For centuries, left-handedness has been a subject of fascination and speculation. In a world largely designed for the right-handed majority, left-handers represent a unique and intriguing minority. This article delves into the compelling question: why are some people left-handed? We will explore the current scientific understanding of handedness, examining the interplay of genetics, environmental influences, and evolutionary pressures that contribute to this enduring human trait. Understanding why some individuals favor their left hand over their right requires a multifaceted approach, considering factors from our ancient past to modern neuroscience.
The Prevalence and History of Left-Handedness
To understand the evolutionary forces at play in handedness, it’s crucial to recognize its historical and geographical variations. Handedness isn’t a simple binary choice; it exists on a spectrum. While definitive categories of ‘left-hander’ and ‘right-hander’ are often used, individuals exhibit hand preference in varying degrees across different tasks. For instance, someone might write with their right hand but prefer their left for throwing. The strength of correlation between hand preference across tasks increases with task complexity and expertise in those tasks. Therefore, when studying handedness across diverse populations, selecting tasks relevant to various cultures is vital. Tasks like writing, common in Western societies, may be culturally biased and not universally applicable. Social and religious influences can also modify hand preference for certain activities. For example, historical social pressures in China favoring right-handed writing and eating significantly reduced left-handedness for these specific tasks.
Archaeological evidence provides insights into handedness in ancient humans. Analysis of skeletons, tools, and artifacts suggests a long history of handedness polymorphism with a right-hand dominance. Studies on Neanderthal arm bone length indicate right-hand prevalence dating back approximately 35,000 years. Tool analysis, through replication studies of knapping patterns, infers handedness in toolmakers from 300,000 to 400,000 years ago, showing a majority of right-handers. Tool wear and shape from axes and cleavers dating back roughly 1 million years also point to a majority of right-hand users. Dental marks on ancient remains further support the existence of handedness polymorphism in Neanderthals. Cave paintings, particularly negative handprints from the Upper Paleolithic period, consistently show a higher prevalence of left hands, indicating a larger proportion of right-handed painters.
Artistic representations have also been used to investigate handedness. Prehistoric cave paintings, like those in Altamira Cave (14,000–18,500 BP), suggest handedness polymorphism based on artistic style differences between right and left-handers. While depictions of handedness in art can be informative, cultural, religious, and aesthetic biases must be considered. Despite methodological challenges, archaeological and artistic records consistently point to a long-standing polymorphism of hand use in human populations, with right-handers consistently being more numerous. This persistence over evolutionary time suggests selective forces are at play. Further evidence for selection pressures comes from the geographical variation in right- and left-hander frequencies around the world.
Geographical variation in handedness is well-documented. Studies focusing on tasks like throwing or hammering across various countries reveal a wide range of left-handedness frequencies (5–25.9%). Writing hand preference studies across 17 countries showed left-handedness ranging from 2.5–12.8%. Traditional societies exhibit similar variations, with left-hander frequencies between 3.3 and 26.9% across eight societies. Consistently, left-handers are less frequent than right-handers globally. Furthermore, in many populations, a lower proportion of women are left-handed compared to men, indicating sex-related influences on handedness. This persistent polymorphism across all studied human populations suggests evolutionary mechanisms maintain this diversity. For natural selection to operate on handedness, it must be a heritable trait, which we will explore in the next section.
The Role of Genetics in Handedness
Family studies offer initial insights into the mechanisms of handedness by assessing its transmission across generations. Tables showing the frequency of left-handedness in families reveal a clear familial component. Parents who are both right-handed are less likely to have left-handed children than parents with other handedness combinations. Notably, couples where both parents are left-handed have the highest proportion of left-handed offspring, approximately 30–40%. This suggests a potential genetic or learned transmission of hand preference from parents to children.
Interestingly, there’s a higher incidence of left-handedness in children of right-handed fathers and left-handed mothers (RxL matings) compared to left-handed fathers and right-handed mothers (LxR matings), suggesting stronger maternal influences. This could indicate a sex-linked genetic effect or greater maternal social influence. However, some studies have shown this maternal effect only in sons and a paternal effect in daughters, potentially supporting an X-linked genetic determination. Heritability estimates for handedness vary from 0.23 to 0.66. Sex-specific heritability estimates further complicate the picture, showing varying paternal and maternal heritability contributions in males and females.
While familial patterns suggest heritability, shared environments within families also play a role. Adoption studies help differentiate genetic from environmental transmission. Studies on adopted children show no correlation between handedness of adopted children and their adoptive parents, while control groups show significant correlation with biological parents. Twin studies further clarify the genetic component. Monozygotic (identical) twins are significantly more concordant for handedness than dizygotic (fraternal) twins. Dizygotic twins’ handedness concordance aligns with binomial expectations, whereas monozygotic twins show higher concordance than chance. This data strongly suggests a genetic contribution to handedness. However, the discordance in handedness among monozygotic twins also highlights the role of cultural and environmental influences. Despite the complexities, evidence convincingly points to a significant heritability of handedness, allowing for natural selection to act upon this trait.
Genetic models attempting to explain handedness face several challenges: cultural biases in hand use, handedness discordance in monozygotic twins, and the lower-than-expected rate of left-handedness in children of two left-handed parents. Genetic models often assume a genetic basis for both laterality and hemispheric asymmetry. Simple Mendelian models fail to fully explain observed patterns. Layton’s research on mice with the iv mutation, causing random organ placement, significantly influenced handedness models by introducing the concept of random genetic factors in asymmetry. Single-gene models, such as Annett’s ‘right-shift theory’ and McManus’ model, propose a single gene with two alleles, incorporating chance factors during development to account for the complexities. These models set a theoretical 50% threshold for left-handedness prevalence to explain observed low rates in LxL offspring and monozygotic twin discordance. Other models include X-linked three-allele models and random-recessive models. However, many observed associations remain difficult to fit into simple genetic models. Segregation analysis in Hawaiian families suggests a limited genetic contribution (10–20%) to handedness phenotype, with environmental factors dominating (80–90%). The failure of simple genetic models suggests a complex genetic basis involving multiple genes or other unidentified factors.
Molecular studies using genome-wide approaches have begun to pinpoint potential genetic regions associated with handedness. Studies on left-handed brothers suggested linkage between a marker on the X chromosome (Xq21) and hand skill. Genome-wide screens identified quantitative trait loci (QTLs) for hand skill on chromosome regions 2p11.2–12 and 17p11–q23, with the 2p12–q11 QTL being further confirmed and showing parent-of-origin effects. Linkage for handedness on chromosome region 10q26 and chromosome region 12q21–23 have also been reported in other studies. The variation in identified genomic regions across studies likely reflects differences in handedness measurement, suggesting multiple genes influence handedness. Large-scale studies with comprehensive genome coverage are needed to definitively identify genes involved in hand skills and preferences. Empirical and family studies robustly indicate a significant genetic component in hand preference. Understanding the selection pressures on handedness also requires examining environmental factors, which we explore next.
Developmental and Environmental Factors in Handedness
Developmental factors, particularly the in utero environment, significantly influence handedness. The association between left-handedness and certain health issues has led to the concept of ‘pathological left-handedness’ arising from developmental stresses, contrasted with ‘familial left-handedness’ linked to genotype. This hypothesis suggests some left-handedness results from developmental disruptions. Satz and colleagues proposed that early brain injury might cause individuals to switch to the non-dominant hand, explaining the increased frequency of left-handers in populations with central nervous system disorders like schizophrenia, epilepsy, and learning disabilities. Any stressor or pathological factor disrupting typical development and inducing hand preference switching could result in a higher prevalence of left-handedness.
Lateralized behavior appears early in development and is susceptible to in utero environmental influences. Fetal arm movements as early as 9-10 weeks gestation show a majority (75%) exhibiting more right arm movements. Similar right-side bias is observed in fetuses from 12 to 27 weeks. From 15 weeks, fetuses show a preference for sucking their right thumb, and fetal thumb-sucking behavior correlates with hand preference later in childhood (10–12 years). Head turning preference to the right relative to the body is observed from 38 weeks of gestation.
Hormonal factors, particularly prenatal sex hormones, are proposed to influence handedness. The Geschwind-Galaburda theory posits that high prenatal testosterone levels or heightened sensitivity to these hormones disrupt neural development, leading to physiological changes and increased likelihood of left-handedness and weaker lateralization. The theory suggests the left hemisphere matures later and is more vulnerable to adverse in utero environments. High prenatal testosterone may slow neuronal growth in the left hemisphere, weakening its dominance. This theory is not mutually exclusive with genetic hypotheses, as in utero testosterone levels have a genetic component.
The Geschwind-Behan-Galaburda theory links dyslexia, immune disorders, and left-handedness to elevated prenatal testosterone. It suggests testosterone acts independently on the thymus and brain, potentially favoring compensatory mechanisms that might explain specific talents associated with left-handedness. Testosterone can retard the development of immune structures, increasing susceptibility to immune disorders like asthma, eczema, and allergies. Studies comparing handedness in patients with immune disorders versus control populations with non-immune disorders have yielded mixed results, with both positive and negative correlations reported. The association between left-handedness and various diseases remains unclear, and the Geschwind-Galaburda model remains controversial.
Directly testing prenatal testosterone effects in humans is challenging. However, studies show a negative correlation between the 2D:4D finger length ratio in right hands and adult testosterone levels in men. Digit ratios are established in utero, suggesting the 2D:4D ratio may reflect prenatal testosterone exposure. The difference in 2D:4D ratio between hands (left-right) correlates with relative hand skill, with a higher left hand ratio and lower right hand ratio associated with better left-hand performance. Furthermore, the number of CAG repeats in the androgen receptor gene on the X-chromosome explains a portion of the genetic variance in handedness, suggesting androgen sensitivity may be involved. Thus, in utero environmental influences on handedness may stem from heritable factors related to hormonal secretion and sensitivity.
Developmental instability, stemming from polygenetic inheritance of developmental factors, offers another perspective on handedness variation. Yeo and Gangestad’s research suggests increased minor physical anomalies and fluctuating asymmetries (markers of developmental instability) in both left-handers and extreme right-handers. They propose minimal developmental instability is near the median of relative hand skill distribution. They also predicted and observed that extreme right-handers are more likely to have left-handed parents due to genotypes predisposing them to developmental instability. Deviation from moderate right-handedness, in this view, reflects imprecise developmental expression due to instability, not necessarily brain damage, but regional variations in fetal growth rates. Left-handers, consistent with this, show greater brain symmetry and reversed asymmetry compared to right-handers. Developmental instability could have a genetic basis, potentially linked to polygenic homozygosity, specific human leukocyte antigen alleles, and pathogen resistance.
Left-handedness is more prevalent in disorders associated with developmental abnormalities, including neural tube defects, autism, psychopathy, cleft palate syndrome, stuttering, and schizophrenia, although negative findings have also been reported. Coren and Searleman proposed that mild birth stressors or atypical intrauterine environments might lead to left-handedness as a behavioral marker of minor neurological developmental abnormalities.
Birth stress, particularly perinatal left hemisphere neurological damage due to oxygen deficiency, has been proposed as a cause of left-handedness by Bakan. He argued birth stress (premature birth, prolonged labor, etc.) leads to hypoxia, more damaging to the left hemisphere. Evidence shows an excess of left-handers among babies with birth stress history and individuals with neurological impairments. However, numerous studies have failed to support this birth stress hypothesis. Coren and Porac found higher average maternal age for mothers of left-handed children, and Smart and colleagues observed more left-handed children in older primiparous mothers. However, these findings regarding maternal age are also inconsistent across studies. Twins, regardless of zygosity, exhibit higher rates of left-handedness compared to singletons, possibly due to unique in utero conditions in multiple pregnancies.
Birth weight has been proposed as a unifying factor in ‘pathological left-handedness’. Low birth weight is associated with perinatal complications, neurological problems, and adult pathologies. There’s evidence for increased left-handedness among extremely low birth weight babies. Low birth weight increases the risk of early brain damage. Alternatively, fetal brain development might be interrupted by birth itself, leading to differences in cortical growth stage in premature or low birth weight babies. Specifically, the planum temporale, typically larger in the left hemisphere in right-handers, is often less asymmetric in left-handers. Brain structural asymmetries develop in utero and are statistically related to handedness, though the relationship is not absolute.
If ‘pathological’ and ‘familial’ left-handedness are distinct, birth weight distribution in left-handers might show bimodality, with lower overall and greater variance in birth weight compared to right-handers. While developmental and perinatal problems can be environmentally influenced, they are also significantly heritable, indicating a genetic contribution and suggesting a negative selection pressure associated with left-handedness.
Cultural Influences on Handedness Expression
Cultural factors also exert significant influence on handedness. Laland and colleagues criticized handedness models for often overlooking cultural influences, despite substantial evidence of their importance. Attitudes towards left-handedness vary widely across cultures. Cultural and environmental factors can modify hand preference in several ways: changing hand use for specific activities (like writing or eating), reducing the strength of hand preference across tasks, or even altering the overall preferred hand, depending on the intensity of cultural pressure.
Mikheev and colleagues found that highly skilled right-handed judo wrestlers more frequently used their left hand for certain judo movements compared to controls. They suggest motor skill acquisition, like long-term judo training, can modify lateral preferences through neuroplasticity. Alternatively, lower asymmetry might be advantageous in judo, leading individuals with less asymmetry to become more proficient wrestlers. Genetic, developmental, and environmental factors all contribute to hand preference determination. Hand preference is heritable and varies across populations, suggesting evolutionary forces are shaping this trait.
Evolutionary Forces Maintaining Handedness Polymorphism
To understand the evolutionary forces driving handedness polymorphism, we need to examine variations in morph frequencies. Polymorphism maintained across all populations is unusual for a neutral trait, as genetic drift often eliminates it in some populations. The consistent presence of handedness polymorphism suggests it’s not neutral and selective forces are at play. Directional selection alone would favor one morph and eliminate polymorphism. The ancient and widespread handedness polymorphism points to balancing selection, potentially due to situation-dependent benefits. Therefore, identifying costs and benefits associated with left-handedness is crucial to understanding its evolutionary role.
Left-handedness as a potential costly trait is suggested by varying frequencies across age classes, possibly reflecting changing social norms, particularly regarding writing hand preference. Studies using writing hand preference might find lower mean age at death for left-handers due to historical social pressures, even if actual longevity is similar. However, even when using other hand preference measures, some studies have reported reduced longevity in left-handers, although contradictory evidence also exists.
Potential explanations for reduced longevity in left-handers include: increased prenatal and perinatal birth stressors, potential genetic or hormonal effects reducing immune system effectiveness, and higher risk of lethal accidents. However, direct fitness cost measurements are lacking, limiting our understanding of evolutionary significance. Higher accident risk for left-handers, particularly in Western societies, might be due to industrialized environments designed for right-handers. Studies suggest a portion of lifespan difference is due to accidental death and warfare. Aggleton and colleagues suggest left-handers’ increased accidental death risk is due to navigating a right-handed world. However, a remaining difference after removing accidental deaths suggests other factors may contribute to left-handed disadvantage. De Agostini and colleagues proposed upper limb injuries causing preferred hand disuse could lead to mixed-handedness and an association with accident frequency. A study in Brazil found dextral individuals more vulnerable to accidental death. Further research is needed with reliable death cause data and birth cohorts. Survival before and during reproductive years is crucial for fitness, and post-menopausal survival is also increasingly recognized for women’s reproductive value. Currently, longevity and handedness research primarily focuses on men, and the impact of longevity differences on handedness evolution remains unclear.
Another potential cost is lower body size observed in left-handers. Body size is an important component of selective value in humans, particularly for male reproductive success. Studies suggest an association between delayed physical maturation and left-handedness, although contradictory results exist. Delayed sexual maturity could also impact reproductive success, potentially representing a fitness cost for left-handers. Further research is needed to quantify this influence. Higher left-handedness frequency among homosexual men has been reported. As homosexual men may have lower reproductive success, this could introduce a fitness bias. However, the association between handedness and sexual orientation is debated, and the proportion of homosexual men is relatively low, likely limiting this factor’s evolutionary impact on handedness frequencies. While potential fitness costs are suggested, their actual impact on fitness and evolutionary significance remain to be fully determined.
Left-handedness as a beneficial trait is supported by evidence of greater intermanual coordination and smaller hand skill asymmetries in left-handers compared to right-handers. Left-handers also show less language dominance lateralization. Smaller right-left differences and higher intermanual coordination might stem from greater bi-hemispheric control. Some studies suggest better interhemispheric transfer in non-right-handers, possibly related to a larger corpus callosum observed in some studies, although this is debated, and the relationship between callosal morphology and degree of lateralization, rather than direction, may be more relevant. Increased intermanual coordination in left-handers, requiring bi-hemispheric control, may be facilitated by more efficient information exchange via callosal pathways. A larger corpus callosum has also been linked to superior verbal fluency and memory, potentially benefiting left-handers.
Creativity has also been linked to left-handedness, particularly in men. A higher proportion of left-handers has been observed in gifted children. Some studies suggest left-handers possess enhanced musical or mathematical abilities, although the latter is controversial. These potential advantages could influence social status. Socio-economic status and cognitive ability studies suggest potential socio-economic advantages for left-handers in certain professions. Studies have shown higher hourly earnings for left-handed men, particularly those with higher education, though findings for women are less consistent. Socio-economic status differences could impact reproductive success, as socio-economic status influences mate choice and offspring benefits.
A significant potential benefit of left-handedness is a strategic advantage in sports, with an overrepresentation of left-handers in elite levels of interactive sports like tennis, baseball, and fencing. This advantage is tactically explained by the rarity of left-handers. Right-handers are more accustomed to facing other right-handers, while left-handers are used to right-handed opponents. This “surprise effect” gives left-handers an advantage, amplified by their lower frequency. Interactive sports, offering strategic advantages to rarer left-handers, show higher left-handedness frequencies compared to non-interactive sports where frequencies mirror the general population. This strategic advantage in interactive sports and fights could represent a strong selective advantage, leading to increased survival or higher social status and mate access. This advantage is frequency-dependent, stronger when left-handers are rarer. Theoretical models support frequency-dependent selection maintaining opposite asymmetrical morphs as an evolutionary stable strategy. The frequency-dependent strategic advantage of left-handers in fights could be a key balancing selection force maintaining handedness polymorphism.
Discussion and Future Directions
The correlation between left-handedness frequency and homicide rates suggests frequency-dependent selection favoring left-handers in violent interactions. However, without counterbalancing costs, frequency-dependent advantage alone would lead to 50% left-handedness prevalence at equilibrium. The consistently lower frequencies of left-handedness worldwide indicate associated costs. Costs studied in Western societies are often attributed to right-handed-centric technology. However, left-handedness frequencies remain below 30% in traditional societies, suggesting costs exist even in non-industrialized environments.
Increased health risks and problems in left-handers are reported, but it’s unclear if these apply to all left-handers or a ‘pathological’ subgroup. Distinguishing ‘pathological’ and ‘familial’ left-handers is challenging without identified major genes. Single-gene models are based on ad hoc assumptions rather than empirical genetic data. Language lateralization heterogeneity in left-handers hints at their possible heterogeneity. While 97% of right-handers show left hemisphere language dominance, only 60% of left-handers do, with 30% showing bi-hemispheric and 10% right hemisphere dominance. Characterizing left-hander categories is crucial for understanding handedness evolution.
Data on left-handedness frequency dynamics over time are critical to identify evolutionary forces. Writing handedness frequency changes in the 20th century due to cultural shifts. For other hand preferences, longitudinal studies are scarce. Arm-waving analysis in Victorian England films and modern Google images suggests increased left-handedness frequency in England over the last century, but the reliability of this measure is questionable. Further investigation of left-handedness frequency across generations is needed to determine polymorphism stability.
If handedness polymorphism is near stable equilibrium, fitness differences between left- and right-handers are expected to be small and difficult to detect empirically. At equilibrium, fitness is equal, though fitness components may differ. Advantages for certain traits may exist for each handedness category, potentially explaining discrepancies in handedness studies. Identifying left-hander categories is crucial for better estimating fitness costs and benefits. Investigating current evolution of left-handedness frequencies and examining selection types, particularly frequency-dependent selection, in diverse environments are pivotal for future research.
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