Developmental origins of variation in human hand preference

An examination of the influence of prenatal sex hormones on handedness: Literature review and amniotic fluid data

Competing theories have posited roles for foetal androgen exposure in the development of human handedness. However, due to practical and ethical considerations, few studies have used hormonal measures to examine this possibility. The current paper reviews this literature and reveals a generally inconsistent pattern of results. We also present data from a longitudinal study of prenatal sex hormone exposure and subsequent handedness. More specifically, we examine correlations between testosterone and estradiol measured from second trimester amniotic fluid and hand preference (Dutch language version of the Edinburgh Handedness Inventory) and hand skill asymmetry (pegboard task) measured at 15 years of age. Prenatal sex hormone exposure was not associated with the direction of hand preference in either males or females. However, in females, high levels of prenatal testosterone were associated with weaker lateralisation of hand skill, and high levels of prenatal estradiol were associated with weaker hand preference. In addition, high levels of prenatal testosterone were associated with increased task duration (i.e., slow hand speed) for the right and left hands of males. The pattern of results observed here is not entirely consistent with any of the main theories linking sex hormones with handedness, suggesting that an association between these variables may be more complex than initially thought.

A Pre-registered Meta-analysis Based on Three Empirical Studies Reveals No Association Between Prenatal (Amniotic) Cortisol Exposure and Fluctuating Asymmetry in Human Infants

Developmental instability (DI) reflects an organism’s inability to develop an ideal phenotype when challenged by genetic and environmental insults. DI can be estimated via the proxy measure of fluctuating asymmetry (FA), i.e., the small random deviations from perfect bilateral symmetry observed in the morphology of paired traits. The mechanisms involved in the genesis of FA in human populations are relatively unknown, though animal research indicates that hormonal processes may be involved. As maternal stress during pregnancy is detrimental to various developmental processes, elevated prenatal cortisol may represent a causal factor in the subsequent emergence of an asymmetrical phenotype. The main purpose of this pre-registered meta-analysis based on three empirical studies was to investigate whether mid-trimester amniotic cortisol levels predict subsequent FA in finger lengths of infants from Germany, Portugal, and the UK. No statistically significant relationships were observed, and meta-analytic combination of the effect size estimates yielded a null result. We did, however, detect significant positive correlations between the cortisol present in the amniotic fluid and maternal plasma in the Portuguese cohort, and observed that FA in the German cohort was significantly lower at 70-months than at either 9- or 20-months. Taken together, the current findings run contrary to animal research showing that elevated prenatal corticosterone exposure leads to increased FA. However, this may be because a single cortisol assay obtained via amniocentesis is an inadequate proxy for average gestational exposure, and/or that prenatal cortisol levels at an earlier (i.e., first rather than second trimester) stage of pregnancy is what explains variance in subsequent FA.

Evolution, genes, and inter‐disciplinary personality research

Most commentaries welcomed an evolutionary genetic approach to personality, but several raised concerns about our integrative model. In response, we clarify the scientific status of evolutionary genetic theory and explain the plausibility and value of our evolutionary genetic model of personality, despite some shortcomings with the currently available theories and data. We also have a closer look at mate choice for personality traits, point to promising ways to assess evolutionarily relevant environmental factors and defend higher‐order personality domains and the g‐factor as the best units for evolutionary genetic analyses. Finally, we discuss which extensions of and alternatives to our model appear most fruitful, and end with a call for more inter‐disciplinary personality research grounded in evolutionary theory. Copyright © 2007 John Wiley & Sons, Ltd.

Foundations of a Functional Theory of Human Handedness

A functional theory of human handedness is introduced based on early childhood experiences and the neural plasticity of the brain. It assumes that very different factors may be involved in the development of an individual left-handedness. Imitation and social learning are major factors in the formation of both right- and left-handedness in normal early childhood development. However, the frequently claimed relation between left-handedness and pathology as well as exceptional giftedness can only be understood by taking into account the function or meaning of left-handedness for personality development. The functional approach conceives left-handedness as a ubiquitous symbolic aid for coping creatively with conflict in early childhood. A childrearing style that clashes with the child's body-mind needs can lead the socioculturally less favored left hand to serve as a symbolic frame of reference for a meaning-making action repertoire that is fundamental in protecting and stabilizing the childhood self and personality development. Suggestions are derived regarding brain lateralization and handedness and for the diagnosis of early childhood disorders.

Strong rightward lateralization of the dorsal attentional network in left-handers with right sighting-eye: an evolutionary advantage

Hemispheric lateralization for spatial attention and its relationships with manual preference strength and eye preference were studied in a sample of 293 healthy individuals balanced for manual preference. Functional magnetic resonance imaging was used to map this large sample while performing visually guided saccadic eye movements. This activated a bilateral distributed cortico-subcortical network in which dorsal and ventral attentional/saccadic pathways elicited rightward asymmetrical activation depending on manual preference strength and sighting eye. While the ventral pathway showed a strong rightward asymmetry irrespective of both manual preference strength and eye preference, the dorsal frontoparietal network showed a robust rightward asymmetry in strongly left-handers, even more pronounced in left-handed subjects with a right sighting-eye. Our findings brings support to the hypothesis that the origin of the rightward hemispheric dominance for spatial attention may have a manipulo-spatial origin neither perceptual nor motor per se but rather reflecting a mechanism by which a spatial context is mapped onto the perceptual and motor activities, including the exploration of the spatial environment with eyes and hands. Within this context, strongly left-handers with a right sighting-eye may benefit from the advantage of having the same right hemispheric control of their dominant hand and visuospatial attention processing. We suggest that this phenomenon explains why left-handed right sighting-eye athletes can outperform their competitors in sporting duels and that the prehistoric and historical constancy of the left-handers ratio over the general population may relate in part on the hemispheric specialization of spatial attention.

Quantifying cerebral asymmetries for language in dextrals and adextrals with random-effects meta analysis

Speech and language-related functions tend to depend on the left hemisphere more than the right in most right-handed (dextral) participants. This relationship is less clear in non-right handed (adextral) people, resulting in surprisingly polarized opinion on whether or not they are as lateralized as right handers. The present analysis investigates this issue by largely ignoring methodological differences between the different neuroscientific approaches to language lateralization, as well as discrepancies in how dextral and adextral participants were recruited or defined. Here we evaluate the tendency for dextrals to be more left hemisphere dominant than adextrals, using random effects meta analyses. In spite of several limitations, including sample size (in the adextrals in particular), missing details on proportions of groups who show directional effects in many experiments, and so on, the different paradigms all point to proportionally increased left hemispheric dominance in the dextrals. These results are analyzed in light of the theoretical importance of these subtle differences for understanding the cognitive neuroscience of language, as well as the unusual asymmetry in most adextrals.

Behavioral genetic variation, adaptation and maladaptation: an evolutionary perspective

Evolutionary psychology seeks to understand the adaptive, evolved nature of humans by considering the forces of natural selection that gave rise to it. Individual humans also exhibit maladaptation, however, and some of the variation in maladaptive conditions is heritable. An evolutionary perspective can shed light on these phenomena too. Recent work suggests that developmental imprecision importantly affects the fitness of organisms and some of the genetic influences have been identified. Both theory and evidence that developmental imprecision underlies human cognitive maladaptation has begun to accumulate, although the manner in which the developing brain is affected remains largely unknown.

Understanding left-handedness

BACKGROUND

The human cerebrum is asymmetrical, consisting of two hemispheres with differing functions. Recent epidemiological and neurobiological research has shed new light on the development of the cerebral lateralization of motor processes, including handedness. In this article, we present these findings from a medical perspective.

METHOD

We selectively searched the PubMed online database for articles including the terms "handedness," "left handedness," "right handedness," and "cerebral lateralization." Highly ranked and commonly cited articles were included in our analysis.

RESULTS

The emergence of handedness has been explained by physiological and pathological models. Handedness arose early in evolution and has probably been constitutive for the development of higher cognitive functions. For instance, handedness may have provided the basis for the development of speech and fine motor skills, both of which have played a critical role in the evolution of mankind. The disadvantages of certain types of handedness are discussed, as some cases seem to be associated with disease.

CONCLUSION

The consideration of handedness from the epidemiological, neurobiological, and medical points of view provides insight into cerebral lateralization.

Left-Handedness and Male-Male Competition: Insights from Fighting and Hormonal Data

Male-male competition can shape some behavioral or morphological traits of males. Here we investigate if this competition could play a role in the persistence of the polymorphism of handedness in human populations. A negative frequency-dependent selection mechanism has been hypothesized, based on the fact that left-handed men may benefit from a “surprise” advantage during fighting interactions because they are rare in human populations. This advantage may thereby enhance the probability of survival of left-handed men and/or their reproductive success through an increase in social status. In this study, we first explored the association between hand preference and lifetime fighting behavior in a population of 1,161 French men. No effect of hand preference on the probability of fighting was detected, suggesting that the innate propensity to fight does not differ between left- and right-handers. However, among men who had been involved in at least one fight during their lifetime, left-handers reported significantly more fights than right-handers. To explore the biological basis of this behavior, we also investigated the testosterone concentration in saliva samples from 64 French university students. Consistent with frequencies of fights, we found a significantly higher average testosterone concentration in left-handers than in right-handers. We suggest that these behavioral and hormonal differences may be acquired throughout life due to previous experiences in a social context and may favor the persistence of left-handers in humans.

Handedness and Eye-dominance: A Meta-analysis of Their Relationship

About one in ten people is left-handed and one in three is left-eyed. The extent of the association of handedness and eyedness is unclear, as some eyedness measures are potentially contaminated by measures of handedness. A meta-analysis of hand-eye concordance in 54,087 subjects from 54 populations, found that concordance was 2.69 x greater in questionnaire rather than performance studies, 1.95 x greater in studies using unimanual monocular performance measures, and 6.29 x greater in studies using non-sighting measures of eye-dominance. In the remaining studies, which seemed to show no evidence of bias, the odds-ratio for hand-eye concordance was 2.53 x; in a population with 9.25% left-handedness and 36.53% left-eyedness, 34.43% of right-handers and 57.14% of left-handers are left-eyed. This pattern of hand-eye association poses problems for genetic models of cerebral lateralisation, which must invoke pleiotropic alleles at a single locus or epistatic interactions between multiple loci. There was no evidence that the incidence of eyedness, or the association between eyedness and handedness, differed between the sexes.

Epigenesis of behavioural lateralization in humans and other animals

Despite several decades of research, the epigenesis of behavioural and brain lateralization is still elusive, although its knowledge is important in understanding developmental plasticity, function and evolution of lateralization, and its relationship with developmental disorders. Over the last decades, it has become clear that behavioural lateralization is not restricted to humans, but a fundamental principle in the organization of behaviour in vertebrates. This has opened the possibility of extending descriptive studies on human lateralization with descriptive and experimental studies on other vertebrate species. In this review, we therefore explore the evidence for the role of genes and environment on behavioural lateralization in humans and other animals. First, we discuss the predominant genetic models for human handedness, and conclude that their explanatory power alone is not sufficient, leaving, together with ambiguous results from adoption studies and selection experiments in animals, ample opportunity for a role of environmental factors. Next, we discuss the potential influence of such factors, including perinatal asymmetrical perception induced by asymmetrical head position or parental care, and social modulation, both in humans and other vertebrates, presenting some evidence from our own work on the domestic chick. We conclude that both perinatal asymmetrical perception and later social modulation are likely candidates in influencing the degree or strength of lateralization in both humans and other vertebrates. However, in most cases unequivocal evidence for this is lacking and we will point out further avenues for research.

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.

On the genetics and measurement of human handedness

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 and the motor asymmetries found in many nonhuman primate species, and perhaps in lower mammals as well. From this perspective, one might see the study of human laterality as relevant to evolutionary biology. Investigators of both human and nonhuman asymmetries have to grapple with such difficult measurement issues as the relations between preference and performance, and the influences of postural and task demands, and have much to offer each other in the quest for the nature of laterality. Our recent work seems to indicate that the various sensory and motor lateralities may be related, in humans, but not in a simple way. In future work, the challenge will be to identify the relations between the various laterality ''profiles'' and patterns of functional cerebral organisation.

An epigenetic induction of a right-shift in hippocampal asymmetry: selectivity for short- and long-term potentiation but not post-tetanic potentiation

In humans, it is well established that major psychological functions are asymmetrically represented between the left and right cerebral cortices. The developmental origin of such functional lateralization remains unknown. Using the rat as a model system, we examined whether exposing neonates briefly to a novel environment can differentially affect synaptic plasticity in the left and right hippocampi during adulthood. During the first 3 weeks of life, one half of the pups from a litter spent 3 min daily away from their familiar home environment (Novel) while their littermates remained in that familiar environment (Home). At adulthood (7-months old), post-tetanic potentiation (PTP) of excitatory post-synaptic potentials (EPSPs), a very short-lasting form of plasticity, was greater among the Novel than the Home rats in both left and right hippocampi. In contrast, the novelty-induced increases in short- and long-term potentiation (STP, LTP), two relatively longer-lasting forms of plasticity, were found only in the right hippocampus. These findings demonstrate that a phase-selective asymmetry in hippocampal synaptic plasticity can be induced epigenetically by seemingly small systematic differences in early life environment. The selectivity of this asymmetry for the longer-lasting forms of synaptic plasticity suggests that the observed asymmetry in plasticity may contribute specifically to an asymmetric learning process which, in turn, may contribute to a functional asymmetry in the neocortex.

Comparative and familial analysis of handedness in great apes

Historically, population-level handedness has been considered a hallmark of human evolution. Whether nonhuman primates exhibit population-level handedness remains a topic of considerable debate. This paper summarizes published data on handedness in great apes. Comparative analysis indicated that chimpanzees and bonobos show population-level right handedness, whereas gorillas and orangutans do not. All ape species showed evidence of population-level handedness when considering specific tasks. Familial analyses in chimpanzees indicated that offspring and maternal (but not paternal) handedness was significantly positively correlated, but this finding was contingent upon the classification criteria used to evaluate hand preference. Overall, the proportion of right handedness is lower in great apes compared with humans, and various methodological and theoretical explanations for this discrepancy are discussed.

Schizotypy and mixed-handedness revisited

Although some previous studies assert that an association between schizotypy and loss of hand dominance is well established, the prevailing use of student populations, small effect sizes and arbitrariness of handedness classification suggest that this tentative association merits further investigation. The association of schizotypy and loss of hand dominance was examined using four samples. The first comprised 353 randomly selected individuals from the general community, the second comprised 131 screened volunteers participating as control subjects in a family study of schizophrenia, the third included 97 full siblings of schizophrenia patients, and the fourth consisted of 176 schizophrenia patients from the same study. The samples of screened volunteers and nonpsychotic siblings were used to replicate results from the community sample and to test the hypothesis that an increase in genetic liability is related to the association of schizotypal traits and mixed handedness. The results demonstrated that mixed handedness and schizotypy traits were unrelated in the representative sample from the community. This finding was replicated in the sample of screened volunteers, while siblings of schizophrenia patients showed a trend in the direction of the hypothesised relationship. In contrast, there was an expected significant but low in magnitude association between loss of hand dominance and the Schizotypal Personality Questionnaire factor of Cognitive Perceptual Dysfunction in schizophrenia patients.

Quantitative assessment of five behavioural laterality measures: distributions of scores and intercorrelations among right-handers

Five behavioural laterality measures (degree of right-handedness, ear dominance, ocular dominance, line bisection performance, and lateral preference as assessed by preferred direction of conjugate lateral eye movements) were analysed in 1171 right-handers by merging data from 15 different samples. All laterality aspects were assessed quantitatively by using well controlled performance measures instead of questionnaires. The distributions and intercorrelations of the measures as well as their test-retest reliabilities were examined. Results show moderately high reliabilities for all measures, and clearly demonstrate considerable variability among dextrals in the direction and magnitude of the obtained behavioural laterality measures. Most importantly, independence of the different measures questions some common assumptions in laterality research. It seems unlikely, for instance, that handedness and other aspects of laterality may be a product of one single underlying mechanism of brain lateralisation.

Seasonal trends in the birth of sinistrals

This study examined the proposition that hand preference may change with season of birth in a group of 523 students born in southern Australia. Hand preference and performance measures revealed a higher incidence of sinistrality or weak dextrality for individuals born in winter and autumn compared to summer and spring. These seasonal effects tended to be more pronounced for females compared to males. Similar seasonal patterns have been observed in the northern hemisphere. Previous research has accounted for seasonal changes in hand preference in terms of changes in the frequency of viral infections. It is proposed that seasonal variations in hand preference can also be accounted for by seasonal variations in hormone levels. Higher levels of testosterone in spring compared to autumn have been reported in females. Exposure to higher levels of testosterone during the first trimester of foetal development is thought to promote the development of the right hemisphere. Individuals born in winter would have been exposed to the high testosterone levels associated with spring during the first critical trimester of foetal brain development. The lack of seasonal effect observed for males may be related to endocrinological differences between the male and female foetus.

The Effect of Familial Sinistrality and Academic Experience on Cognition in Right-Handed Women

Research in sex, brain lateralization, and cognition suggests that right-handed women vary in cognitive ability according to handedness inheritance patterns. Right-handed college women with positive familial sinistrality (FS+; i.e., the presence of at least one left-handed biological relative; n=30) were compared with right-handed women with negative familial sinistrality (FS-; n=30) by means of visuospatial (the Rey-Osterrieth Complex Figure Test and the Wechsler Adult Intelligence Scale--Revised Block Design subtest), verbal (the California Verbal Learning Test and Animal Naming), and motor performance tests (Finger Tapping and Grooved Pegboard). FS+ women outperformed FS- women on spatial tasks and used more efficient spatial strategies. The FS- group showed no corresponding verbal advantage. Spatial differences were not accounted for by motor skill, intellectual ability, or academic major.

Family size, miscarriage-proneness, and handedness: tests of hypotheses of the developmental instability theory of handedness

The handedness theory of Yeo and Gangestad (1993) posits that moderate right-handedness is the modal manifestation of genetic handedness and that ''developmental instability'' produces deviations from modal handedness. It is also suggested (1) that sinistral parents may produce fewer offspring than do dextral parents; and (2) that sinistral mothers may be more prone to miscarriages than are dextral mothers. In line with these hypotheses, Gangestad et al. (1996) reported that a human leukocyte antigen (B8) was related to both left-handedness and to reduced offspring number in their study. They also found that left-handedness was related to the human leukocyte antigen DR3, and Yeo and Gangestad (1998) noted that this antigen has been found by Christiansen et al. (1996) to be associated with an increased risk of spontaneous abortion in women. We assessed the first hypothesis through a study of the family sizes of 2083 families with two right-handed parents and 502 families having one or more left-handed parents; we assessed the second hypothesis from miscarriage history data supplied by 300 dextral and 52 sinistral mothers. Results supported the developmental instability theory with respect to the hypothesis regarding family size, but not with respect to the hypothesis regarding miscarriage-proneness.

Developmental instability and right shift theory hypotheses concerning correlates of familial sinistrality: negative findings

Both the right shift (Annett, 1985) and developmental instability (Yeo & Gangestad, 1993) theories of handedness predict that familial sinistrality (FS) should relate to hand skill asymmetries (HSA) on peg-moving tasks. Annett also hypothesises that both HSA and a procedure she derived for classifying individuals according to the pattern of their hand preferences for different manual activities can index the genotypes posited in her theory, i.e. she believes these variables are highly correlated. These hypotheses were tested in a sample of 280 dextral college students. Results failed to support the finding of Gangestad and Yeo (1994) that subjects showing greater hand skill asymmetry deviations from typical asymmetry had a greater probability of having a left-handed parent. There was also no support for Annett's finding that greater dextrality, as defined by her hand preference classification system, was negatively associated with familial sinistrality. Additionally, the relationship between HSA and Annett's hand preference classes was found to be exceedingly weak, contrary to Annett's theorising. These and other failures to find highly replicable differences between FS- and FS+ dextrals cast doubt on the validity of genetic or partial-genetic theories of handedness that posit a ''penetrance'' of a recessive non-dextrality-favouring gene that causes presumed dextral heterozygotic persons to differ from presumed homozygotic dominant persons in cognitive or manual skills.

Season of birth and fluctuating asymmetry

Fluctuating asymmetry (FA) refers to random, small deviations from perfect bilateral symmetry in morphological traits. These minor deviations from the ideal phenotype reflect environmental and genetic perturbations experienced during ontogeny. FA has been associated with negative health outcomes and many developmental disorders in humans. The prevalence of developmental disorders and adult health vary according to the month of birth, suggesting that seasonal stressors may leave enduring signs in the adult body, marked by high FA. The current study examined the relationship between FA and birth season. Data were collected for 205 males and females (average age = 20.39 years) on FA of 10 bilateral traits (second, third, fourth, and fifth digit length, palm height, wrist diameter, elbow width, ear height, foot breadth, and ankle circumference). Additional relationships were also investigated among FA, testosterone (T), and birth order. Results indicate that ear FA was lower for fall births compared to winter births in males. In females, palm FA was lower for fall births compared to those of the spring. FA of the digits was positively associated with T in males. Average FA, excluding the digits, decreased as the number of maternal siblings increased for both sexes. T concentrations in males were positively associated with the number of younger brothers. Our results generally confirm previous research on seasonal variation in adult longevity and neurological and psychiatric disorders, suggesting that winter and spring births are at risk for asymmetric developmental trajectory.

Evaluation of empirical methods and methodological foundations of human left-handedness

The methodological and empirical foundations underlying the theories on human left-handedness are examined, including the definition and classification of left-handedness, its relation to pathology, and the results on family, adoption, and twin research. It is argued that major research findings on left-handedness are based on inadequate empirical methods, and the concrete definitions and assessment procedures do not meet validity and reliability criteria. This also casts doubt on the relationship between left-handedness and pathology reported in the literature. Furthermore, findings from family, adoption, and twin studies do not allow interpretation of possible direct genetic contributions to explaining left-handedness. It is concluded that the methodological limitations of a causal explanation should encourage an examination of the epistemological and methodological premises of theory formulation. The functional method and a theoretical framework based on early childhood experiences and the neural plasticity of the brain may be more appropriate for the selection of empirical methods in research of human left-handedness.

Motoric dominance and sporting excellence: training versus heredity

In the past two decades, laterality in sports has become clearly established. In particular, the concept of motoric dominance in sports has been a fertile area for laterality research. The literature is consistent with the view that in certain sporting skills, sporting competitors with specific motoric-dominance patterns enjoy an advantage over other competitors and are overrepresented in some sports. Two theoretical interpretations have been offered to explain this imbalance of motoric-dominance distribution among sporting individuals: hypotheses of "innate superiority" and "strategic advantage." This paper presents an overview the two hypotheses, along with relevant work so far reported and identifies directions for further empirical research. The theoretical and practical implications of research on laterality in human motor performance are discussed.

Developmental Instability Is Associated With Neurocognitive Performance in Heterosexual and Homosexual Men, but Not in Women

Little is known about the neurodevelopmental nature of human cognitive abilities. This investigation presents evidence consistent with a hypothesis that interindividual and within-sex cognitive variations are associated with vulnerabilities to environmental sources of developmental stress. A large sample of healthy heterosexual and homosexual men and women (N=240) completed a series of visuospatial and verbal tests. A composite fluctuating asymmetry (FA) measure was computed from the lengths of the finger digits. In heterosexual men, higher FA scores were associated with poorer line orientation judgment; and in homosexual men, with poorer verbal fluency and perceptual speed. No associations were found in heterosexual or homosexual women. These results suggest that developmental instability is linked to neurocognitive integrity in men, but not in women.

Early life environment modulates 'handedness' in rats

Right handedness is one of the most prominent markers of human functional brain asymmetry. Deviation from this norm appears to be associated with certain developmental disorders. While many studies have dealt with the genetic contribution to the determination of handedness, few have examined whether environmental factors that are subtler than forced hand switching can modulate the development of handedness. In this study, we exposed rats to a novel environment for 3 min daily during their first 3 weeks of life and found that their paw preferences during both infancy and adulthood showed a leftward shift compared with the controls. This result suggests that 'handedness' can be modified by rather subtle early environmental manipulation. Since exposure to a novel environment does not involve a direct asymmetric activation of the sensory--motor system underlying paw-use, mechanisms beyond this paw-specific system must exist to mediate the observed modulation of 'handedness'.

Birth order and left-handedness revisited: some recent findings in chimpanzees (Pan troglodytes) and their implications for developmental and evolutionary models of human handedness

This paper examines the relationship between parity, pregnancy outcome, and handedness in a sample of captive chimpanzees (Pan troglodytes). The relation between parity, maternal age and positive or negative pregnancy outcome was assessed from life history data for 536 chimpanzees housed at the Yerkes Regional Primate Research Center. The incidences of negative pregnancy outcome (notably spontaneous abortions and stillbirths) were significantly higher in parities of 8 or higher compared to all other parities. In a sub-sample of 165 chimpanzees, the relation between parity, maternal age and handedness was assessed to determine whether left handedness may serve as a marker of prenatal pathology. These analyses indicated that left-handedness was more prevalent in 1st and 8 or higher parities compared to parities between 2 and 7, respectively. Possible prenatal hormonal and periparturitional factors are discussed as possible mechanisms for the observed findings.

Inflammatory bowel disease and laterality: is left handedness a risk?

BACKGROUND

Left handedness has been associated with inflammatory bowel disease (IBD) and autoimmune diseases.

AIMS

To determine whether left handedness is associated with IBD in two prospective national birth cohorts.

METHODS

Subjects with Crohn's disease (CD) and ulcerative colitis (UC) were identified from two national longitudinal birth cohorts at age 26 years (1970 British Cohort Study (BCS70), born in 1970) and age 33 years (National Child Development Study (NCDS), born in 1958). Laterality was determined at age 10 (BCS70) or seven (NCDS) years, based on hand preference for writing and foot preference for kicking a ball (BCS70 only). Multiple logistic regression was used to assess the relationship of handedness with CD, UC, and IBD in the cohorts combined and adjusted for sex.

RESULTS

Both cohorts combined showed increased adjusted relative odds of 2.13 (95% confidence interval (CI) 0.97--4.65; p=0.059), 2.13 (95% CI 0.92--4.91; p=0. 077), and 2.13 (95% CI 1.20--3.78; p=0.010) for CD, UC, and IBD, respectively in left handers.

CONCLUSIONS

The study suggests a link between IBD and left handedness which may be genetic and/or environmental in origin.

Dermatoglyphic fluctuating asymmetry and atypical handedness in schizophrenia

Atypical handedness and dermatoglyphic abnormalities are hypothesized to reflect a neurodevelopmental disturbance in schizophrenia. Developmental instability, indexed by dermatoglyphic fluctuating asymmetry (FA), reflects the degree to which an individual's ontogenetic program is maintained and provides a useful framework in which to consider atypical handedness in schizophrenia. Thirty patients diagnosed with schizophrenia were compared with 37 matched healthy controls on levels of dermatoglyphic FA, a demonstration task determining hand preference and a test of relative hand skill. Multivariate analyses established that patients demonstrated greater FA and more atypical hand skill compared with controls. In patients, but not in controls, there was a strong positive association between a measure of FA and a measure of atypical hand skill, suggesting that these markers of neurodevelopmental disturbance are related in schizophrenia. On a measure of hand preference, patients were more likely than controls to be classified as mixed handed than either right or left handed. Results from the present study support the conjecture of greater developmental instability in schizophrenia affecting neurodevelopmental processes, including those conferring manual dominance.

Genetic influence on the expression of hand preferences in chimpanzees (Pan troglodytes): evidence in support of the right-shift theory and developmental instability

Genetic mechanisms have been proposed to explain the pervasive representation of right-handedness in humans, whereas random, nongenetic factors have been posited to explain the lack of population-level right-handedness in nonhuman primates. We report evidence that hand preferences in chimpanzees are heritable, even among related individuals raised in different environments. Furthermore, we report that the degree of heritability is modified by factors associated with developmental instability, notably, offspring parity. The data are interpreted to reconcile both genetic models for handedness and hypotheses suggesting that developmental instability influences variation in handedness.

Developmental instability and working memory ability in children: a magnetic resonance spectroscopy investigation

This study of children (ages 7 through 12) wishes to determine (a) whether variation in frontal lobe brain chemistry, determined from proton magnetic resonance spectroscopy (1H-MRS), is related to performance on a working memory task in children, and (b) whether developmental instability (DI; the imprecise expression of the genetic plan for development due to several known genetic and environmental effects) underlies phenotypic variation in brain chemistry. 1H-MRS assessed neurometabolites in a right frontal white matter voxel. The Visual Two-Back test assessed working memory. A composite measure of DI was created from measures of minor physical anomalies, fluctuating asymmetry of body characteristics, and fluctuating asymmetry of dermatoglyphic features. Greater DI strongly predicted lower concentrations of creatine-phosphocreatine (Cre) and choline-containing compounds, whereas Cre and N-acetyl-aspartate positively correlated with working memory skills. Working memory skills thus seem related to frontal lobe energy metabolism, which in turn is related to DI.

Sexual orientation and handedness in men and women: a meta-analysis

Recent findings suggest that sexual orientation has an early neurodevelopmental basis. Handedness, a behavioral marker of early neurodevelopment, has been associated with sexual orientation in some studies but not in others. The authors conducted a meta-analysis of 20 studies that compared the rates of non-right-handedness in 6,987 homosexual (6,182 men and 805 women) and 16,423 heterosexual (14,808 men and 1,615 women) participants. Homosexual participants had 39% greater odds of being non-right-handed. The corresponding values for homosexual men (20 contrasts) and women (9 contrasts) were 34% and 91%, respectively. The results support the notion that sexual orientation in some men and women has an early neurodevelopmental basis, but the factors responsible for the handedness-sexual orientation association require elucidation. The authors discuss 3 possibilities: cerebral laterality and prenatal exposure to sex hormones, maternal immunological reactions to the fetus, and developmental instability.

Do left-handed competitors have an innate superiority in sports?

This study assessed handedness distributions among (a) sporting competitors (n = 1,112) and nonsporting university students (n = 1,112), (b) sporting competitors engaging in interactive (n = 576) and in noninteractive sports (n = 536), and (c) sporting competitors engaging in direct interactive (n = 219) and indirect interactive (n = 357) sports. Chi-squared showed that there were statistically significant differences in proportions of left-handed persons in (a) sporting competitors and nonsporting university students, (b) sporting competitors engaging in interactive and noninteractive sports, (c) sporting competitors engaging in interactive sports and nonsporting university students, and (d) sporting competitors engaging in direct interactive and indirect interactive sports. It appears that left-handers are more common among those who engage in competitive manual activities. This superiority of the left-handers may be fully explained by a consideration of tactical or strategic factors associated with handedness during sporting interactions. The results with important implications for the measurement and evaluation of handedness are discussed in the light of the current findings on laterality.

Nonshared environment: a theoretical, methodological, and quantitative review

When genetic similarity is controlled, siblings often appear no more alike than individuals selected at random from the population. Since R. Plomin and D. Daniels' seminal 1987 review, it has become widely accepted that the source of this dissimilarity is a variance component called nonshared environment. The authors review the conceptual foundations of nonshared environment, with emphasis on distinctions between components of environmental variance and causal properties of environmental events and between the effective and objective aspects of the environment. A statistical model of shared and nonshared environmental variables is developed. A quantitative review shows that measured nonshared environmental variables do not account for a substantial portion of the nonshared variability posited by biometric studies of behavior. Other explanations of the preponderance of nonshared environmental variability are suggested.

The evolutionary genetic underpinnings of schizophrenia: the developmental instability model

The importance of genes in the etiology of schizophrenia is well known, but the manner in which the relevant genomic factors influence neural development and the nature of selection forces operating on these factors are poorly understood. In several prominent papers, Crow has provided a unique and comprehensive theory that attempts to deal with these issues. A central aspect of his theory is that a single gene leads to reduced cerebral lateralization, increased ventricular size, and risk for developing schizophrenia. He relies greatly on Annett's right shift theory of individual variation in handedness. An alternative approach, based on the construct of developmental instability, provides a different way to conceptualize genetic influences, selection forces, and atypical lateralization in schizophrenia. We suggest that the developmental instability model has stronger empirical support and is better grounded in contemporary evolutionary genetics.

Evolutionary psychology and genetic variation: non-adaptive, fitness-related and adaptive

Behavioural variation across individuals can be substantial. A broad generalization emerging from three decades of behavioural genetic studies is that most psychological individual differences have moderate broad heritabilities (30-60%). There are at least three possible scenarios for this genetic variation. First, it may be adaptively neutral and not subject to selection. Second, it may be related to fitness despite selection. Third, it may be maintained by selection for alternative adaptations. Some authors favour the first of these possibilities, but the latter two cannot be ruled out. First, temporally varying selection pressures (e.g. pathogens) can maintain fitness-related genetic variance in a population despite current selection pressures. Moreover, direct and indirect evidence on humans support the notion that some phenotypic variance is fitness related. Second, while adaptive alternatives are unlikely to be found at a level of highly complex design, frequency dependent selection can maintain variation at finer, quantitative levels. One potential example is discussed. Because of their particular relevance to evolutionary psychology, fitness-related and adaptive genetic variance deserve further attention.

Meningitis and the evolution of dominance of righthandedness

In bacterial meningitis, forced deviation of the head and/or the eyes towards either the left or the right side reflects an ipsilateral focal inflammation of the brain. Also, the symptom of forced deviation is associated with a high fatality and sequelae rate. In this study it was observed that, among 2124 Dutch patients with bacterial meningitis, forced deviation was significantly more often to the right than to the left. Because the right hemisphere is dominant for motor functions in natural lefthanders, it was concluded that bacterial meningitis might have been a selection mechanism favouring the evolution of a dominance of righthandedness.

The mixture distribution of left minus right hand skill in men and women

The distribution of left minus right (L - R) hand skill (peg moving) was studied in 436 men and 247 women. In total sample, the distribution was continuous and a sum of two Gaussian curves. One of them was constituted by subjects with better right hand (79.0%) and better left hand (21.0%). The other curve (2.5% of total) was exclusively due to the subjects with better left hand. In men and women, the total curve was a sum of three overlapping Gaussian curves. Two of them were mostly due to the subjects with better right hand. There was a third curve representing almost entirely the subjects with better left hand. Women were more strongly right-handed than men. There was no significant sex difference at the sinistral side. The overall results were compatible with the right shift theory, but there was not a single Gaussian curve representing a chance distribution with a mean of zero. These results suggested that the distribution of manual asymmetry in skill is continuous, which may be described by probabilities due to subjects with better right and left hands; there is no place for chance in human handedness.