Handedness and aggressive behavior in an Ntumu village in southern Cameroon

Handedness in humans is possibly maintained by a frequency-dependent advantage during aggressive interactions. To contribute to relevant data related to this idea, functional handedness has been measured in a traditional ethnic group (Ntumu) from southern Cameroon, and confronted to the level of aggressive behavior. Functional handedness was measured by the hand holding the machete, an asymmetric tool used every day. Aggressive behavior was assessed by the frequency of occurrence of the traditional tribunal for solving internal conflicts. There was a low percentage of left-handers (8.1%), with no significant sex differences, although females below 50 year old displayed a significant increase of left-handedness with age. The level of aggressive behavior was low, as the traditional tribunal took place only seven times during the last five years. Results are discussed in the context of the evolution of handedness and the possible recent societal changes following European colonisation. Received: 20 August 1999 / Received in revised form: 22 October 1999 / Accepted: 6 November 1999     

Maintenance of handedness polymorphism in humans: a frequency-dependent selection model

Frequency-dependent selection is an important process in the maintenance of genetic variation in fitness. In humans, it has been proposed that the polymorphism of handedness is maintained by negative frequency-dependent selection, through a strategic advantage of left-handers in fighting interactions. Using simple mathematical models, we explore: (1) whether it is possible to predict the range of left-handedness frequencies observed in human populations by the frequency and the violence of fighting interactions; (2) the consequences of the sex differences in the probability of transmission of hand preference to offspring. We show that a wide range of values of the frequency of left-handers can be obtained with realistic changes of the parameters values. Our models reinforce the idea that negative frequency-dependence may have played a role in maintaining left-handedness in human populations, and provide further support for the importance of fighting interactions in the evolution of hand preference. Moreover, they suggest an explanation for the occurrence of left-handedness among women in this context, namely an indirect selective advantage through their male offspring.     

Handedness, homicide and negative frequency-dependent selection

Humans exhibit hand preference for most manual activities in which they are specialized. Right- and left-handers have coexisted at least since the Upper Palaeolithic, and left-handers are in the minority in all human populations. The persistence of the polymorphism of handedness is a puzzle because this trait is substantially heritable and several fitness costs are associated with left-handedness. Some countervailing benefit is required to maintain the polymorphism. Left-handers may have a frequency-dependent advantage in fights--the advantage being greater when their frequency is lower. Sports data from Western societies are consistent with this prediction. Here, we show that the frequency of left-handers is strongly and positively correlated with the rate of homicides across traditional societies. It ranges from 3% in the most pacifistic societies, to 27% in the most violent and warlike. This finding is consistent with a frequency-dependent selection mechanism maintaining left-handedness in these societies.     

Effects of handedness on visual sensitivity in perihand space

Recent studies have shown that changes in visual processing in perihand space are limited to the area around the right hand, at least in right-handers. One explanation for these findings is that perception is altered at locations where action is more likely to occur. To test this notion, we asked both right- and left-handers to perform an unspeeded visual discrimination task under four hand-position configurations: Left hand, right hand, both hands, or no hands near the display. Compared to the no-hands (control) condition, visual sensitivity (d') was higher in the dominant-hand condition for right-handers and higher in the dominant- as well as the non-dominant hand condition for left-handers. When both hands were near the display, sensitivity was similar to that in the dominant-hand condition for right-handers and to that in the non-dominant hand condition for left-handers. This shows that performance differed between the two handedness groups when their non-dominant hand was near the display (both alone and accompanied by their dominant hand). Thus, the pattern for left-handers did not correspond to a mirror image of the pattern for right-handers. In line with studies on bimanual action control, visual processing in perihand space seems to be determined by the different ways in which left- and right-handers use their hands.     

Handedness and socioeconomic status in an urban population in Uzbekistan

The persistence of left-handers in every human population studied to date is an evolutionary puzzle in light of evidence of survival costs associated with left-handedness. Associations between left-handedness and socioeconomic advantages have been observed in Western countries and could provide left-handers fitness benefits through higher survival chances and greater reproductive success. We aimed to explore the generality of this result in another culture. For this purpose, we investigated several socioeconomic status indicators and the number of children alive for 917 men and women in Uzbekistan and compared results for two different measures of handedness: hand preferences for writing and for knife use. Among both men and women, left-handed writers were significantly more likely to own a car, own a washing machine and have a bank account. Left-handed women (using both measures) had a higher income than right-handed women. Among men, left-handers for knife use had a higher income than right-handers. The results of our study suggest that the previously observed socioeconomic advantage of left-handers in Western populations also applies to non-Western populations, at least in the urban environment studied. However, we did not detect any difference in the number of children. We discussed how the frequency-dependent socioeconomic status advantage could be responsible for the persistence of left-handers throughout human evolution.     

Motivation and motor control: hemispheric specialization for approach motivation reverses with handedness

Background

According to decades of research on affective motivation in the human brain, approach motivational states are supported primarily by the left hemisphere and avoidance states by the right hemisphere. The underlying cause of this specialization, however, has remained unknown. Here we conducted a first test of the Sword and Shield Hypothesis (SSH), according to which the hemispheric laterality of affective motivation depends on the laterality of motor control for the dominant hand (i.e., the “sword hand," used preferentially to perform approach actions) and the nondominant hand (i.e., the “shield hand," used preferentially to perform avoidance actions).

Methodology/Principal Findings

To determine whether the laterality of approach motivation varies with handedness, we measured alpha-band power (an inverse index of neural activity) in right- and left-handers during resting-state electroencephalography and analyzed hemispheric alpha-power asymmetries as a function of the participants'' trait approach motivational tendencies. Stronger approach motivation was associated with more left-hemisphere activity in right-handers, but with more right-hemisphere activity in left-handers.

Conclusions

The hemispheric correlates of approach motivation reversed between right- and left-handers, consistent with the way they typically use their dominant and nondominant hands to perform approach and avoidance actions. In both right- and left-handers, approach motivation was lateralized to the same hemisphere that controls the dominant hand. This covariation between neural systems for action and emotion provides initial support for the SSH.     

Actigraphic investigations on the activity-rest behavior of right- and left-handed students

The aim of this study was to explore differences between left-and right-handed subjects in sleep duration. Sleep and activity patterns were continuously registered for 12 days using actometers on 20 left-handed and 20 right-handed medical students in Berlin. Handedness was determined by a modified version of the Edinburgh handedness inventory. Each participant wore one actometer on each wrist. Actiwatch^® Sleep Analysis Software (CNT, UK) was used to evaluate the data, and statistical calculations were performed with a non-parametric variance analysis. A significant difference in mean sleep duration between left-handers (7.9 h) and right-handers (7.3 h) was determined (p=0.025 for measurement made on the dominant hand and p=0.013 for ones made on the non-dominant hand). In contrast, the maximal phase of daily activity (acrophase) did not show any difference between the two groups. The difference in sleep duration might be caused by either the greater effort required for left-handers to cope in a right-handed world or by structural brain differences.     

Developmental origins of variation in human hand preference

Though right-handedness is a prominant characteristic within all human societies, a substantial and stable proportion of individuals are left-handed. Any comprehensive approach to the origin of variation in handedness must account for substantial evidence that left-handedness is associated with reduced fitness, neurodevelopmental disorders, and reduced neuroanatomical asymmetry. In this paper we investigate the hypothesis that developmental instability in early fetal development underlies variation in handedness. In two studies we note an increased incidence of minor physical anomalies (MPAs) and fluctuating asymmetries in both left-handers and extreme right-handers. Moreover, extreme right-handers were more apt to have left-handed parents than moderate right-handers. These data suggest that deviation from moderate right-handedness reflect imprecise expression of a near-universal design due to developmental instability. Preliminary attempts to elucidate the mechanisms underlying developmental instability suggest that both polygenic homozygosity and particular HLA alleles may be important factors. These observations are discussed with respect to current genetic theories of handedness and human evolution.     

Cortical activations in humans grasp-related areas depend on hand used and handedness

Background

In non-human primates grasp-related sensorimotor transformations are accomplished in a circuit involving the anterior intraparietal sulcus (area AIP) and both the ventral and the dorsal sectors of the premotor cortex (vPMC and dPMC, respectively). Although a human homologue of such a circuit has been identified whether activity within this circuit varies depending on handedness has yet to be investigated.

Methodology/Principal Findings

We used functional magnetic resonance imaging (fMRI) to explicitly test how handedness modulates activity within human grasping-related brain areas. Right- and left-handers subjects were requested to reach towards and grasp an object with either the right or the left hand using a precision grip while scanned. A kinematic study was conducted with similar procedures as a behavioral counterpart for the fMRI experiment. Results from a factorial design revealed significant activity within the right dPMC, the right cerebellum and AIP bilaterally. The pattern of activity within these areas mirrored the results found for the behavioral study.

Conclusion/Significance

Data are discussed in terms of an handedness-independent role for the right dPMC in monitoring hand shaping, the need for bilateral AIP activity for the performance of precision grip movements which varies depending on handedness and the involvement of the cerebellum in terms of its connections with AIP. These results provide the first compelling evidence of specific grasping related neural activity depending on handedness.     

Brief communication: Captive gorillas are right‐handed for bimanual feeding

Predominance of right‐handedness has historically been considered as a hallmark of human evolution. Whether nonhuman primates exhibit population‐level manual bias remains a controversial topic. Here, we investigated the hypothesis that bimanual coordinated activities may be a key‐behavior in our ancestors for the emergence and evolution of human population‐level right‐handedness. To this end, we collected data on hand preferences in 35 captive gorillas (Gorilla gorilla) during simple unimanual reaching and for bimanual coordinated feeding. Unimanual reaching consisted of grasping food on the ground, while bimanual feeding consisted of using one hand for holding a food and processing the food item by the opposite hand. No population‐level manual bias was found for unimanual actions but, in contrast, gorillas exhibited a significant population‐level right‐handedness for the bimanual actions. Moreover, the degree of right‐handedness for bimanual feeding exceeds any other known reports of hand use in primates, suggesting that lateralization for bimanual feeding is robust in captive gorillas. The collective evidence is discussed in the context of potential continuity of handedness between human and nonhuman primates. Am J Phys Anthropol 2010. © 2009 Wiley‐Liss, Inc.     

Study on pottical type, palmar and plantar digital formulae, hand clasping, arm folding, handedness, leg folding and stride type in the Daur population, China

The pottical type, palmar and plantar digital formulae, hand clasping, arm folding, handedness, leg folding and stride type have been investigated on a sample of 143 male and 160 female students of the Daur population of Molidawa Banner, Inner Mongolia. The results of this study are the following: 1. the frequency of the hyperextensive pottical type is 49.17%, the relative length of index over annularis 12.21%, right hand clasping 45.87%, right arm folding 49.50%, right handedness 94.39%, right leg folding 72.28% and right stride type 44.88%, 2. pottical type, hand clasping, handedness, leg folding and stride type do not show significant sex differences, 3. there are some relations between hand clasping and arm folding as well as between arm folding and stride type, 4. compared with other population groups, the Daur population shows a low frequency of right hand clasping, a moderate frequency of right arm folding and a low frequency of left handedness.     

Memorization of sequences of right and left hand movements in right- and left-handers: Vector coding

Using studies of the right and left hemisphere’s specialization for positional and vector coding, we analyzed the errors made by right- and left-handers while reproducing sequences of right and left hand movements in a task that activates vector coding by changing the order of movements in memorized sequences. The task was performed first with one hand (starting) and then with the other (continuing). Both right- and lefthanders were found to use information about previous movements of the starting hand only when the dominant hand was starting. After changing the hand, right-handers used information about previous movements of the continuing hand, while left-handers did not. The results were compared with data from earlier experiments wherein positional coding was activated. The comparison showed that vector coding was predominantly involved in memorizing sequences of movements made by the dominant hand, while positional coding was used in the case of the opposite hand in both right- and left-handers. Patterns of errors after changing the hand differed between right- and left-handers, and the conclusion was made that skills are transferred in different ways in right- and left-handers, depending on the type of coding.     

Genetic and environmental contributions to the expression of handedness in chimpanzees (Pan troglodytes)

Most humans are right‐handed and, like many behavioral traits, there is good evidence that genetic factors play a role in handedness. Many researchers have argued that non‐human animal limb or hand preferences are not under genetic control but instead are determined by random, non‐genetic factors. We used quantitative genetic analyses to estimate the genetic and environmental contributions to three measures of chimpanzee handedness. Results revealed significant population‐level handedness for two of the three measures—the tube task and manual gestures. Furthermore, significant additive genetic effects for the direction and strength of handedness were found for all three measures, with some modulation due to early social rearing experiences. These findings challenge historical and contemporary views of the mechanisms underlying handedness in non‐human animals.     

Sex difference in chimpanzee handedness

Chimpanzees at Mahale, Tanzania, show strong individual hand preferences when they use bimanual actions in processing the fruit of Saba florida and Citrus lemon. The direction of hand preference differs between the sexes: most males are left-handed, whereas most females are right-handed. Monkeys and apes are considered to lack "handedness," in the sense of a population mode of left- or right-hand preference; they are normally ambidextrous. Indeed, strong individual preferences were previously seldom found in natural tasks. We propose that lateralization of manual actions becomes advantageous in bimanual tasks, which involve role differentiation between the hands and a need to combine power and precision. If the pattern of lateralization found here reflects the ancestral state, common to chimpanzees and humans, this may explain why, in modern humans, women tend more strongly to be right-handed than men, who include a larger minority of left-handers.     

Maternal effect of low temperature on handedness determination in C. elegans embryos

C. elegans embryos, larvae, and adults exhibit several left-right asymmetries with an invariant dextral handedness, which first becomes evident in the embryo at the 6-cell stage. Reversed (sinistral) handedness was not observed among > 10,000 N2 adults reared at 16°C or 20°C under standard conditions. However, among the progeny of adults reproducing at 10°C, the frequency of animals with sinistral handedness was increased to ∼0.5%. Cold pulse experiments indicated that the critical period for this increase was in early oogenesis, several hours before the first appearance of left-right asymmetry in the embryo. Hermaphrodites reared at 10°C and mated with males reared at 20°C produced sinistral outcross as well as sinistral self-progeny, indicating that the low temperature effect on oocytes was sufficient to cause reversals. Increased frequency of reversal was also observed among animals developed from embryos lacking the egg shell. Possible mechanisms for the control of embryonic handedness are discussed in the context of these results, including the hypothesis that handedness could be dictated by the chirality of a gametic component. © 1996 Wiley-Liss, Inc.     

Handedness in comparison with the asymmetry of the upper extremities]

In 1990 221 males (88 children, age 3.5-7.25 years and 133 adults, age 20-44 years) were investigated. The sample was divided into two groups based on handedness; 140 individuals were right-handed and 81 left-handed. 10 measurements were taken at the upper limbs. In addition, marked handedness, handclasping and armfolding were determined. The last three traits showed no statistically significant connection. It was determined that measurements of length, breadth and circumference are more often significantly asymmetric in favor of the right side in right-handers than in left-handers. Further, skinfolds of right-handed individuals are larger on the left side. Partially reverse results were found for left-handers as a whole, however, the results are more irregular. A difference in the degree of asymmetry exists between children and adults. Asymmetries of breadth and circumference measurements as well as forearm skinfold are larger in adults. Taking into account the error of measurement, frequencies of R > L, R = L and L > R for right- and left-handed individuals were evaluated using the chi 2-test for significance. Preponderance of measurements on the dominant side could often be secured statistically.     

Patterns of lateralized hand use in an arboreal primate, Simias concolor

Studies of hand use in nonhuman primates suggest that several species exhibit hand preferences for a variety of tasks. The majority of studies, however, focus on the lateralized hand use of captive nonhuman primate populations. Although captive settings offer a more controlled environment for assessing hand preferences, studies of wild populations provide important insights into how handedness is affected by natural environmental conditions and thus potential insights into the evolution of handedness. To investigate handedness in a population of wild nonhuman primates, we studied patterns of lateralized hand use during feeding in four simakobu monkeys (Simias concolor), an arboreal species inhabiting the Mentawai Islands, Indonesia. Our data show that individual variation in hand preferences for feeding existed among our study animals. In addition, each simakobu expressed a significant hand preference for supporting itself on a branch during feeding, an uncoordinated bimanual task. This bias was most prevalent when the branch used for support was a main branch rather than a terminal branch. When both hands were employed in a coordinated bimanual feeding activity (bimanual manipulation), only two subjects showed a significant bias for feeding. Our data suggest that these individuals are more likely to express significant hand preferences when feeding from stable, rather than precarious, positions within the canopy.     

The intertriradial dermal ridges and their relation to handedness

Palm prints of 394 right-handers and 356 non-right-handers (left-handers and ambidextrous) were evaluated regarding intertriradial ridge counts. Comparison between the handedness groups produced no significant differences. There were found partly contrary tendencies in males and females. The values of an elected group of extremely left-handed females indicate a relation between a "graded variety of handedness-distinction" and a "graded distinction of dermatoglyphic traits". Earlier indications that bilateral asymmetry is reduced in left-handers could not be confirmed generally.     

Hand dominance and bilateral asymmetry in the structure of the second metacarpal

Bilateral asymmetry in the structure of the second metacarpal was examined in relation to functional hand dominance in a large, clinically nonselected, healthy population sample from the Baltimore Longitudinal Study of Aging. Bilateral bone measurements were made from anteroposterior hand radiographs of a total of 992 individuals, 609 males and 383 females, with an age range of 19–94 years. Hand dominance was determined on the basis of personal impression. Total width and medullary width at the midshaft of the second metacarpal were measured to 0.05 mm using a Helios caliper. These two measurements were used to derive cortical thickness, cortical bone area, periosteal (total) area, medullary area, percent cortical area, and the second moment of area in the mediolateral plane. In both right and left-handed individuals, statistically significant side differences were found in the calculated bone areas and the second moment of area, with the dominant hand being larger. Cortical thickness did not show significant side-related differences for either handedness. These results show that functional handedness leads to periosteal and endosteal expansion of the second metacarpal cortex on the dominant side, increasing bone strength without increasing cortical thickness. This is the first time this pattern of asymmetry has been reported in left-handers as well as right-handers. Our results argue for the primacy of environmental (mechanical) effects in determining bilateral asymmetry of limb bone structural properties. © 1994 Wiley-Liss, Inc.