My last post speculated about what some conditions which manifest impaired theory of mind could tell us about the evolution of ToM. Of these conditions autism was one which could be the most informative when it comes to looking at the genetics of how ToM evolved, in this post I will look at what autism could tell us, not only about theory of mind, but also about other aspects of the language faculty.
Dorothy Bishop has recently written a paper exploring the above average co-occurrence of Specific Language Impariment (SLI) and Autistic Spectrum Disorders (ASD).
SLI is a condition where a child fails to develop spoken language on the normal schedule, for no observable or obvious reason (Bishop and Norbury 2008). Whilst ASD and SLI are regarded as distinct conditions, these disorders co-occur at above chance levels.
Bishop (2010) explores why this might be. Bishop begins her paper by painting a textbook example of a child with SLI. This example is of a child with normal social interaction and nonverbal communication, but with specific difficulties in mastering structural aspects of language, especially syntax and phonological skills. So this typical picture is not one of an autistic child in that one of the defining features of autism is a limited capacity for normal social interaction and a child is much more likely to be deficient in pragmatic skills than syntactic or phonological skills.
Bishop states that despite the fact that according to conventional diagnostic frameworks, SLI andASD are mutually exclusive diagnoses, similarities exist between the two conditions and these include:
- They are both highly heritable
- Identical, monozygotic twins are significantly more concordant than fraternal, dizygotic twins for autism and SLI
- In both conditions rates of impairment in first degree relatives are higher than in the general population
- First degree relatives of affected individuals of both conditions often manifest sub-threshold symptoms
- These conditions correspond to points on a continuum of impairment, rather than all-or-none diseases
So any model of causation for either condition must take into account the following considerations:
- Above chance levels of comorbidity between SLI and ASD
- Rates of language impairment in relatives of probands with SLI and ASD
- Molecular genetic findings of shared genetic risk factors for ASD and SLI
Now the article goes on to explore etiological models which explain these considerations with varying degrees of success. I'm not going to pretend to understand these models as I have only ever been formally taught in linguistics and so I'm a bit stumped by genetic psychology. If you're much smarter than me you can read the article yourself here:
So what I got from this article was that the genetic factors involved in autism can not only cause the characteristics typical of a person with autism (pragmatic impairments) but also other language impairments which are typical of a person with a Specific Language Impairment. Specifically the CNTNAP2 gene has been found in independent samples to be associated with both ASD and SLI. This is interesting because it could show that gene mutations which cause improved social abilities could have also caused changes in our linguistic ability on a syntactic or phonological level.
Disclaimer: Sorry if I've made too many assumptions in the conclusion I've just drawn. As I said above I know next to nothing about genetic psychology but I just felt this research would have interesting consequences in the field of language evolution. I'd love to hear the thoughts of people who know better than I do.
Bishop DVM, Norbury CF (2008) Speech and language disorders. In: Rutter M, Bishop DVM, Pine D, Scott S, Stevenson J, Taylor E, Thapar A (eds) Rutter’s child