and suggest differences in personality as a novel mechanism to rationalise L’s behaviour.

In praise of ambidexterity: How a continuum of handedness predicts social adjustment

This paper estimates the relationship between handedness and social adjustment in children. In addition to binary measures of hand preference, we also use a continuous measure of relative hand skill. Outcomes at ages 7, 11 and 16 are studied. The data used is the British 1958 Birth. Using a partially linear semi-parametric regression estimator, it is shown that non-right-handedness (as hand preference) is associated with poorer social adjustment but this effect weakens as individuals age into their teens. The continuous measure of hand skill has a non-monotonic effect on social adjustment with poorer social adjustment in the tails of the continuum. The results are consistent with a growing body of evidence which shows that it is the consistency or degree of laterality (rather than direction) that is important for many outcomes.

Cognitive benefits of right-handedness: a meta-analysis

Hand preference - which is related to cerebral dominance - is thought to be associated with cognitive skills; however, findings on this association are inconsistent and there is no consensus whether left- or right-handers have an advantage in either spatial or verbal abilities. In addition, it is not clear whether an interaction between sex and hand preference exists in relation to these cognitive abilities. As these matters are relevant from a neurodevelopmental perspective we performed a meta-analysis of the available literature. We searched PubMed and Embase, and included 14 studies (359,890 subjects) in the verbal ability meta-analysis and 16 studies (218,351 subjects) in the spatial ability meta-analysis. There was no difference between the full sample of left and right-handers for verbal ability, nor was there a hand preference-by-sex interaction. Subgroup analysis of children showed a small right-hand benefit. Our results further revealed a modest but significant effect favouring right-handedness for overall spatial ability, which was more pronounced when analysis was restricted to studies applying the mental rotation test. We could not identify a specific interaction with sex. Our results indicate that there is a small but significant cognitive advantage of right-handedness on spatial ability. In the verbal domain, this advantage is only significant in children. An interaction effect with sex is not confirmed.

Intrahousehold resource allocation: do parents reduce or reinforce child ability gaps?

Do parents invest more or less in their high-ability children? We provide new evidence on this question by comparing observed ability differences and observed investment differences between siblings living in the United States. To overcome endogeneity issues, we use sibling differences in handedness as an instrument for cognitive ability differences. We find that parents invest more in high-ability children, with a 1 standard deviation increase in child cognitive ability increasing parental investments by approximately one-third of a standard deviation. Consequently, differences in child cognitive ability are enhanced by differential parental investments.

Nature's experiment? Handedness and early childhood development

In recent years, a large body of research has investigated the various factors affecting child development and the consequent impact of child development on future educational and labor market outcomes. In this article, we contribute to this literature by investigating the effect of handedness on child development. This is an important issue given that around 10% of the world's population is left-handed and given recent research demonstrating that child development strongly affects adult outcomes. Using a large, nationally representative sample of young children, we find that the probability of a child being left-handed is not significantly related to child health at birth, family composition, parental employment, or household income. We also find robust evidence that left-handed (and mixed-handed) children perform significantly worse in nearly all measures of development than right-handed children, with the relative disadvantage being larger for boys than girls. Importantly, these differentials cannot be explained by different socioeconomic characteristics of the household, parental attitudes, or investments in learning resources.

Why are some people left-handed? An evolutionary perspective

Since prehistoric times, left-handed individuals have been ubiquitous in human populations, exhibiting geographical frequency variations. Evolutionary explanations have been proposed for the persistence of the handedness polymorphism. Left-handedness could be favoured by negative frequency-dependent selection. Data have suggested that left-handedness, as the rare hand preference, could represent an important strategic advantage in fighting interactions. However, the fact that left-handedness occurs at a low frequency indicates that some evolutionary costs could be associated with left-handedness. Overall, the evolutionary dynamics of this polymorphism are not fully understood. Here, we review the abundant literature available regarding the possible mechanisms and consequences of left-handedness. We point out that hand preference is heritable, and report how hand preference is influenced by genetic, hormonal, developmental and cultural factors. We review the available information on potential fitness costs and benefits acting as selective forces on the proportion of left-handers. Thus, evolutionary perspectives on the persistence of this polymorphism in humans are gathered for the first time, highlighting the necessity for an assessment of fitness differences between right- and left-handers.

Is the decreased longevity among left-handers related to an increase in heart disease?

Many studies report that left-handers have a shorter longevity than right-handers, and the present study may provide a possible explanation for that finding. In a Cardiac Rehabilitation Unit for the elderly with a mean age of 75.2 years the prevalence of left-handers was 16.7%. This latter value was significantly different (p = 0.0028) from the 6.7% in controls of similar age. These data suggest that heart disease may be one reason for a reduced longevity among left-handers. Left-handers use the right hemisphere for movement, and unilateral activation of that hemisphere in the form of EEG desynchronization and deactivation in the form of EEG slow waves are both related to cardiac abnormalities.