A Unique Child: Cognitive development - Left-handed and right-handed

Scientists have long known that our brain's two hemispheres are asymmetrical, with the left hemisphere governing the right side of the body and vice versa. This is why, when our left hemisphere becomes increasingly specialised during childhood, most of us become right-handed.

For right-handers, the left hemisphere specialises in several language functions and fine detailed processing, whereas the right hemisphere deals predominantly with emotion and global types of processing. Ninety per cent of the world's population are right-handed, but this is not present at birth; it becomes consolidated over developmental time.

Although foetuses display some degree of handedness - visible using ultrasound to assess the hand they use to grasp the umbilical cord and the thumb they suck - the link between foetal preferences and handedness isn't strong. Indeed, infants/toddlers show a high degree of ambidexterity, and it isn't until the third/fourth year that parents can accurately determine whether their child is right- or left-handed. But even then, handedness continues to become fully consolidated between the ages of three and seven, indicating that it requires a substantial amount of experience.

ARE HUMANS THE ONLY RIGHT-HANDED SPECIES?

Although it used to be thought that handedness was specific to humans, we are not the only species to prefer one limb over the other. Studies reveal the existence of brain asymmetry in our primate cousins, including chimpanzees, bonobos and orang-utans. Until recently, it was thought that gorillas were an exception. Surprisingly, they didn't seem to show an obvious preference for one limb over another. Yet it was widely assumed that right-handedness arose during evolution in response to the brain's specialisation for the sequential manipulation of tools, and since gorillas spontaneously use tools, they too should show a limb preference.

A team of scientists recently decided to examine this more closely. They studied 12 Western lowland gorillas at Port Lympne Wild Animal Park, Kent, to determine whether they could detect any right or left limb preference. Although some individual differences were found, on the whole the gorillas used their right limb when manipulating objects but both limbs when touching other gorillas in social interactions. So it seems that like other great apes, gorillas show a preference for the right limb when manipulating inanimate objects. However, the fact that they use both limbs for socially driven acts suggests that handedness may be influenced by the type of activity involved.

To investigate this further, scientists studied chimpanzees to ascertain whether right limb preference is restricted to tool use or is also a feature of the chimp's gestural system. A team of scientists from the University of St Andrews studied the spontaneous gestures of wild chimpanzees in Uganda's Budongo Forest Reserve. They identified more than 60 different spontaneous gestures used in contexts of grooming, contact, play, tool use and feeding, each of which was coded according to the limb preferentially used.

The right limb was used for a large percentage of gestures towards objects, whereas those produced for social purposes tended, as in gorillas, to be bilateral. Furthermore, certain gestures were more lateralised than others: the right limb was used for shaking, moving and scratching objects, but less for simply touching or pointing towards them. Male chimps were more right-limbed than females, and this became increasingly consolidated over age as juvenile chimps reached adulthood, much as it does in human children. So, in the use of our hands, we are more similar to our primate cousins than we might think.

IS THE LACK OF RIGHT-HANDEDNESS DETRIMENTAL TO DEVELOPMENT?

By the time they start nursery, most children show a tendency for right-handedness, most noticeable in behaviours involving tools, self-feeding, drawing and balancing objects. This is also the time when most children show a preference for one foot (kicking a ball) or one eye (peering through a hole) over the other.

As mentioned, only a small minority (about 10 per cent of the world's population) consistently use the left side of their bodies for such functions. Even rarer (about 1 per cent) are those who can use their right and left appendages interchangeably. Since right-handedness in humans is clearly the norm, lateralisation probably had a strong evolutionary benefit and offers some developmental advantages to children.

So how do right-handers and non-right-handers differ in early life, and are there long-term consequences?

A large body of research now exists highlighting the links between handedness and low-birth weight, ADHD, and autism (particularly in boys), with the conclusions that more than left-handedness, it is a lack of consistent handedness that poses a certain risk to developmental outcomes.

Handedness can be affected by several environmental factors. For example, a large Scandinavian study concluded that maternal stress during pregnancy results in an increased risk of left- or mixed handedness. Several other studies are beginning to reveal that although not correlated with reading abilities, non-right-handedness raises the risk of lower scores in other domains of cognitive development.

Of course, these just represent general trends at the group level. There are many exceptions at the individual level. Some of our most brilliant minds are left-handed, but they are rarely mixed-handed, except if they were forced to change to the right hand during childhood. Nonetheless, there seems to be a difference in brain functioning between those who are strongly right-lateralised and those who are not. One thing is clear: children should never be forced to use their right hand if they are obviously left-handed, because a strongly lateralised brain turns out to be positive for development.

ARE MOST HUMANS REALLY FULLY RIGHT-HANDED?

Until recently, handedness in humans was tested only with objects. A child/adult was deemed right- or left-handed on the basis of which hand they used for holding a pencil, a spoon, a cup, a hammer and so on. But with the primate research revealing the influence of type of behaviour on handedness, researchers are now looking at human limb preference with fresh eyes. Devising new tests, they are starting to investigate whether young children distinguish between animate beings and inanimate objects when using one or other of their hands.

In one study, a group of children were asked to complete various tasks involving either inanimate objects or social gesturing. The children, all right-handed, consistently used their right hand when manipulating objects. However, they switched to their left or to both hands for social interactions such as tapping someone to get their attention, stroking the face or patting the head. The one exception to this was handshaking, which we have all been culturally programmed to do with our right hand.

Handshakes aside, this new research suggests that the way we use our limbs is affected by the type of gesture produced and is not just a function of our inherent 'handedness'. So, in spontaneous situations, humans often opt for either both or their 'less preferred' hand when carrying out socially directed actions. Once again, we turn out to be similar to our primate cousins.

WHAT ABOUT OUR PREHISTORIC ANCESTORS?

A final question: How recent is right-handedness in humans? The ape findings suggest that our prehistoric ancestors might already have been right-handed. But how can the scientist find out? Using fossil bones, teeth and the stone tools, a recent study showed that hemispheric specialisation was already present in Neanderthals. This was ascertained from ancient tools by analysing the way in which flints were indented from repeated use over time, revealing that the right hand was used to hold tools while the left hand played the role of support, much as most of us hold a hammer in the right hand and stabilise the nail with the left.

Furthermore, of the 21 samples of fossil body bones examined, 18 turned out to be right-handed, two left-handed and one mixed-handed. Determining asymmetries in fossil bones, it became clear that the right side of most of our ancestors' bodies was employed during tool use (just as the bones of tennis players become more asymmetrical from constant racket practice).

In terms of handedness, then, human children, the great apes and prehistoric man are really quite similar: they predominantly use a single hand/limb (typically the right) for goal-directed actions on objects, and both hands for interacting socially with conspecifics. It is also clear that right-handedness predates the human/ape split across evolutionary time, so cannot, as some have speculated, explain the hemispheric specialisation of human language, which instead was probably deeply entwined with the development of tool use over evolutionary time.

REFERENCES

• Forrester, GS, Leavens, DA, Quaresmini, C, Vallortigara, G (submitted). Target animacy influences gorilla handedness. Animal Cognition

• Forrester, GS (2008). A multidimensional approach to investigations of behavior: Revealing structure in animal communication signals. Animal Behavour, 76: pp1,749-1,760.

• Geschwind, N & Behani, P (1982). Left-handedness: Association with immune disease, migraine, and developmental learning disorder. PNAS, Vol. 79, pp5,097-5,100

• Hepper, PG (2007). Prenatal development. Introduction to Infant Development, 2nd edition. Slater, A & Lewis, M (eds), Oxford University Press: Oxford, pp41-62

• Hobaiter, C & Byrne, RW (2011). The gestural repertoire of the wild chimpanzee. Animal Cognition

• Johnston, DW et al (2009). Nature's Experiment? Handedness and early childhood development. Demography, 46(2), pp281-301

• McManus, IC et al (1988). Development of handedness in children. British Journal of Developmental Psychology, 6(3), pp257-273.

MOTHER AND DAUGHTER

Annette Karmiloff-Smith studied in Geneva with Jean Piaget, where she completed her doctorate, and is now a Professorial Research Fellow at the Centre for Brain and Cognitive Development, Birkbeck, University of London

Kyra Karmiloff has a BSc in Anthro-pology and an MSc in Psychological Research Methods from University College London and was a research assistant at the Centre for Studies in Language at Cambridge University. She is a novelist and co-author with her mother of three books on child development and on language acquisition