What is ambidextrous

Ambidextrous or mixed-handedness is the ability to use both hands to do everything really easily and well.

Ambidextrous facts

Handedness is an important aspect of human psychology, however, its origins, neurobiological substrates, and function are not well understood ¹. Apart from obvious functional differences, subtle cognitive and behavioral differences have been demonstrated in relation to various handedness measures ² but their significance is uncertain. Handedness has been found to be associated with structural and functional cerebral differences. Left handedness and ambidextrous (mixed handedness) also appear to be associated with an elevated risk of some developmental and immunological disorders that may contribute to pathological processes developing in ageing. Inconsistent reports show that left handedness may be more prevalent in early-onset as well as late-onset Alzheimer’s disease, but might also be associated with slower decline. Such inconsistencies may be due to handedness being usually modeled as a binary construct while substantial evidence suggests it to be a continuous trait. The findings of this study ¹ suggest that mixed but not left handedness is associated with greater hippocampal and amygdalar atrophy. This effect may be due to genetic, environmental, or behavioral differences that will need further investigation in future studies.

A number of competing theories have been developed to account for handedness differences in humans. A main genetic origin of handedness is widely accepted and Annett’s and McManus’ theories of a single gene, two-allele determinant of handedness have accumulated substantial supporting evidence ³. Annett ⁴ proposed that a gene responsible for handedness phenotype could present either with a dominant allele for handedness direction (RS+), which shifts handedness to the right or a neutral allele (RS−), which leaves direction of handedness to chance. Thus, according to Annett, carrying two RS+ alleles would be associated with a shift of approximately two standard deviations to the right while one allele would be associated with a one standard deviation shift. The McManus’ ⁵ theory is very similar to Annett’s but diverges in that it suggests the RS+ allele (called D in McManus’ theory, for “Dextral”) to be co-dominant with the RS− allele (called C for “Chance”). Thus, D homozygous individuals are, according to McManus’ theory, 100% right handed. C homozygous carriers have a 50% chance of being left handed. While CD carriers have 25% chance of being left handed. Both theories fit the existing epidemiological and inheritance data for this trait showing that approximately 10% of humans are left handed, that 26% of individuals with two left-handed parents are also left handed while only 20% of those with one left-handed parent and 10% of those with two right-handed parents are left handed. These theories are also consistent with large genetic investigations in twins showing that approximately 25% of the variance in handedness is accounted for by genetic variation ⁶, but not all studies support this view ⁷. In this context other possible origins of handedness, such as early developmental abnormalities or trauma ⁸, or prenatal hormonal variation ⁹, may make some contribution to handedness variability but their influence is uncertain.

There is good evidence showing that variation in handedness is related to some anatomical ¹⁰ and behavioral ¹¹ measures. In addition, the fact that handedness/laterality is also detectable in animals (Annett 2006) and therefore has an origin that can likely be traced back in millions of years ¹² suggests that it is not a recent evolutionary effect and that behavioral laterality and left handedness must provide some advantage in order to be preserved through selective processes. In support of this notion, left-handed individuals appear to be over represented in professional musicians and other artistic professions ¹³, have a slight advantage in some physical activities ¹⁴, have somewhat better mathematical abilities, and have been found to have lower rates of arthritis and ulcers ¹⁵.

However, there is also substantial evidence showing that left handedness might be associated with important developmental and health differences. For instance, increased prevalence of certain health problems in left-handed individuals has been reported for cardiovascular disease, high blood pressure, thyroid disorder, motor coordination disorder, dyslexia, asthma, multiple sclerosis, type 1 diabetes ¹⁶, but decreased prevalence in left handedness has also been found in type 2 diabetes ¹⁷ and other studies have failed to replicate some of these effects. Some evidence suggesting an association between handedness and mental illness is also available ¹⁸.

In contrast, associations between cognitive performance and handedness have been investigated in large cohorts and have shown only small or no effect ¹⁹. A recent study that investigated cognitive decline in a prospective study of ageing also found no effect of handedness ²⁰ but a cross-sectional investigation of 1669 individuals aged 55–95 years found that poor cognitive function was more likely in nonright-handed individuals ²¹. Adding further complexity, Doody et al. ²² showed that age of onset of Alzheimer’s disease occurred earlier in left-handed individuals but was followed by a slower rate of decline. These findings were consistent with those of another study ²³ demonstrating that left-handed individuals were overrepresented in early-onset Alzheimer’s disease, but partly contradicted another that found a reduced frequency of left handedness in late-onset dementia and no association between severity of impairment and strength of handedness ²⁴.

It has been argued that these somewhat inconsistent findings are likely due to the way handedness is assessed and classified with most investigations using an oversimplified binary measure despite available evidence suggesting important differences between consistent handedness (left or right) and inconsistent and mixed handedness ²⁵. A more sensitive way of assessing handedness involves measuring hand preference using a typical questionnaire (e.g., Edinburgh Inventory) that yields a handedness score (usually ranging from −1 to +1) but instead of reducing the measure to a binary variable, it is decomposed into direction (left/right) and strength (absolute value of the handedness score) components that are used in analyses together thus not losing any variance of the original measure. Studies which have considered not only the direction but also the strength of handedness have found that mixed-handed but not strongly left-handed individuals had lower cognitive measures ²⁶, scored higher on schizotipic scales ²⁷, had poorer physical ²⁸ and mental health ²⁶, and had higher rates of asthma ²⁹, ADHD ²⁶, and dyslexia ²⁹. However, de Leon and colleagues ³⁰ found no association between severity of impairment and strength of handedness in late-onset dementia. Recently, Luders and colleagues ³¹ also showed that mixed handedness, but not left handedness per se, was associated with corpus callosum thickness. In addition, Leask and Crow ³² found that when behavioral as opposed to self-reported handedness measures were considered, peak performance occurred somewhere between mixed handedness and strong handedness. Thus, it is possible that genetic or early developmental differences in left-handed but more likely in mixed-handed individuals might be associated with life-long influences that may have implications for healthy ageing.

What percentage of people are ambidextrous?

Only about 1% of all people are truly ambidextrous.

How to become ambidextrous

A large amount of available evidence supports the view that handedness preferences develop very early and are linked to cerebral development differences, findings that are more consistent with either genetic causes or trauma in the first trimester of pregnancy (e.g., due to bacterial infections, alcohol exposure) or hormonal influences ¹. For instance, handedness has been shown to be genetically determined to a large extent ⁶, the majority of fetuses suck their right thumb in the womb as early as in the fifteenth gestational week ³³, thumb sucking in utero is strongly associated with hand preference 10–12 years later ³⁴ and cerebral asymmetry which is correlated with handedness has been shown to have an important genetic component in twins ³⁵. Thus, it is possible that a common genetic factor predisposes to mixed handedness as well as to certain anatomical differences that might be associated with a higher long-term disease risk. Interestingly, the size of the left hemisphere appears to be less influenced by genetics than that of the right ³⁵, which might provide a rationale for one hemisphere being more affected by certain pathological factors such as those observed in the present study. That is, if the effects detected in the present study have genetic origins they may have a greater influence on the hemisphere more genetically determined while the reverse might be true if the origins are environmental.

There is limited evidence supporting the view that differences in behavior between left- and right-handed individuals might be associated with higher exposure to noxious environments or traumatic injuries with some notable exceptions. In a population of 2180, 13–17 year olds, a greater proportion of left-handed individuals, again without information on handedness strength, presented with permanent incisors injuries ³⁶. While in another sample of 5033 individuals the risk of some bone fractures was found to be higher in left-handed, but most of all, in mixed-handed individuals when compared to right-handers ³⁷. Thus, it may be that behavioral differences in mixed- or non-right-handed individuals expose them to a higher risk of trauma either because their interaction with the world is in some circumstances less adaptive or because it is somewhat more hazardous for a left-handed person to live in a world generally designed for a right-handed population.

The possibility that early traumatic injuries of the central nervous system might predispose to left handedness or mixed handedness also finds some support in the literature. For instance, a twin study showed that the sibling with lower birth weight was at higher risk of lower IQ but only in the context of left handedness, suggesting an association between prenatal pathological events and handedness ³⁸. A recent prospective study following 1714 children before and after birth found that mothers’ depressive symptoms and critical life events (including interpersonal loss, financial difficulties, illness, or injury) before birth were associated with mixed handedness, which in turn was associated with a higher risk of language difficulties and ADHD symptoms at age 5 ³⁹.

Overall, the available evidence suggests that left handedness but particularly mixed handedness is associated with a number of risk factors, most of which have a strong genetic origin, which could lead to smaller hippocampal and amygdala volumes and greater atrophy in ageing ¹. A link with pathological processes is further strengthened by the fact that mixed handedness was associated with prospective atrophy but not wave 1 volumes in the present investigation ¹.

Ambidextrous personality traits

Dr Alina Rodriguez and colleagues from Imperial College London, and other researchers from the UK, Finland and the US carried out a research called the Northern Finland Birth Cohort 1986 ⁴⁰, to see whether there was a link between children being ambidextrous (“mixed-handed”) and their risk of having mental health issues, difficulties with language, scholastic, and mental health problems in childhood and that persist into adolescence. Previous studies ²⁶, ⁴¹, ²⁹, ²⁷ have suggested that such a link may exist in younger children. The researchers thought that if they could establish a link between ambidexterity and these problems, it might provide a way to identify children at risk of such problems. In this case, the researchers did not specifically believe that ambidextrous or mixed-handedness directly causes the language or behavioral problems seen, rather that differences in the brain could contribute to both traits. The researchers concluded that “mixed-handed children have a greater likelihood of having language, scholastic and mental health problems in childhood”, and that “these persist into adolescence” ⁴⁰. They say that ambidextrous (mixed-handedness) could be used to identify children at risk of having persistent problems. The researchers also added that more research is needed to explain why there might be a link between hand dominanace and mental health problems including ADHD symptoms.

The Northern Finland Birth Cohort 1986 study provided data on 9,479 children from the two most northerly provinces in Finland who were expected to be born between July 1 1985 and June 30 1986 ⁴⁰.

In the current analysis, the researchers assessed children’s school performance, behavior, and whether they had any language difficulties at eight and 16 years of age. To assess these measures, the researchers sent questionnaires to the children’s parents at both ages, and to their teachers at age eight only. The children also completed a questionnaire on themselves when they were aged 16. Data was excluded for children with intellectual disabilities (IQ scores of 70 or less) or those who did not agree to their data being used. In all, data from 7,871 children was included in the current analysis.

Parents provided information on their children’s hand dominance at age eight by answering a single question about whether the children were right-handed, left-handed or ambidextrous. Parents answered questions about whether the children had any language problems, including whether they had problems with sounds, stuttered or made phonetic mistakes that interfered with learning words. Parents also estimated how their child’s speech compared to that of their peers (four possible answers ranging from ‘clearly weaker’ to ‘better’).

At age eight, teachers reported whether the children had problems with reading, writing or maths, and estimated the standard of their overall school performance (below average, average or above average). At age 16, the children reported on their own school performance in the Finnish language and maths relative to their peers (better than average, average, below average or very poor).

The children’s behavior at age eight was rated by their teachers, based on a recognized scale. At age 16, their level of ADHD symptoms was assessed using another recognized scale, the Rutter scale. A score above a certain threshold indicated “probable psychiatric disturbance”. Children with the highest 5% of scores on the three ADHD-related parts of the Rutter scale (inattention, hyperactive-impulsive behavior, or both) were considered to have problems in these areas. The researchers also looked at the severity of each child’s symptoms.

The researchers compared school performance, behavior, and language difficulties in the group of all non-right-handed children (i.e. left-handed and ambidextrous combined). They then compared all of those who were ambidextrous with those who were right-handed. In their analysis they took into account three factors that could potentially affect the outcomes of the research: gender, birth weight, and gestational age (how far into the pregnancy they were born).

The researchers found that of the 7,871 children analyzed, the majority (90.9%) were right-handed, 8% were left-handed (632 children), and 1.1% were ambidextrous (87 children). At age eight, 15.3% of the total group of children were reported by their parents to have speech problems. Teachers reported that 9.7% of the children had weaker school performance than their peers, while their teachers’ behaviour assessments suggested that 13.4% had probable psychiatric disturbance. By age 16, problems were reported to be less common.

Based on parental reports at age eight, ambidextrous children were twice as likely as right-handed children to have weaker speaking ability than their peers (odds ratio 2.44). After adjusting for confounding factors that could affect the results, the researchers found no significant differences in other language assessments at age eight.

Odds ratios are used to compare the relative odds of the occurrence of the outcome of interest (e.g. disease or disorder), given exposure to the variable of interest (e.g. health characteristic, aspect of medical history). The odds ratio can also be used to determine whether a particular exposure is a risk factor for a particular outcome, and to compare the magnitude of various risk factors for that outcome.

• Odds ratio=1 Exposure does not affect odds of outcome
• Odds ratio>1 Exposure associated with higher odds of outcome
• Odds ratio<1 Exposure associated with lower odds of outcome

Reports from teachers suggested that, compared to their peers, ambidextrous children were around twice as likely as right-handed children to have weaker overall school performance (odds ratio 2.16). After adjusting for potential confounders, there were no significant differences between the groups in behavioral measures at age eight.

At age 16, ambidextrous children were around twice as likely to report that they had poor performance in Finnish language at school compared to right-handed pupils (odds ratio 2.16). There was no significant difference in their report of maths performance.

There was no significant difference in the proportion of ambidextrous and right-handed children receiving high scores on the hyperactive-impulsive behavior scale at age 16. However, ambidextrous children were more likely than right-handed children to receive high scores on the inattention and combined inattention/hyperactivity scale (inattention subscale: odds ratio 2.96; combined subscale: odds ratio 2.67).

Conclusion

This research, the Northern Finland Birth Cohort 1986 ⁴⁰ has intriguing findings, but there are a number of limitations to consider:

• The small number of ambidextrous children studied (87) means that the results are more likely to be affected by chance, thus reducing their reliability.
  Although the study took into account some factors that could affect the results (gender, birth weight and gestational age), there are likely to be other confounding factors that could have affected the results.
• It was not clear whether the methods used to assess handedness, language problems and school achievement had been tested and shown to be valid ways of measuring these characteristics. For example, the children’s hand dominance was reported by their parents at age eight, and the presence of individual language problems was only appraised as either ‘yes’, ‘no’ or ‘cannot say’.
• At each age, there was only one source of information about certain aspects of the children’s behaviour and performance (either parents, teachers or the children themselves). Some of the measures used were relatively subjective (for example, problems with language), and their reliability could have been improved by asking more than one source (e.g. the parents and teachers).
• The study carried out a number of statistical tests, which can increase the likelihood that significant differences will be found by chance, and not because a true difference exists.
• Some of the children were scored in the range of “probable psychiatric disturbance”, based on teacher scores of their behaviour. However, this does not necessarily mean that these children would be diagnosed with psychiatric problems if they were assessed by mental health professionals.

The researchers do not think that ambidexterity directly causes the language or behavioral problems seen. Instead, they think that differences in the brain that affect hand dominance could also affect both these traits. At this stage, due to the limitations of this study, its results should be seen as very preliminary, and will require confirmation by other studies. These results should not concern parents who have ambidextrous children.

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