Is there any scientific research on why there are a minority of left-handed people? (On another note: I am an identical twin, but I’m left-handed and he is right-handed. I guess they call us “mirror-imaged.” Not sure how common this is.)
@patrick, this ScienceAlert summary of the science article is genuinely bad. It is example of epigenetics woo. It is a prime example, of why you need to cite the original article, which is far more sensible. The ScienceAlert article is downright wrong and illogical. The actual science article, is far more reserved, but also banal…
Epigenetic regulation of lateralized fetal spinal gene expression underlies hemispheric asymmetries
Lateralization is a fundamental principle of nervous system organization but its molecular determinants are mostly unknown. In humans, asymmetric gene expression in the fetal cortex has been suggested as the molecular basis of handedness. However, human fetuses already show considerable asymmetries in arm movements before the motor cortex is functionally linked to the spinal cord, making it more likely that spinal gene expression asymmetries form the molecular basis of handedness. We analyzed genome-wide mRNA expression and DNA methylation in cervical and anterior thoracal spinal cord segments of five human fetuses and show development-dependent gene expression asymmetries. These gene expression asymmetries were epigenetically regulated by miRNA expression asymmetries in the TGF-β signaling pathway and lateralized methylation of CpG islands. Our findings suggest that molecular mechanisms for epigenetic regulation within the spinal cord constitute the starting point for handedness, implying a fundamental shift in our understanding of the ontogenesis of hemispheric asymmetries in humans.
This is how the journalistic article puts it:
The researchers, moreover, traced the cause of asymmetric gene activity. Epigenetic factors appear to be at the root of it, reflecting environmental influences. Those influences might, for example, lead to enzymes bonding methyl groups to the DNA, which in turn would affect and minimise the reading of genes. As this occurs to a different extent in the left and the right spinal cord, there is a difference to the activity of genes on both sides.
This equivocates developmental influences with environmental influences. It equivocates environment with differences in epigenetic, which may or may not have anything to do with organismal or cellular environment.
This is an excellent review article, that is worth reading. It includes a section on hand discordant twins:
The study of twins could clarify the relative contribution of genetic and environmental factors. Monozygotic twin pairs are significantly more likely to be concordant for handedness than dizygotic pairs (Sicotte et al . 1999). Table 4 shows that dizygotic twins do not differ from binomial expectations in their hand preference, whereas monozygotic twins show more concordance than would be expected by chance. These data therefore suggest that there is a genetic contribution to handedness. However, discordance of handedness among monozygotic twins also raises the question of cultural influence on hand preference. Even if the genetic and cultural contributions to transmission of hand preference are not fully determined, these results provide convincing evidence for a significant heritability, allowing the action of natural selection on this trait.
Three major problems have arisen for determining a genetic aetiology of handedness: (i) cultural biases influence the practice of hand usage, (ii) despite identical genotypes, approximately 18 per cent of monozygotic twins are discordant for handedness and (iii) only 30–40% of children from LxL couples are left-handed. Models of handedness generally assume a genetic basis to both laterality and hemispheric asymmetry (Levy & Nagylaki 1972; Annett 1985; McManus 1991). Conventional Mendelian genetic models fail to fit the data.