Human handedness – inherited or developed?

This post is a little off the topic of what I normally write about, but its something I’ve been wondering about lately as the little munchkin grows up. She’s 6 months old now and is starting to use her hands a lot, going through that grabby stage where anything within arms length is grabbed and shoved in the mouth.  I’ve noticed that more often than not she’s using her left hand for this exploring.  It’s her left hand that grabs her left foot, and her left hand that grabs the spoon during my attempts to introduce her to the wonderful world of solid food.

Does this mean she’s going to be left-handed?  Coming from an extended family of determinedly right-handed people this comes as a bit of a surprise, and makes me wonder if there is an inherited component to handedness, or if there is some trigger during development that makes babies favour one hand over the other.

Delving into the developmental psychology literature, I have discovered that there does appear to be a distinct genetic component to handedness, but as with eye colour inheritance, it’s far from simple and clear-cut.  The percentage of left-handed people in the human population hovers around 10%. Where both couples are right-handed, about 90% of children are right handed; left-handed couples produce 40-50% left-handed children, and mixed couples about 80% right-handed children.

These figures suggest that right-handedness is a dominant trait.  Current genetic models of handedness propose a gene with two alleles – one “right-shift” allele that produces a bias for the right hand, and a second allele that doesn’t impose any bias.  Individuals homozygous for a right-shift allele will be strongly right-handed, heterozygous individuals will be mostly right-handed, and individuals homozygous for the other allele will be either right-handed or left-handed.  The location of this putative gene is unknown, although a recent study found a locus called LRRTM1 on chromosome 2p12 that was associated with the degree of handedness.  However, it seems unlikely that LRRTM1 is the only gene involved, and several other chromosomal regions have also been identified as candidates for handedness genes, suggesting there may be more than one gene involved.

These genetic effects can show up very early – even in the womb.  A 2005 study by Hepper and colleagues found that 90% of babies tend to suck their right hand in the womb and retain this preference when they are older.  However, young infants often don’t show a clear preference, or they actually appear to favour the left hand.  A recent study by Françoise Morange-Majoux and Georges Dellatolas on 4 month old infants found a left-handed dominance for approach movements (where an infant extends a hand out to an object, but doesn’t touch it), and a right-hand dominance for grasping objects.  As grasping develops later, the infant initially appears to favour the left hand.

Hand use preference begins to stabilize after 6 months, but often a clear preference for the dominant hand doesn’t show until around 2-3 years of age, when motor skills are more developed.  And can parents (unwittingly or not) influence the handedness of their babies?  Well, most of the research seems to suggest that handedness is a not learned trait, so constantly putting toys in the right hand of a left-handed child is unlikely to make a difference in the long run.

References:

Corballis MC (2009) The evolution and genetic of cerebral asymmetry. Phil. Trans. R. Soc. B vol. 364 no. 1519 867-879 . doi:10.1098/rstb.2008.0232

Francks C, Maegawa S, Lauren J, et al. (2007) LRRTM1 on chromosome 2p12 is a maternally suppressed gene that is associated paternally with handedness and schizophrenia. Mol Psychiatry 12, 1129-1139. doi:10.1038/sj.mp.4002053

Hepper PG, Wells DL, & Lynch C (2005). Prenatal thumb sucking is related to postnatal handedness. Neuropsychologia, 43, 313–315. doi:10.1016/j.neuropsychologia.2004.08.009

F. Morange-Majoux, G. Dellatolas (2010) Brain and Cognition 72: 419–422. doi:10.1016/j.bandc.2009.12.002

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