Fitness costs and benefits associated with hand preference in humans: A large internet study in a Dutch sample

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).

The effects of sex and handedness on masturbation laterality and other lateralized motor behaviours

Masturbation is a common human behaviour. Compared to other unimanual behaviours it has unique properties, including increased sexual and emotional arousal, and privacy. Self-reported hand preference for masturbation was examined in 104 left-handed and 103 right-handed women, and 100 left-handed and 99 right-handed men. Handedness (modified Edinburgh Handedness Inventory, EHI), footedness, eyedness, and cheek kissing preferences were also measured. Seventy nine percent used their dominant hand (always/usually) for masturbation, but left-handers (71.5%) were less consistently lateralized to use their dominant hand than right-handers (86.5%). Hand preference for masturbation correlated more strongly with handedness (EHI), than with footedness, eyedness, or cheek preference. There was no difference in masturbation frequency between left- and right-handers, but men masturbated more frequently than women, and more women (75%) than men (33%) masturbated with sex aids. For kissing the preferred cheek of an emotionally close person from the viewer's perspective, left-handers showed a left-cheek preference, and right-handers a weaker right-cheek preference. The results suggest that hemispheric asymmetries in emotion do not influence hand preference for masturbation but may promote a leftward shift in cheek kissing. In all, masturbation is lateralized in a similar way to other manual motor behaviours in left-handed and right-handed men and women.

Is imitational learning a driving factor for the population bias in human hand preference?

Lateral preference is a widespread organizational principle in human and nonhuman animals. In humans, the most apparent lateralized trait (handedness) is unique in the animal kingdom because of a very pronounced bias towards right-handedness on a population level. In this study, based on previous experiments, we test the hypothesis that this bias was-among other factors-shaped by evolution through the facilitation of social learning. We exposed 134 subjects to footage of right- or left-handed knot making and analyzed whether concordant handedness between instructor and student facilitated quicker and more successful imitation. We used a set of nautical knots of different difficulty levels in order to test whether the potential effect of concordance became stronger with increasing knot difficulty. For all three performance measures (time until correct completion, number of attempts needed and correct imitation), we found hand congruency and difficulty level to be significant predictors but not the interaction of the two. We conclude that concordance of handedness between teacher and student of a motor skill enhances the speed and accuracy of imitation, which may have been a beneficial trait for selection to act upon, thereby shaping the human population bias in handedness.

No association between adult sex steroids and hand preference in humans

BACKGROUND

There is ongoing debate about the effects of hormones on the lateralization of the developing brain. In humans, there are conflicting theories of how testosterone during development should affect lateralization. Empirical studies linking prenatal and postnatal testosterone levels to hand preference (a proxy for lateralization) are similarly mixed. Links between hand preference and health may also suggest a mediating role of steroid hormones such as testosterone and estradiol. Studies to date of adult steroid hormones and handedness have been hindered by samples that contain small numbers of non-right-handers.

RESULTS

In the largest study of the phenomenon to date, I find that the testosterone (n = 7290) and estradiol (n = 3700) levels of left- and mixed-handed adults are no different to those of right-handers. All Bayesian 95% highest density intervals contained 0.

DISCUSSION AND CONCLUSIONS

The results have implications for studies that show elevated risk of hormonal-related mental and physical disorders in left-handed individuals.

Where next for laterality research? Looking back and looking forward

While paying attention to the recommendations of Ocklenburg, Berretz, Packheiser, and Friedrich (2020) in the target article, researchers in the field of laterality should attempt to: (1) solve the long-standing puzzle of the relationship between handedness and language lateralization; (2) further explore the genetic bases of manual and cerebral asymmetry and of their associations with psychiatric and neurodevelopmental conditions; (3) explore the adaptive significance of laterality for humans and non-humans and elucidate the relationships of asymmetry across species; and (4) embrace developing technologies to investigate the interaction between the hemispheres during the performance of everyday tasks.

Left-handedness is associated with greater fighting success in humans

Left-handedness is a costly, sexually dimorphic trait found at low frequencies in all human populations. How the handedness polymorphism is maintained is unclear. The fighting hypothesis argues that left-handed men have a negative frequency-dependent advantage in violent intrasexual competition giving them a selective advantage. In support of this, many studies have found that left-handed men are overrepresented among modern professional fighters, but studies typically find no difference in fighting success between left and right-handed fighters. We studied over 13,800 professional boxers and mixed martial artists of varying abilities in three of the largest samples to test this hypothesis to date, finding robust evidence that left-handed fighters have greater fighting success. This held for both male and female fighters, and for both percentage of fights won and an objective measure of fighting ability. We replicated previous results showing that left-handed fighters are strongly overrepresented in professional combat sports, but left-handed fighters did not show greater variance in fighting ability, a hypothesis suggested in previous studies. Overall we find strong evidence consistent with the fighting hypothesis.