Left-handers are often excluded from study cohorts in neuroscience and neurogenetics in order to reduce variance in the data. However, recent investigations have shown that the inclusion or targeted recruitment of left-handers can be informative in studies on a range of topics, such as cerebral lateralization and the genetic underpinning of asymmetrical brain development. Left-handed individuals represent a substantial portion of the human population and therefore left-handedness falls within the normal range of human diversity; thus, it is important to account for this variation in our understanding of brain functioning. We call for neuroscientists and neurogeneticists to recognize the potential of studying this often-discarded group of research subjects.

/pdf/on-the-other-hand-including-left-handers-in-cognitive-5ahkbpiy3r.pdf

On the other hand: including left-handers in cognitive neuroscience and neurogenetics

The measurement of handgrip strength (HGS) is an indicator of an individual’s overall strength and can serve as a predictor of morbidity and mortality. This study aims to investigate whether HGS is associated with handedness in young adults and if it is influenced by anthropometric characteristics, body composition, and sport-related parameters. We conducted a cross-sectional study on a sample of 544 young Italian adults aged 18–30 years. We measured HGS using a dynamometer and collected data on handedness and physical activity, along with anthropometric measurements. In both sexes, the HGS of the dominant side was significantly greater than that of the non-dominant side. Furthermore, in ambidextrous individuals, the right hand was stronger than the left. A comparison between the lowest and the highest tercile of HGS highlighted its significant association with anthropometric and body composition parameters in both sexes. Moreover, sex, dominant upper arm muscle area, arm fat index, fat mass, and fat-free mass were found to be significant predictors of HGS by multiple regression analysis. Our findings suggest that HGS is especially influenced by body composition parameters and handedness category. Therefore, HGS can be used as a proxy for unhealthy conditions with impairment of muscle mass, provided that the dominance in the laterality of the subject under examination is taken into account.

https://www.mdpi.com/1660-4601/17/12/4273/pdf

Handgrip Strength in Young Adults: Association with Anthropometric Variables and Laterality

http://personperceptionlab.org/Carey%20Cortex%202009.pdf

The bi-pedal ape: Plasticity and asymmetry in footedness

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.

/pdf/intrahousehold-resource-allocation-do-parents-reduce-or-3exm8uy0cf.pdf

Intrahousehold Resource Allocation: Do Parents Reduce or Reinforce Child Ability Gaps?

The number of penises vary in the insect suborder Forficulina (order Dermaptera; earwigs). Males of the families Diplatyidae, Pigidicranidae, Anisolabididae, Apachyidae, and Labiduridae have two penises (right and left), while those of the Spongipohridae, Chelisochidae, and Forficulidae have a single penis. The proposed phylogenetic relationships among these families suggest that the single-penis families evolved from an ancestor possessing two penises. To date, examinations of double-penis earwig species have found that only a single penis is used per single copulation. These diversities in structural and behavioral aspects of genitalia raises the following intriguing questions: How are the two penises used? Why did a penis degenerate in several earwig families, and which one was lost? To address these questions, structural and behavioral asymmetries were examined in detail for a representative species Labidura riparia (Labiduridae). Although there was no detectable morphological differentiation between the right and left penises, male L. riparia predominantly used the right one for insemination. This significant "right-handedness" developed without any experience of mating and was also manifested in the resting postures of the two penises when not engaged in copulation. However, surgical ablation of the right penis did not influence the insemination capacity of males. In wild-caught males, only about 10% were left-handed; within this group, abnormalities were frequently observed in the right penis. These lines of evidence indicate that the left penis is merely a spare intromittent organ, which most L. riparia males are likely never to use. Additional observations of five species of single-penis families revealed that the left penis degenerated in the common ancestor of this group. Considering the proposed sister relationship between the Labiduridae and the single-penis families, it is possible that such behavioral asymmetries in penis' use, as observed in L. riparia, are parental to the evolutionary degeneration of the infrequently used left penis.

http://www.bio-nica.info/Biblioteca/Kamimura2006Dermaptera.pdf

Right‐handed penises of the earwig Labidura riparia (Insecta, Dermaptera, Labiduridae): Evolutionary relationships between structural and behavioral asymmetries

Lateral preferences in humans and other primates may be a key to many important issues in biology and psychology. There is strong evidence that the representation of language functions in the cerebral cortex is different in left-handed (LH) people than in right-handed (RH) people, and an understanding of handedness may lead to valuable clues as to how the brain becomes organised in the way that it does. Although there are clearly cultural effects that influence manual activities, there is nevertheless evidence that human handedness and other lateralities are at least in part genetically determined: compared with two RH parents, one RH and one LH parent are 2-3 times more likely to have an LH child, and two LH parents are 3-4 times more likely to have an LH child. Thus, one might wish to investigate the genetics of laterality with the goal of understanding the biological mechanisms that lead to the preferential use of one hand (or eye or foot). One may also see links between human handedness and footedness...

On the Genetics and Measurement of Human Handedness

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On the Genetics and Measurement of Human Handedness