Autism and birth order

Being resident in the United Kingdom with its often quirky customs and practices, I know a thing about the power of birth order. Whilst offering no opinion on the rights and wrongs of our monarchy system, I do know that first born (sons) tend to get the lion's share when it comes to 'ruling over us' (with the same offer soon to be granted to first born daughters apparently).

The science of birth order has found some interest for lots of different things; potentially relating to our measured intellectual ability and our personality and disposition for example. I don't quite know if I believe such sweeping generalisations from this collected body of research, but I guess many of these more psychological differences might reflect the trials and tribulations of the family dynamic and that bullying big brother or sister.

Health has also come under the birth order spotlight. The idea that first born children are more likely to suffer with allergies, including food allergies than subsequent siblings, is an interesting concept potentially tied into things like the hygiene hypothesis. Parents of more than one child probably know already about the concept of 'bringing infections home to share' with siblings and what joys that can bring alongside the 'first baby being wrapped in cotton wool' effect.

A recent paper by Turner and colleagues* suggests that there may be more than a passing association between birth order and 'risk' of autism. The theory goes that depending on where you are in relation to your sibling/s, might determine some risk of autism or facet of autism. This is nothing new. For quite a few years now, various groups have suggested that in some cases, there may be a 'pattern' of risk based on your position in the sibling stakes. Some have linked this back to parental age; others have talked about genetics; others have talked about other factors.

Turner and colleagues approached this question in a slightly more 'mathematical' way than has been previously applied. The methods and results, bearing in mind that the paper is open-access:

• Three different cohorts were included for analysis, based on the AGRE, NIMH and Simons Simplex collections. AGRE and NIMH were primarily composed of multiplex families (families with more than one child presenting with autism) whilst the Simons Simplex was comprised of one child with autism, the other sibling not. The total number of families and children included was notable.
• Statistics were applied (rank-sum test, inverse rank-sum test, chi-squared) and suggested that for multiplex families (more than 1 child with autism) there was an inverse-V shape pointing towards the middle ranks being more likely to be diagnosed (particularly the second child) and in simplex families, risk is linear and increases with each additional birth, also potentially linked to gender. 

Before we get too carried away with these results (and the various biases included), it is important to understand that mathematical models and statistics are fine for looking at risk and trends in data in a theoretical sense, but when it comes to real life, the whole thing becomes a lot more complicated. I think back to the recent article on low birth weight covered in this post and the subsequent comments left by many parents of children with autism on many websites indicating their child's birth weight did not fall into that category. Lots of factors involved.

Still data like this, alongside other information, all adds to the body of literature. In these days of biological phenotypes of autism, this could represent an interesting variable. There is one little detail mentioned in the paper which I find very interesting: a possible role for epigenetics - changes in gene activity without changes in the genetic code - or to you and me, environmental factors. It works in flies and most probably works for humans too; although at this stage, I make no comparisons with the topic in question.

* Turner T. et al. Quantifying and modeling birth order effects in autism. PLoS ONE. October 2011.