Handedness

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Questions Received:

  1. What determines whether a baby will develop into a left-handed or right-handed person?

  2. At what age is it pretty safe to say you can determine that your child will be left handed or right handed? We notice that our 10 month old uses his left hand more frequently than his right.

  3. Is there a known correlation between handedness and the relative sizes of the two halves of the body? For example, my left hand is significantly larger than my right hand (the same relationship exists for legs, feet, and muscle groups). And, except for writing, I have more dexterity with my left hand in most tasks.

  4. Can handedness be used to determine the reaction of a primate to a novel object?

  5. My question is about left-handedness: is it genetically inherited & how if yes? Also what has the brain lateralization do with it.

Responses:

What determines whether a baby will develop into a left-handed or right-handed person?

6th February 1999

A preference for the left or right hand is shown by humans and many other species, including mammals, birds, and amphibia (Bisazza et al, 1996). In the case of the human, the preference first appears several months before birth and can be recognised on ultrasound scanning (Hepper et al, 1990). Approximately 5-8% of babies are born left-handed and the remainder right-handed. How these preferences arise is a topic of continuing discussion, but clues have been found in studies of embryos and even in the structure of biological molecules such as proteins.

Vertebrate embryos are considered to have a symmetrical arrangement when they begin development, so the question could be expanded into how and when embryonic symmetry is broken and then how preferential handedness arises somewhat later in development. The first physical signs of asymmetry in the embryo have been detected at the primitive streak stage - for example, the node region at the future cranial end of the streak in chick embryos is deviated to the left (Cooke, 1995), and in mouse embryos the cilia of cells lining the node beat in an anticlockwise pattern (Vogan and Tabin, 1999). It is proposed that this early asymmetry may be a result of the ‘handedness’ (chirality) of the molecules that drive the cilia, and result in a cascade of events which produce asymmetric development of the internal organs. Subtle asymmetries in gene expression have also been detected in early embryos before any morphological differences become apparent: two genes called nodal and lefty are expressed only on the left side of the midline (Beddington, 1996).

Although the human body has a symmetrical appearance when viewed externally, many of the internal organs are arranged asymmetrically. So for example the heart is biased to the left, and the liver to the right, while the right kidney is positioned lower than the left kidney. Occasionally, someone is born in whom all the asymmetries are the other way round, a mirror-image arrangement which is called situs inversus. This occurs in about 1 in 10,000 people (Galloway, 1990). Complete reversal like this does not necessarily create any additional problems for the person. It is interesting that in conjoined twins who are joined at the thorax or abdomen, one of the twins will show situs inversus (Galloway, 1990). This probably results from cross-signalling between the two primitive streaks in the embryonic disc (Levin et al, 1996). However, if there is only a partial reversal of position, as for example when the heart is biased to the right (dextrocardia) and all the other organs are in their usual positions, then clinical problems can arise because of the changed inter-relationships within the body. In mouse embryos, a mutation in the gene called inv causes situs inversus (Mochizuki et al, 1998), and is associated with a reversal of the normal distribution of nodal and lefty. It is becoming clear that the asymmetries of the body are tightly controlled by genes that have been conserved during evolution, and are implemented by cascades of signalling molecules (Ryan et al, 1998).

The preference for a particular hand appears to be linked with an asymmetry in the brain that controls it. White et al (1994) found that the regions of the cerebral cortex which control each upper limb do indeed differ in volume by about 7%. Thus, in right-handed people the left somatomotor cortex controlling the right upper limb is about 7 % larger than the corresponding cortex of the right side of the brain (motor pathways to the limbs cross the midline), although it is difficult to decide whether this difference is causing the preference or a result of the preferential use of one hand. There is also evidence for a link between hand preference and speech: in most people, the speech centre is located in the left cerebral hemisphere, the same side as the cortex controlling the preferred hand (Connor, 1991). Thirty percent of left-handed people have their speech centre located in the right hemisphere (only 5% in right-handers).

So, internal asymmetries arise early in embryonic development, perhaps triggered by preferred molecular configurations and/or genetic events, and in some way lead to behavioural bias in the use of the upper limbs several months before birth.

References

At what age is it pretty safe to say you can determine that your child will be left handed or right handed? We notice that our 10 month old uses his left hand more frequently than his right.

16th March 1999

Although many babies begin to show hand preferences before they are born, others take longer to become either left-handed or right-handed, and a small number of people remain truly ambidextrous throughout their lives. Preferences in hand use are linked with asymmetries in brain organisation, so for example the part of the brain controlling the preferred hand generally has a larger volume than the corresponding region on the other side that controls the non-dominant hand. In the human there is also a link between language development and handedness, so it could be that your son’s hand preference will become more obvious as his language skills emerge. In right-handed people the dominant right hand is controlled by the left cerebral hemisphere, and language development tends to be localised on the same side of the brain. In left-handed people the picture is less clear-cut: 30% of left-handers have language functions localised in the right hemisphere, which is controlling their preferred left hand, while in the remainder language is either bilateral or left-sided.

McCartney and Hepper (1999) studied arm movements during the middle third of pregnancy. Throughout all periods of observation, 83% of fetuses showed more right-arm than left-arm movements. The number of arm movements observed peaked at 15 to 18 weeks' gestation and then declined rapidly in mid-gestation. The presence of lateralized arm movements in early gestation was suggested to be a trigger for later asymmetries in the brain, and thought to have a genetic origin.

Left-handedness occurs in about 8% of the human population. It is more likely in a child if one or both parents are left-handed, and since adopted children do not show similar links this is taken as evidence of a genetic rather than an environmental origin (McManus, 1991). However, environmental factors may influence hand preference: there is some evidence that a difficult birth may delay the establishment of preferential hand use and slightly increase the probability of left-handedness (Liederman and Coryell, 1982).

References

Is there a known correlation between handedness and the relative sizes of the two halves of the body? For example, my left hand is significantly larger than my right hand (the same relationship exists for legs, feet, and muscle groups). And, except for writing, I have more dexterity with my left hand in most tasks.

17th April 1999

Yes, there is a correlation between handedness and body asymmetries. However, this is more marked and clearcut in right-handers than in left-handers.

Many studies have shown asymmetries from one side of the body to the other. Generally the right hand is larger than the left, regardless of whether the person is right-handed or left handed (Plato, Wood, and Norris, 1980; Neumann, 1992). However, in right-handed people the difference is more marked, and in left-handed people the hands are generally more nearly symmetrical. We note in your case that your left limbs are your preferred limbs and that they are significantly larger than the right, indicating that although generalisations can be made there is still a wide range of possibilities for individual cases. This is often the case for biological systems. It is interesting that you write with your right hand - perhaps this is a consequence of experience in school where there may have been an expectation for you to learn to write with your right hand. If so, it would be an example of how social pressures can over-ride biological tendencies. Alternatively, it might be because in your case language is lateralised to your left cerebral hemisphere, the same hemisphere that controls your right hand. Asymmetries of the skull are also more marked in right-handers than left-handers (LeMay 1977). So it seems that asymmetries in left-handed people arise by different developmental rules compared with the more marked asymmetries of right-handers (Purves, White, and Andrews, 1994).

Anatomical and functional asymmetries are linked with individual differences in cerebral organization (Kertesz et al, 1992). For right-handed people, the dominant hand is controlled by the left cerebral hemisphere, and for them the volume of cortex involved in controlling the right hand is approximately 7% greater than for the other side (White et al, 1994). The expansion of hand motor cortex in the dominant hemisphere may allow the development of an enhanced repertoire of motor skills for the preferred hand (Volkmann et al, 1998).

It is interesting to think about the way that these asymmetries arise. At this stage we do not know which comes first - the structural asymmetries in brain and limbs giving the potential for preferred use, or the predominant use of a preferred limb followed by changes and enhancements in the regions of the nervous system controlling them. Probably it will turn out to be an iterative process involving both.

References

Can handedness be used to determine the reaction of a primate to a novel object?

20th April 1999

There is an on-going debate about handedness in non-human primates - not everyone is yet convinced that monkeys and apes show hand preferences in the way that humans do. However, a relationship between hand preference and approach-avoidance behaviour in chimpanzees has been reported by Hopkins and Bennett (1994). Right-handed subjects approached and touched novel objects significantly faster than non-right-handed subjects. There was also a gender difference in that males touched the objects significantly faster than did females. The overall results were in line with theoretical models linking hemispheric specialization with the expression of affective behaviours. The same authors also found a genetic component in the expression of hand preferences in chimpanzees (Hopkins, Bales, and Bennett, 1994). In an interesting study by Morris et al (1993), two language-trained chimpanzees exhibited a right-hand preference for precise manual tasks.

It appears that Professor W.D. Hopkins at the Division of Behavioral Biology, Yerkes Regional Primate Research Center, Emory University, Atlanta, Georgia 30322 would be a good person to contact on this subject.

References

My question is about left-handedness: is it genetically inherited & how if yes? Also what has the brain lateralization do with it.

23rd November 1999

In the general population about 8% of people are left-handed. Within a family, left-handedness is more common if one parent - particularly the mother - is left-handed, and even more common if both parents are left-handed. This does not apply to adopted children, suggesting that the link is genetic rather than environmental. However, as yet there is no clear explanation of how genetic factors can influence the choice of preferred hand. With regard to brain lateralisation: in most people the left side of the brain controls the movements of the right upper limb and speech. It has been proposed that in evolutionary terms there is a link between handedness and language development. In left-handers, although you might expect a simple mirror-reversal of this arrangement, the situation is less clearcut. The preferred left limb is controlled by the right hemisphere as expected, but in only about 30% is speech controlled by the same right side.

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