There have been plenty of claims about what being left-handed means, and whether it changes the type of person someone is – but the truth is something of an enigma.
Myths about handedness appear year after year, but researchers have yet to uncover all of what it means to be left-handed.
So why are people left-handed?
The truth is we don’t fully know that either. What we do know is that only around 10% of people across the world are left-handed – but this isn’t split equally between the sexes. About 12% of men are left-handed but only about 8% of women. Some people get very excited about the 90:10 split and wonder why we aren’t all right-handed.
But the interesting question is, why isn’t our handedness based on chance?
Why isn’t it a 50:50 split?
It is not due to handwriting direction, as left-handedness would be dominant in countries where their languages are written right to left, which it is not the case.
Even the genetics are odd – only about 25% of children who have two left-handed parents will also be left-handed.
Being left-handed has been linked with all sorts of bad things.
Poor health and early death are often associated, for example – but neither are exactly true.
The latter is explained by many people in older generations being forced to switch and use their right hands.
This makes it look like there are less left-handers at older ages.
The former, despite being an appealing headline, is just wrong.
Positive myths are also abound. People say that left-handers are more creative, as most of them use their “right brain”.
This is perhaps one of the more persistent myths about handedness and the brain.
But no matter how appealing (and perhaps to the disappointment of those lefties still waiting to wake up one day with the talents of Leonardo da Vinci), the general idea that any of us use a “dominant brain side” that defines our personality and decision making is also wrong.
Brain lateralisation and handedness
It is true, however, that the brain’s right hemisphere controls the left side of the body, and the left hemisphere the right side – and that the hemispheres do actually have specialities.
For example, language is usually processed a little bit more within the left hemisphere, and recognition of faces a little bit more within the right hemisphere.
This idea that each hemisphere is specialised for some skills is known as brain lateralisation.
However, the halves do not work in isolation, as a thick band of nerve fibres – called the corpus callosum – connects the two sides.
Being left-handed has been linked with all sorts of bad things. Poor health and early death are often associated, for example – but neither are exactly true. The image is in the public domain.
Interestingly, there are some known differences in these specialities between right-handers and left-handers.
For example, it is often cited that around 95% of right-handers are “left hemisphere dominant”.
This is not the same as the “left brain” claim above, it actually refers to the early finding that most right-handers depend more on the left hemisphere for speech and language.
It was assumed that the opposite would be true for lefties.
But this is not the case.
In fact, 70% of left-handers also process language more in the left hemisphere.
Why this number is lower, rather than reversed, is as yet unknown.
Researchers have found many other brain specialities, or “asymmetries” in addition to language.
Many of these are specialised in the right hemisphere – in most right-handers at least – and include things such as face processing, spatial skills and perception of emotions.
But these are understudied, perhaps because scientists have incorrectly assumed that they all depend on being in the hemisphere that isn’t dominant for language in each person.
In fact, this assumption, plus the recognition that a small number of left-handers have unusual right hemisphere brain dominance for language, means left-handers are either ignored – or worse, actively avoided – in many studies of the brain, because researchers assume that, as with language, all other asymmetries will be reduced.
How some of these functions are lateralised (specialised) in the brain can actually influence how we perceive things and so can be studied using simple perception tests.
For example, in my research group’s recent study, we presented pictures of faces that were constructed so that one half of the face shows one emotion, while the other half shows a different emotion, to a large number of right-handers and left-handers.
Usually, people see the emotion shown on the left side of the face, and this is believed to reflect specialisation in the right hemisphere.
This is linked to the fact that visual fields are processed in such a way there is a bias to the left side of space.
This is thought to represent right hemisphere processing while a bias to the right side of space is thought to represent left hemisphere processing. We also presented different types of pictures and sounds, to examine several other specialisations.
Our findings suggest that some types of specialisations, including processing of faces, do seem to follow the interesting pattern seen for language (that is, more of the left-handers seemed to have a preference for the emotion shown on the right side of the face).
But in another task that looked at biases in what we pay attention to, we found no differences in the brain-processing patterns for right-handers and left-handers.
This result suggests that while there are relationships between handedness and some of the brain’s specialisations, there aren’t for others.
Left-handers are absolutely central to new experiments like this, but not just because they can help us understand what makes this minority different.
Learning what makes left-handers different could also help us finally solve many of the long-standing neuropsychological mysteries of the brain.
The aim of this review is to identify the evolutionary forces involved in human handedness. Two main questions are critical to improve our understanding of the evolution of human handedness: (i) why is hand use asymmetric (i.e. why do individuals show a preference for one hand in most manual tasks)? and (ii) why is hand use asymmetry polymorphic (i.e. why do some individuals prefer the left hand and some the right for a similar task)?
Most research on handedness has focused on the intersection of these two questions, namely, why are humans right-handed?
As a consequence, the debate on the origin and understanding of handedness has been partially obscured, as left-handedness was historically considered as an anomalous or pathological case, thus ignoring the relatively high proportion of left-handers within human populations.
Previously, asymmetric hand use has been studied extensively. Corballis (2003) suggested that the evolution of human speech implied an association between speech and gesture, and thus brain lateralization for speech may be responsible for asymmetric hand use.
This evolutionary explanation was further elaborated by Vallortigara & Rogers (2005) who evaluated the costs and benefits of brain asymmetry.
The aim of the present review is to clarify the question of the asymmetric hand use polymorphism in humans.
Many diverse studies on left-handedness have been conducted by researchers from various fields such as medicine, neurology and psychology. Although some data remain controversial, several hormonal, cultural and developmental factors have been found to be associated with handedness.
Here, we attempt to review the information available on both proximal and ultimate aspects of handedness in humans to identify the evolutionary mechanism of the persistence of left-handers.
RIGHT AND LEFT HAND USE IN HUMAN POPULATIONS
To understand the evolutionary processes implied in handedness, it is first necessary to have a clear idea of the historical and geographical variation of this trait.
(a) Hand preference assessment
Handedness measures are based on hand use preference or hand performance (McManus 1996). However, there are no two clear categories such as left- and right-handers. For a given manual action, each individual shows a preference for the use of one hand, and it is not always the same hand for two different actions (Salmaso & Longoni 1985).
This suggests that right- or left-handers are not general categories, but rather are defined as a function of the tasks. When the tasks considered are highly skilled and complex, and the individuals tested are specialized in these tasks, there is a very strong correlation between the different tasks (Bryden 1977; Wood & Aggleton 1989; Connolly & Bishop 1992; Marchant et al. 1995; Marchant & McGrew 1998).
To study handedness variations in humans, it is important to choose tasks that are typical tasks among human populations from different cultures. Thus, some tasks commonly used in Western societies to measure handedness, such as writing or teeth brushing, are meaningless in other cultures.
Hand preference for some tasks could also be modified by social or religious influence. For example, Teng et al. (1976) observed in China a strong social pressure for right-handed writing and eating, which has drastically decreased the proportion of left-handers for these tasks compared with other tasks.
Thus, the tasks chosen to measure handedness should be logically related to the biological hypothesis tested. For example, if one wants to assess handedness in the context of the hypothesis of a frequency-dependent advantage of left-handers in fights, tasks should be related to fighting actions.
If the aim is to examine brain lateralization related to language, writing handedness is more relevant. Therefore, despite the complexity of the phenomenon, the biological causes for left- and right-handers still make sense under some circumstances.
(b) Handedness in the past
Handedness in ancient humans has been inferred by analysis of archaeological samples from skeletons, stone tools and various other artefacts (see Steele & Uomini (2005) for a review). By studying arm bone length, Trinkaus et al. (1994) observed a prevalence of right hand dominance in Neanderthal skeleton samples (dating from approx. 35 000 BP). For some tools, modern replication has shown that handedness conditioned the pattern of knapping scatters and that it is thus possible to infer the laterality of the tool maker by studying ancient tools (Rugg & Mullane 2001).
A handedness polymorphism, with a majority of right-handers, has been inferred from tools dating from 300 000 to 400 000 BP.
Data from tool shape or wear, which both reflect tool use, also allowed inferences on ancient human handedness.
Phillipson (1997) reconstructed grip types on hand axes and cleavers dating from approximately 1 Myr ago, and also observed a majority of right users.
However, it is a possibility that these estimations could have been modified by the use of these same tools by different individuals.
Dental marks have also been used to infer hand use for cutting food with a stone tool. These data suggest that the handedness polymorphism existed in Neanderthals (Bermùdez de Castro et al. 1988; Fox & Frayer 1997).
Again, for this task, right-handers outnumbered left-handers. However, these could be biased by dietary or post-mortem marks.
Negative hands painted in caves during the Upper Palaeolithic in Western Europe, or more recently elsewhere in the world, could also be informative on the handedness of the painter.
In all cases, both right and left hands are found, with a higher prevalence of left hands, indicating a higher proportion of right-handers for this task (for a review see Steele & Uomini 2005).
Human representations in artistic samples were also used to track the handedness of artists, or the handedness of individuals depicted by artists.
For the handedness of artists, Perello (1970)showed that representations differ between right- and left-handers and by studying painting from prehistoric caves (e.g. Altamira Cave, 14 000–18 500 BP), he inferred that handedness should be polymorphic.
These methods could be reliable if the characteristic left-hander’s marks on painting or engraving were formally tested on modern humans.
Another approach consists of studying the handedness of the individual’s depicted (e.g. Spennemann 1984).
However, these methods are more problematic, as it has not been shown that handedness frequency in art reflects reality.
In addition, there are known biases in the depiction of handedness, at the cultural, religious and aesthetic levels (Needham 1973; Faurie & Raymond 2004). Thus, the use of these data requires further study.
However, all the above-described studies clearly show a polymorphism of hand use in Hominid populations during prehistoric and historic times, with an overall dominance of right-handers.
The polymorphism thus seems to have persisted over significant evolutionary time, suggesting that selection may play an important role in the persistence of this diversity.
Another indication of selection pressures is provided by the study of the variation of the frequencies of right- and left-handers throughout the world.
(c) Geographical variation
The diversity of the tasks used to test handedness introduced complications in comparing hand preference across populations.
When only one task is considered, there is still substantial geographical variation.
Raymond & Pontier (2004) reviewed 81 studies on handedness that examined throwing or hammering in 14 countries in America, Africa, Europe, Asia and Australia and found a range of 5–25.9%, suggesting an important geographical variation in hand preference.
Such geographical variations have also been observed for writing hand preference: in a survey of 12 000 subjects from 17 countries, 2.5–12.8% were left-handed for writing (Perelle & Ehrman 1994), and among seven ethnic groups based on 255 100 answers to a BBC internet study 7–11.8% were left-handed (Peters et al. 2006).
Studies on traditional societies tend to show a similar range of variations. Faurie et al. (2005b) found a range of left-hander frequencies between 3.3 and 26.9% across eight societies.
The frequency of left-handedness thus seems to be variable among human populations, left-handers being always at a lower frequency than right-handers.
Moreover, in most populations studied, the proportion of left-handers among women was lower than in men (reviewed in Raymond & Pontier 2004), suggesting an important influence of sex in the determinism of hand preference.
This polymorphism is thus present in every human population studied, suggesting that evolutionary mechanisms should be involved in its persistence. However, for selection to take place, hand laterality should be a heritable trait.
Thus, in the following sections, we review the determinism and transmission of hand preference in humans. The exact mechanisms have not yet been clearly characterized, but several influential factors have been identified.
Environmental factors could also exert selection forces on left-handers. Laland et al. (1995)criticized the fact that none of the most often cited models of handedness incorporate cultural influences on handedness, despite widespread evidence of their importance.
Attitudes towards left-handedness vary from one culture to another (Needham 1973; Teng et al. 1976; Harris 1992; Bryden et al. 1993; Mandal et al. 1999). Cultural and environmental factors could change hand preference in three ways, which correspond to different degrees and types of pressure by: (i) changing the hand used for some activities (e.g. writing, eating), with no change for other unimanual activities, (ii) reducing the degree of hand preference, when weak pressure applies to all hand actions, or (iii) changing the overall preferred hand, when strong pressure applies to all hand actions.
- Dellatolas et al. (1988) observed an increase in the frequency of left hand use for writing in France (‘generation’ effect), showing clearly that the educational attitude towards left hand writing has significantly changed in France during the second half of the twentieth century. The same phenomenon has been observed in other countries (e.g. Italy: Salmaso & Longoni 1985; Brazil: Berdel Martin & Barbosa Freitas 2003). Studies of school children in China and Taiwan have found that only 3.5 and 0.7% used their left hand for writing (Teng et al. 1976). This contrasts with a 6.5 per cent figure for Asian school children living in the United States, where cultural pressures have been reduced (Hardyck et al. 1976). In two African countries (Ivory Coast and Sudan), the target activity against left hand use was eating and there remains strong cultural pressure for this activity (De Agostini et al. 1997). In Japan, the proportion using the left hand for writing and eating are only 0.7 and 1.7%, respectively (Shimizu & Endo 1983), again suggesting strong cultural pressures. Coren (1992) reported a reduction of the degree of hand preference as a consequence of environmental influences: subjects who reported attempts to change handedness (by parents, teachers) could switch their hand preference for the targeted tasks. However, he also noted that this cultural influence on one particular task (e.g. writing or eating) did not condition hand preference for other tasks.
- De Agostini et al. (1997) showed that the proportion of subjects with weak handedness is higher among those who reported earlier injuries causing a temporary shift of hand use. Bryden et al. (1993) also observed that positive reinforcement for right hand use can modify patterns of handedness, and alter the relationships among different handedness tasks.
- Bryden et al. (1993) suggested that the Tucano of Amazonia successfully modify preference at an early age through positive reinforcement, and that this switched preference is manifested in all unimanual activities.
Mikheev et al. (2002) have found that highly qualified right-handed judo wrestlers more frequently preferred to perform certain judo movements with the left hand than right-handed controls. They suggest that during motor skill acquisitions (long-term judo training), lateral preferences are modified though neuroplastic development. However, an alternative hypothesis to consider is that less asymmetry is advantageous in judo and so that individuals with low asymmetry are more likely to become qualified judo wrestlers.
Genetic, developmental and environmental components have been identified be involved in hand preference determinism. Hand preference has been shown to be heritable and variable across populations, suggesting that evolutionary processes are acting on this trait.
EVOLUTIONARY FORCES ACTING ON THE POLYMORPHISM OF HANDEDNESS
The variation in morph frequencies for a given trait is essential to characterize the selective forces involved.
A polymorphism maintained in all populations of a given species is a rare case.
It can happen for a neutral trait, but is easily lost by genetic drift, so that at least some populations lose the polymorphism.
The fact that the polymorphism of handedness is maintained in all human populations suggests that handedness is not a neutral trait, and that some selective forces are maintaining this diversity.
Directional selection, if acting alone, would lead to the fixation of the advantageous morph, and eliminate the polymorphism.
The ancient and ubiquitous polymorphism observed for handedness is thus a signal of balancing selection acting on this trait.
This balancing selection could result from a situation-dependent benefit. Hence, we tried to identify, in the existing publications, the deleterious and advantageous traits associated with left-handedness, as they could, respectively, represent fitness costs and benefits playing a relevant evolutionary role.
(a) Left-handedness as a costly trait
This could be interpreted as due to changing patterns of social norms (Hugdahl et al. 1993): for example, writing handedness was submitted to more social pressures in the past than in the present (Dellatolas et al. 1988).
As a consequence, studies using hand preference for writing as the marker of handedness will find that mean age at death is lower for left-handers, even if the longevity of left- and right-handers was actually the same.
Nevertheless, even when hand preference assessment was based on other tasks, longevity has been shown to be reduced in left-handers, from a few months to a few years (Halpern & Coren 1988, 1991; Coren & Halpern 1991; Aggleton et al. 1993), but some contradictory evidence also exists (Wood 1988; Anderson 1989; Harris 1993; Hicks et al. 1994; Berdel Martin & Barbosa Freitas 2003).
Three factors may explain the reduced longevity of left-handers:
(i) prenatal and perinatal birth stressors, more probable in left-handers,
(ii) genetic effects and intrauterine hormones may have reduced the effectiveness of the immune system of left-handers, and
(iii) left-handers may have more lethal accidents. However, fitness costs have not been directly measured yet, so we are still unable to properly evaluate its evolutionary significance.
Concerning the accidental mortality, a higher risk of accidents for left-handers has been observed in Western societies (Halpern & Coren 1991; Daniel & Yeo 1994), probably due to the industrialized environment designed for a right-handed majority (Porac & Coren 1981; Coren & Halpern 1991).
As shown by Aggleton et al. (1993), an important part of the difference in lifespan between left- and right-handers is due to accidental death and death in warfare. According to Aggleton et al. (1993), the most likely explanation for the increase in accidental death among left-handed men concerns their need to cope in a world full of right-handed tools, machines and instruments.
The fact that a difference still remained, after removal from the sample of the known cases of accidental death, suggests, however, that some other factor may produce a left-handed disadvantage.
However, it is also possible that the sources used did not mention all the cases of unnatural death.
Interestingly, De Agostini et al. (1997) suggested that injury on upper limbs that made a person unable to use the preferred hand could lead to mixed handedness and create an association between accident frequency and mixed handedness.
In a study on 556 dead Brazilians, Berdel Martin & Barbosa Freitas (2003) pointed out that dextral and not sinistral individuals manifested an increased vulnerability to accidental death.
Further investigations on these aspects are thus needed with reliable information on death causes and birth cohort.
The importance of survival before and during the reproductive period of life for the fitness of an individual is clear.
Moreover, there is now clear evidence that survival beyond menopause is of great importance for women’s reproductive value (Lahdenperä et al. 2004).
Unfortunately, the link between life expectancy and handedness has as yet only been reliably investigated for men.
The impact of longevity on human fitness is not fully characterized. The importance of reduced longevity in left-handedness evolution is thus still unclear.
This could have fitness consequences as body size is an important component of selective value in humans (Guégan et al. 2000; Nettle 2002b; Silventoinen et al. 2003). For males, it is clearly established that height is correlated with reproductive success (Pawlowski et al. 2000; Mueller & Mazur 2001; Nettle 2002a). Coren et al. (1986) also showed that an association between delay in physical maturation (based on the onset of secondary sexual characteristic, age of menarche and relative body size) and left-handedness (see Eaton et al. (1996) for contradictory results).
A delay in sexual maturity could also have an influence on reproductive success and thus lead to some fitness cost for left-handers. Further investigations are needed to quantify the influence of this factor.
The frequency of left-handers among homosexual men seemed to be higher than in the general population: in their meta-analysis, Lalumiere et al. (2000) reported 39 per cent greater chance of being non-right-handed. As reproductive success has been shown to be lower for homosexual men, this could introduce some fitness bias (Berman 2003). However, the association between handedness and sexual orientation is still being discussed (Bogaert et al. 2007), and the proportion of homosexual men in the general population is low: from 2 to 6 per cent of males in the general population, depending on the study (Kinsey et al. 1948; Sandfort 1998; Berman 2003). Therefore, the effect of this association has probably a low influence on the evolution of handedness frequencies.
Some potential fitness costs are thus suggested by the literature, but proper estimation of the actual consequences on fitness are not yet identified, limiting the evolutionary significance of these results.
(b) Left-handedness as a beneficial trait
Left-handers have indeed smaller asymmetries in hand skills than right-handers (Peters 1989; Curt et al. 1992; Judge & Stirling 2003) and are less lateralized in language dominance (Steinmetz et al. 1991).
Smaller right–left differences as well as higher intermanual coordination in left-handers may be due to greater control of both hemispheres.
A larger corpus callosumhas been detected in non-right-handers (Witelson 1985), but the precise implications for interhemispheric interaction are unclear.
Moreover, there is considerable controversy about the relation to handedness: the degree of hand lateralization rather than its direction may be related to callosal morphology (Beaton 1997).
This is supported by the finding that left-handers, who are less lateralized show significantly higher values than right-handers in intermanual coordination, a performance thought to be more demanding in terms of bihemispheric control, and that may be due to a more efficient exchange of information transmitted by callosal pathways (Gorynia & Egenter 2000).
A larger corpus callosum has also been reported to be associated with superior verbal fluency (Hines et al. 1992) or to confer advantages in some forms of memory (Christman & Propper 2001); two advantages from which left-handers could benefit.
The proportion of left-handers also appeared to be greater in gifted children (IQ>131) than in non-gifted children (Hicks & Dusek 1980), but the importance of this effect at the population level remains to be quantified, owing to the low proportion of gifted children.
A few studies have considered that left-handers could have special talents that could lead to benefits, such as enhanced musical (Aggleton et al. 1994; Kopiez et al. 2006) or mathematical capacities (Casey et al. 1992; Crow et al. 1998), although this last fact seems controversial (Peters 1991). All these advantages may play a significant role in the social status of left-handers.
Some studies concerning socio-economic status and cognitive abilities have tried to estimate differences in social status between right- and left-handers (see Faurie et al. (2008) for a review).
Left-handers are supposed to have particular cognitive capacities that could lead them to particular kinds of professional activities. Faurie et al. (2008) highlighted that left-handers could have some particular socio-economic positions, which could lead to a socio-economic advantage.
A few studies have compared the earnings of right- and left-handers. Denny & O’Sullivan (2007) revealed that the hourly earning of left-handed men was greater (approx. 4%) and observed an opposite result for right-handed women. Ruebeck et al. (2007) also found a significant increase of wage for left-handed men in the group with high levels of education and did not observe any effect of handedness on wages for women.
The differences between right- and left-handers socio-economic statuses could be related to their reproductive success, though the importance of socio-economic status in human mate choice directly benefits the offspring.
Another important benefit of left-handedness could be represented by a strategic advantage of left-handers in sports: a number of studies have noted an excess of left-handers at the top levels of sports such as tennis, baseball and fencing (Annett 1985; Voracek et al. 2006).
The left-handed advantage in many sports can be explained tactically without any hypothetical additional neurological advantage (Wood & Aggleton 1989). As right-handers are more frequent, an individual is always more likely to be confronted by a right-handed opponent, so right-handers are not used to the different gestures of left-handers, whereas left-handers are used to the right-handers way of playing.
Thus, left-handers have a surprise advantage, which increases when their frequency is lower (Raymond et al. 1996).
Left-handedness frequencies in interactive sports (such as fencing, boxing, tennis, baseball, cricket), offering a strategic advantage to the rarer left-hander, appear to be very high, when compared with non-interactive sports (gymnastics, swimming, bowling), where the frequencies are no different from those of the general population (Aggleton & Wood 1990; Goldstein & Young 1996; Raymond et al. 1996; Grouios et al. 2000; Brooks et al. 2003).
This strategic advantage of left-handers in interactive sports could be a marker of a strong selective advantage during fights.
This advantage in fights could indeed lead to direct benefit through increased survival during fighting interaction or indirect benefit through acquisition of higher social status, facilitating access of partners.
It is negatively frequency dependent because it is higher when the left-handers are rarer.
Theoretical approaches have confirmed that the maintenance of two opposite asymmetrical morphs by frequency-dependent selection could be an evolutionary stable strategy (Ghirlanda & Vallortigara 2004; Faurie et al. 2005a).
The frequency-dependent strategic advantage of left-handers in fights could be the source of balancing selection involved in the persistence of the handedness polymorphism in the general population.
Emma Karlsson – The Conversation
The image is in the public domain.