Hand Performance in Jamaican Children
J. T. MANNING, R. L. TRIVERS, R. THORNHILL, AND D. SINGH
Abstract. Testosterone, particularly prenatal testosterone, has been implicated in the aetiology of many extragenital sexually dimorphic traits. It is difficult to test directly for the effect of prenatal testosterone in humans. However, Manning, Scutt, Wilson, and Lewis-Jones (1998b) have recently shown that the ratio of the length of the 2nd and 4th digits (2D: 4D) in right hands negatively predicts testosterone levels in men. As digit ratios are fixed in utero it may be that the 2D: 4D ratio is associated with many prenatally determined sexually dimorphic traits. We tested this for one case by examining the relationship between lateralised hand performance (LHP), as measured by an Annett peg board, and 2D: 4D ratio in rural Jamaican children. 2D: 4D ratio was measured from photocopies and X rays of hands. A low 2D: 4D ratio in the right hand of boys and girls (photocopies) and the right hand of boys only (X rays) was associated with a reduction in rightward performanceasymmetry. In both samples the difference in 2D: 4D ratio between the hands (2D: 4D left hand–2D: 4D right hand) showed the strongest relationship with LHP, i.e., high ratio in the left and low in the right correlated with a tendency towards a fast performance with the left hand. It is suggested that the 2D: 4D ratio may be associated with the expression of other sexually dimorphic behavioural traits.
Prenatal testosterone has been implicated as an important factor in the development of extragenital sexual dimorphism including the differentiation of the nervous system (Bardin & Caterall, 1981; McEwen, 1981; MacLusky & Naftolin, 1981). One such dimorphism may be seen in the expression of hand preferences (Geschwind & Behan, 1982; Geschwind & Galaburda, 1985; Hassler & Gupta, 1993). Geschwind and Galaburda (1985) have hypothesised that testosterone may slow growth within some areas of the left hemisphere and promote growth of certain areas in the right hemisphere. Such a process may mean that high levels of testosterone in utero would be associated with left-handedness and this left-preference could be seen in higher frequencies in males.
The Geschwind and Galaburda model is controversial. The model predicts association between left-handedness and lateralised hand performance and such things as auto-immune disorders, autism, and dyslexia. Not all of