BACKGROUND AND SIGNIFICANCE

Behavioural laterality (which encompasses hand, foot and eye preference and relative skill) is one of the earliest developing and oldest behavioural traits. Hand preference is first demonstrated at between 9-10 weeks gestational age as embryos begin to exhibit single arm movements [1]. From a neuropsychological perspective lateralization in the form of hand or foot preference remains the best behavioural predictor of language and emotional lateralization [2,3]. Heritability estimates from studies of twins and their non-twin siblings have found evidence of moderate genetic effects suggesting 26% of the variance in hand preference at the population level is due to additive genetic gene effects (95% Confidence Intervals: 14.8-29.9%) with the remaining 74% of variance (95%CI: 70.1-78.4%) due to unshared environmental effects [4,5]. Significant associations have been found with the Androgen receptor (Xq11-12) [6] and LRRTM1 (2p12) [7].

While laterality appears to develop prenatally exposure to adverse environments or pathogenic insults has been hypothesised to increase the prevalence of left handedness creating phenocopies [8]. Hemiparesis and gross skeletal injuries are obvious pathological causes of changes in laterality. However, more subtle neurological insults may also result in lasting changes in lateralization without deficits in other neuropsychological domains [9]. A wide range of pathogenic risk factors have been proposed, including but not limited to, maternal illness, anoxia, birth stress, low birth weight, small focused neurological injuries and ultra-sound exposure [8-16]. Unless analysis of laterality data controls for these confounds through the collection and incorporation of high quality peri-natal information it is likely that genetic effects may obscured.