James Steele (Dept of Archaeology, University of Southampton) and Simon Mays (Ancient Monuments Lab, English Heritage)
Human handedness - the consistent preference for one hand in skilled manipulative tasks - is often said to be a defining trait. While other primates may demonstrate individual preferences for the right or left forelimb in reaching and in manipulating objects, only in human populations is there a consistent tendency for the right hand to be the preferred hand. In many societies, the left side is symbolically associated with ill-fortune: and in some societies, cultural pressures for the forced use of the right hand even by 'natural left-handers' drastically reduces the visibility of left-handedness in census databases. Yet there is overwhelming evidence that where such cultural pressures are relaxed, a natural preference for the left hand in skilled tasks develops in as many as one individual in six.
In the last two centuries modern industrial societies have been among those exerting strong cultural pressures for right hand use, a trend strongly associated with the emergence of mass literacy. But as these pressures have been relaxed in the later decades of this century, so the frequency of left-handedness in industrialised societies (as indexed by census data) has increased. This is clearly visible in recent surveys: for instance, in Britain Fleminger et al. (1977) found that in a sample of the population in the 1970s, 3% of the 55-64 age group reported that they had developed patterns of hand usage corresponding to left-handedness, compared with 11% of the 15-24 age group.
A systematic study of the distribution of manipulative skill has been carried out by Marian Annett and colleagues from the Department of Psychology at Leicester University. Annett and Kilshaw (1983) found that in a group of 1480 adolescents and adults, using a simple peg-moving task apparatus to assay hand skill, 82 % were more skilled with the right hand, 3 % were equally skilled with each hand, and 15 % were more skilled with the left hand. When the disparity of skill between the two hands is plotted as a histogram, it becomes apparent that skill asymmetry is normally distributed: there is no clear separation into the two conventional handedness groups (see Figure 1).
Figure 1.The distribution of asymmetry of skill in the peg moving task (R-L), in seconds (n=617 males and 863 females, age range 12-63 years). Data from Annett and Kilshaw (1983).
Does this pattern reflect an underlying frequency of 'natural handedness' in human populations, but one which is only now emerging as cultural pressures against left handedness have relaxed? Until recently, no-one has demonstrated any convincing ways of quantifying the frequencies of left- and right-handedness in historical or prehistoric societies. A simple technique applied by James Steele of the University of Southampton and Simon Mays of the Ancient Monuments Laboratory, English Heritage (1995) promises to open a window onto the history of this interrelationship between culture and human biology. Steele and Mays took as their starting point the observation, made fifty years ago by the anatomist Bo Ingelmark, that the bones of the arm tend to grow to be slightly longer on the side of the dominant hand. The mechanisms of this are increasingly well understood: bone growth responds to loading. If a limb is disused, for example due to paralysis, the bones begin to atrophy. If a limb sustains repeated high levels of mechanical loading, its bones will respond by growing more vigorously and by increasing in density. Human hand preferences develop in early childhood, and so the bones of the arm on the preferred side (which will be subject to consistently greater loading) will tend to grow to be slightly greater in length during childhood and adolescence. This pattern has been documented radiographically in living samples of adolescents whose hand preferences were known, and is the basis for the use made by forensic anthropologists of long bone length as an indicator of the hand preference of individuals they are seeking to identify.
Steele and Mays took measurements of the lengths of the humerus and the radius in adult mediaeval skeletons excavated from the cemetery of the mediaeval village of Wharram Percy, in Yorkshire. They reasoned that this would give the best skeletal indicator of arm length. They then quantified the frequencies with which the left arm was longer, the right arm longer, and the frequencies with which both arms were equally long (within the limits of measurement error). Intriguingly, they found a pattern in the remains of these illiterate peasant agriculturalists which closely replicated Annett and Kilshaw's findings for behavioural handedness in a contemporary British sample. Of the 80 adults whose bones were sufficiently complete to be measured, 81% were longer in the right arm, 3 % were of equal length on each side, and 16% were longer in the left arm. Furthermore, the histogram of arm length asymmetries had the same shape as that plotted for Annett and Kilshaw's contemporary data (Figure 2). It seems, therefore, that the inhabitants of this later mediaeval community had a distribution of hand use preferences which was either unconstrained by, or resistant to, cultural pressures for conformity.
Figure 2.Standardized asymmetry in the adult arms (humerus+radius) from Wharram Percy (n=80).
Steele and Mays now want to gather information from other skeletal samples, to see if this pattern is a universal in human populations from different regions of the world. Earlier studies suggest that this could be interesting. Data from a study in the 1930s of arm length asymmetry in adult human skeletons from different regional populations of the world (Schultz 1937) give a pooled mean of 79% longer in the right arm, 3% of equal length on either side, and 18% longer on the left side - results strikingly similar to those found by Steele and Mays, and by Annett and Kilshaw using the behavioural measure.
Annett, M. and Kilshaw, D. (1983) Right- and left-hand skill II: estimating the parameters of the distribution of L-R differences in males and females. British Journal of Psychology 74: 269-283
Fleminger, J.J., Dalton, R. and Standage, K. (1977) Age as a factor in the handedness of adults. Neuropsychologia 15: 471-473.
Ingelmark, B.E. (1946) Über die Längenasymmetrien der Extremitäten und ihren Zusammenhang mit der Rechts-Linkshändigkeit. Upsala Läkareförenings Förhandlingar N.F. 52:17-82
Schultz, A.H. (1937) Proportions, variability and asymmetries of the long bones of the limbs and the clavicles in man and apes. Human Biology 9: 281-328
Steele, J. and Mays, S. (1995) Handedness and directional asymmetry in the long bones of the human upper limb. International Journal of Osteoarchaeology 5: 39-49