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.     

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.     

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.     

Testosterone levels and their associations with lifetime number of opposite sex partners and remarriage in a large sample of American elderly men and women

Testosterone (T) has been argued to modulate mating and parenting behavior in many species, including humans. The role of T for these behaviors has been framed as the challenge hypothesis. Following this hypothesis, T should be positively associated with the number of opposite sex partners a male has. Indeed research in humans has shown that T is positively related to the number of opposite sex partners a young man has had. Here we test, in both men and women, whether this relationship extends to the lifetime number of sex partners. We also explored whether or not T was associated with current marital status, partnership status and whether or not the participant remarried. Using a large sample of elderly men and women (each sample n > 700), we show that T is positively and sizably associated with the number of opposite sex partners in men. When controlling for potential confounding variables such as educational attainment, age, BMI, ethnicity, specific use of a medication and time of sampling this effect remained. For women, the relationship between T and number of opposite sex partners was positive but did not prove to be robust. In both men and women there was no evidence for an association between T and current marital status and partnership status (being in a relationship or not). However, remarriage was positively associated with T, but only in males. Results are discussed with reference to the literature on T and sex partners, remarriage and more broadly the challenge hypothesis.     

Status,reproductive success,and marrying polygynously

A study of female reproductive histories from nineteenth-century Utah shows that although women who married polygynously had fewer children, their number of grandchildren was equal to that of women who married monogamously. Women who chose to marry high-status men polygynously traded decreased fertility for enhanced reproductive performance of offspring. High status can be associated with low fertility and yet still be consistent with fitness optimization. These results suggest how female reproductive decisions influence social structure and challenge previous assumptions concerning proximate measures of fitness.     

Handedness is a determining factor in lateralized olfactory discrimination

The study aimed to re-investigate differences in olfactory thresholds and odor discrimination between the left and right sides in relation to the handedness of healthy subjects. Twenty left- and 20 right-handers participated; all were in excellent health with no indication of any major nasal or health problems, and all were non-smokers. The two groups were comparable in terms of sex and age (left-handers: 11 women, 9 men, median age 25 years; right-handers: 9 women, 11 men, median age 26 years). Odor thresholds did not differ in relation to handedness. However, in the odor discrimination task the left-handers performed significantly better at the left side compared with the right nostril; this pattern was reversed in the right-handers. The data indicate that, similar to other sensory systems, higher olfactory functions exhibit a certain degree of lateralization.      

Testosterone Levels Are Negatively Associated with Childlessness in Males,but Positively Related to Offspring Count in Fathers

Variation in testosterone (T) is thought to affect the allocation of effort between reproductive and parenting strategies. Here, using a large sample of elderly American men (n = 754) and women (n = 669) we examined the relationship between T and self-reported parenthood, as well as the relationship between T and number of reported children. Results supported previous findings from the literature, showing that fathers had lower T levels than men who report no children. Furthermore, we found that among fathers T levels were positively associated with the number of children a man reports close to the end of his lifespan. Results were maintained when controlling for a number of relevant factors such as time of T sampling, participant age, educational attainment, BMI, marital status and reported number of sex partners. In contrast, T was not associated with either motherhood or the number of children women had, suggesting that, at least in this sample, T does not influence the allocation of effort between reproductive and parenting strategies among women. Findings from this study contribute to the growing body of literature suggesting that, among men, pair bonding and paternal care are associated with lower T levels, while searching and acquiring sex partners is associated with higher T levels.     

Role of Motor Asymmetry and of Sex in Lateralization of Autonomic Effects

The paper presents data on the populational laterality proportions among men and women. Analysis of cardiac rhythm variations in right- and left-handed men and women in rest and during mental loading has shown a more pronounced sympathetic domination in left-handed examinees unlike right-handers. The most significant differences in the autonomic balance during mental loading were revealed between the left-handed and right-handed men. An earlier physiological maturation was established to be more characteristic of left-handed women as compared with right-handers. On the background of low and high levels of estrogens a correlation was revealed between the level of hormones and parameters of autonomic balance of the autonomic nervous system both in right-handed and left-handed women.     

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.     

Morphogenetic variability and handedness in Montenegro and Serbia

This study tries to establish correlations between the types of handedness and several morphophysiological characteristics, controlled by one or a small number of genes with alternative dominant recessive manifestation. The population-genetic homozygosity degree study includes the analysis of the presence, distribution and individual traits combination in left-handed and right-handed persons. It was conducted at three localities in two states, Serbia (SRB) and Montenegro (MNE). Our hypothesis is that a possible genetic load due to increased recessive homozygosity, being a potential population-genetic parameter of left-handedness manifestation, may cause some change in other morpho-physiological characters. The average proportion of 23 studied homozygously-recessive characters (HRC’s) was similar among observed left-handed individuals in the studied localities. It varied from 10.5 ± 0.4 in Serbia to 10.7 ± 0.3 in MNE. The differences were somewhat bigger among right-handed persons, varying from 8.7 ± 0.3 in MNE to 9.1 ± 0.3 HRC’s in Serbia. However, in all localities the average homozygosity was significantly higher among left-handed school children. The number of HRC’s among 400 individuals varied from 2 to 16 among right-handed and from 4 to 19 among left-handed persons. There were no differences in scholar scores between left-handed and right-handed children, although small differences were found between two state samples.     

Handedness in a nonindustrial society challenges the fighting hypothesis as an evolutionary explanation for left-handedness

Handedness is a heritable trait, and left-handedness is related with increased fitness costs. Left-handedness persists, however, as a minority in every human population investigated. One explanation for this persistence has been put forward in the fighting hypothesis, which postulates that left-handers have a frequency-dependent benefit in fights. Support for this has been found in the finding that left-handedness is relatively frequent in populations with high homicide rates, according to estimates of left-handedness partly based on pictures and films made for a different purpose. We measured handedness based on hand preference in 10 ecologically relevant tasks in 621 subjects in the nonindustrial society of the Eipo (Papua, Indonesia) in which homicide rate was very high. This set of tests was validated in 198 Western students. Contrary to the prediction based on the fighting hypothesis, we did not find a high frequency of left-handedness or a difference between men (who participate in warfare) and women (who do not). These findings challenge the idea that fighting is the driving evolutionary force for the persistence of left-handedness in human populations. Furthermore, we found lower percentages of left- and mixed-handers compared to a Western population who executed the same tasks. Since left-handedness is associated with health problems, we suggest that in a society lacking Western health care, selection pressures against left-handedness may be more intense and therefore its frequency may be reduced.     

Wealth, status, and fitness: a historical study of Norwegians in variable environments

Wealth and status covary with lifetime reproductive success in preindustrial human populations. Local ecology is likely to modify this association, but details of this presumed relationship are not yet known. We sought to determine whether local ecology modifies the relationship between status and fitness (number of grandchildren). Our approach to the problem was to measure variation in fitness relative to status (landless or with land) and to local ecology (inland versus coastal communities). We also analyzed life history traits that might explain observed variations in fitness. Our results confirm previous findings that both status (landless=9.9 vs. with land=16.5) and ecology (inland=12.3 vs. coast=14.1) affect the number of grandchildren produced by a female in pre-industrial society. We also found that the differences in number of children between the status groups were less pronounced on the coast (landless=12.0 vs. with land=16.1) than inland (landless=7.8 vs. with land=16.8). Our findings are novel because they suggest that the fitness consequences of human status may depend on details of local ecology. We discuss four different mechanisms that could account for these fitness differences: (1) differential reproductive rate of mothers, (2) differential marriage rate of children (3) differential survival rate of children, and (4) different social practices (breastfeeding, inheritance of property and diet).     

Sinister strategies succeed at the cricket World Cup

Left-handers occur at unexpectedly high frequencies at top levels of many interactive sports. This may occur either because left-handed contestants are innately superior or because they enjoy a negatively frequency-dependent strategic advantage when rare relative to right-handers. We analysed the batting records from the 2003 cricket World Cup and showed that left-handed batsmen were more successful than right-handers, and that the most successful teams had close to 50% left-handed batsmen. We demonstrate that this was because left-handed batsmen have a strategic advantage over bowlers, and that this advantage is greatest over bowlers that are unaccustomed to bowling to left-handers. This provides a clear mechanism for negative frequency-dependent success of left-handed batsmen. Our results may also support a historical role for negative frequency-dependent success in fights and other contests in the maintenance of left-handedness by natural selection.     

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.     

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.     

Crab scars reveal survival advantage of left-handed snails

Biological asymmetries are important elements of the structure and function of many living organisms. Using the Plio-Pleistocene fossil record of crab predation on morphologically similar pairs of right- and left-handed snail species, we show here for the first time, contrary to traditional wisdom, that rare left-handed coiling promotes survival from attacks by right-handed crabs. This frequency-dependent result influences the balance of selection processes that maintain left-handedness at the species level and parallels some social interactions in human cultures, such as sports that involve dual contests between opponents of opposite handedness.     

Sex,status, and reproductive success in the contemporary United States

This paper reexamines the relationship between status and reproductive success (at the ultimate and proximate levels) using data on sex frequency and number of biological children from representative samples of the U.S. population. An ordered probit analysis of data from the 1989–2000 General Social Survey (GSS) shows that high-income men report greater frequency of sex than all others do. An OLS regression of data from the 1994 GSS shows that high-income men have more biological children than do low-income men and high-income women. Furthermore, more educated men have more biological children than do more educated women. Results also show that intelligence decreases the number of offspring and frequency of sex for both men and women.     

Three palmar dermatoglyphic traits and their asymmetry in Bulgarian right-, mixed- and left-handers

Summary: In a sample, comprising 264 right-, 246 mixed- and 360 left-handers (RH, MH and LH, correspondingly), the atd-angle, the a-b ridge count and the hypothenar radial arch were investigated, the asymmetry of both quantitative traits differentiated into directional (DA) and fluctuating (FA) one. Except for the FA of the a-b ridge count in females, which decreased significantly from RH to LH, the trends observed in the relations between the investigated dermatoglyphic values and handedness were not significant. In both quantitative traits the most important finding was the categorical left-palm excess over the right palm, since it was significantly related to sex and handedness, being much more expressed in females than in males and in the non-right-handers than in the right-handers. The hypothenar radial arch, along with its considerably higher frequency in females than in males and on the right palm than on the left, as well as its rarity combined with a very high symmetry, displayed another interesting peculiarity. The pattern was 3.9-fold more frequent in the MH and 3.4-fold more frequent in the LH as compared to RH. As witnessed by the odds ratio, if a given palm belongs to a non-right-hander, the probability that it bears a hypothenar radial arch is nearly 4-fold higher than if it were a palm of a right-hander. Arguments are adduced that left-handedness, although not a pathological character is, to say the least, a modified condition and that, similarly, the hypothenar radial arch is a subnormal dermatoglyphic finding. If such is the case, their relationship found by the present study is not surprising, although its causal background still remains unclear.     

The association of three indicators of developmental instability with mating success in humans

Developmental instability (DI) has been proposed to relate negatively to aspects of evolutionary fitness, like mating success. One suggested indicator is fluctuating asymmetry (FA), random deviations from perfect symmetry in bilateral bodily traits. A meta-analytically robust negative association between FA and number of lifetime sexual partners has been previously shown in men and women. We examined the relationship between bodily FA across twelve traits and indicators of quantitative mating success in 284 individuals (141 males, age 19–30 years). Two further indicators of DI, minor physical anomalies (MPAs) and asymmetry in palmar atd angles, were also assessed. For men, no significant associations were detected, whereas for women, unexpected positive relationships of FA with the number of lifetime sexual partners and one-night stands emerged. Thus, in a large sample and using a more highly aggregated FA index, our study fails to replicate previous findings, though equivalence testing also did not support deviation from previous meta-analytic estimates, especially for men. No associations were found for MPAs and FA in atd angles in either sex.     

Personality and long-term reproductive success measured by the number of grandchildren

Personality, that is, individual behavioral tendencies that are relatively stable across situations and time, has been associated with number of offspring in many animals, including humans, suggesting that some personality traits may be under natural selection. However, there are no data on whether these associations between personality and reproductive success extend over more than one generation to numbers of grandchildren. Using a large representative sample of contemporary Americans from the Health and Retirement Study (n = 10,688; mean age 67.7 years), we studied whether personality traits of the Five Factor Model were similarly associated with number of children and grandchildren, or whether antagonistic effects of personality on offspring number and quality lead to specific personality traits differently maximizing short and long-term fitness measures. Higher extraversion, lower conscientiousness, and lower openness to experience were similarly associated with both higher number of children and grandchildren in both sexes. In addition, higher agreeableness was associated with higher number of grand-offspring only. Our results did not indicate any quality–quantity trade-offs in the associations between personality and reproductive success. These findings represent the first robust evidence for any species that personality may affect reproductive success over several generations.