Handedness, Birth Order & Autism

It is widely accepted that genes play a role in the etiology of autism. Evidence for this derives, in part, from twin data. However, despite converging evidence from gene-mapping studies, aspects of the genetic contribution remain obscure. In a sample of families selected because each had exactly two affected sibs, we observed a remarkably high proportion of affected twin pairs, both MZ and DZ. Of 166 affected sib pairs, 30 (12 MZ, 17 DZ, and 1 of unknown zygosity) were twin pairs. Deviation from expected values was statistically significant (P<10(-6) for all twins); in a similarly ascertained sample of individuals with type I diabetes, there was no deviation from expected values. We demonstrate that to ascribe the excess of twins with autism solely to ascertainment bias would require very large ascertainment factors; for example, affected twin pairs would need to be, on average, approximately 10 times more likely to be ascertained than affected non-twin sib pairs (or 7 times more likely if “stoppage” plays a role). Either risk factors (related to twinning or to fetal development) or other factors (genetic or nongenetic) in the parents may contribute to autism. (Greenberg, D. A., Hodge, S. E., Sowinski, J. & Nicoll, D. (2001). Excess of twins among affected sibling pairs with autism: Implications for the etiology of autism. American Journal of Human Genetics, 69(5), 1062-7.)

Twins and handedness are related, with lefthandedness being more common among twins than non twins, particularly identical twins. (I’m not sure of this, I’m going my memory.)

Handedness is genetic to a large degree. Individuals with left handed mothers are more likely to be left handed than if they have left handed fathers. If they have one left handed parent they are more likely to be left handed than if they have none.

According to some studies there are those that are autistic that come from older mothers, and those that are first born. I’m suggesting higher testosterone levels compels one, a lack of alloparents and/or older siblings causes the other. A question I have is, do first borns and later borns both exhibit elevated rates of left handedness?  Is there a difference in handedness percentages between first born or later born autistics?

I’m looking for what differences there might be between these two groups of autistics. Is it possible that first born autistics only occur in family with higher rates of left handedness? I assume that a genetic predisposition makes a child more likely to get autism if not provided an environment that encourages theory of mind. What if an environment with few opportunities to develop theory of mind encouraged maturational delay and left handedness? That seems backwards, but interesting.

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