Specific Language Impairment, Autism and Language Evolution

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. (2010) Overlaps Between Autism and Language Impairment:
Phenomimicry or Shared Etiology? Behavior Genetics 40:5, 618-629

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

  • Dorothy Bishop

    Thanks, Hannah, for your blog. It is good to find people outside behavioural genetics being interested in the paper. The most interesting stuff ideas usually come from trying to cross disciplinary boundaries, hard though that is.

    As you anticipated, I think there are some problems with the implications you draw from the work. There are two issues. The first is that the variants of CNTNAP2 associated with language level are not mutations. You would usually only use that term in the case where most people had the same DNA sequence in a gene, but rare individuals had a different DNA sequence. FOXP2 is a case in point: there is a family, the KE family, who have a mutation affecting around half the family members, where the DNA sequence is changed. For most people in the general population, and for most people with SLI, the FOXP2 sequence is the same.

    The CNTNAP gene is very different. The DNA sequence has different versions in different people, and one version, which is pretty common in the general population, is associated with a small decrease in language abilities, but most people with this version would not be recognised as having any language impairment. Most researchers now think that SLI is probably the result of the combined effect of many genes, each of which may nudge language ability up or down a bit. In this regard, language ability is rather like height: there are rare mutations that may make a person drastically tall or short, but most variation in height arises from combined effect of many small influences of genes that show DNA variation in the normal population.

    The second issue concerns the evidence for CNTNAP2 being involved in both SLI and autism. Many people in the field do think this means that the same gene that can cause SLI can also cause autism, and that the only difference is that people with autism have additional difficulties going beyond language – what I have termed the ‘autism as SLI plus’ model. I supported that model in the past, but there are some facts that are hard to square with it. First, although many people with autism have structural language problems (affecting grammar and phonology) similar to those in SLI, not all of them do. So people with high-functioning autism or Asperger syndrome may have well-developed skills in syntax and phonology, while still having difficulties with pragmatics. The second point, which is a big problem for a simple genetic account, is that whereas the relatives of people with SLI often have some difficulties with structural language, we don’t usually see that in relatives of people with autism, even if the person with autism has poor language skills. It was this latter point that I was particularly keen to try and explain in my paper. The bottom line is that to explain the pattern of data we need to think in terms of interactions between genes (technically known as epistasis). So there are genetic variants that increase risk of autism, and others that increase risk of SLI. Most of these will have an individually small effect. However, if you have a risk variant for a gene influencing SLI (such as CNTNAP2) in the context of having a genetic risk for autism, the effect on language will be much worse. According to this model CNTNAP2 doesn’t affect both social cognition and language; rather it affects language, but that effect will get multiplied if the person also has risk factors for autism.

    I’m currently drafting a blog that discusses some relevant aspects of genetics, especially focusing on the idea that we need to get away from thinking in terms of single genes with big effects. I will let you know when it appears.

    Just in case you want to read more, you may find these papers useful (and I hope not too daunting)

    1. Keller MC, Miller G. Resolving the paradox of common, harmful, heritable mental disorders: Which evolutionary genetic models work best? Behavioral and Brain Sciences. 2006;29:385-452.
    1. Bishop DVM. Genes, cognition and communication: insights from neurodevelopmental disorders. The Year in Cognitive Neuroscience: Annals of the New York Academy of Sciences. 2009;1156:1-18.

  • Hi Dorothy. That's so interesting! Thanks so much for taking the time to put me straight on those points. I really appreciate it.

    I think one of the main problems with people from outside of genetics is that the temptation is to approach things with regards to a single gene with big effects. Especially in language evolution where FoxP2 is a big deal. So that blog post would be very interesting and useful. I'll take a look at the papers you've suggested there and try and get my head round it all!

    Thanks again for the input and I will blog again about genetics once I'm sure I understand it all.


  • Richard Fauth

    Hi Dorothy, Hannah and anyone else who ends up here. My son has a variant in the CNTNAP2 gene (or allel if you prefer) he also has two variants in the Shank3 gene. He has Autism-and is primarily non verbal. He can speak many words and phrases but usually he is either repeating or wants something. We believe he understands much language but expresses little. When he was sequenced for 3 and only 3 Autism suspect genes the results were astounding. The other sequenced was CDKL 5 and no variants found. One of the Shank 3 variants does not result in an amino acid change. The other shank 3 variant is a frame shift in Exon 11 that many believe is causitive. However, one top researcher has published a paper arguing that while exon 11 transcribes, it does not actually code for the protein. Part of the argument is my wife (and apparentally others) also have the frame shift-while the remaining argument cites the genetics of other mammals. This opinion is that the variant is not causitive.

    I recently attended the Phelan Mcdermid syndrome symposium which focused on the Genetics of Shank3 and also of Autism. CNTNAP2 was also discussed. One of the main take aways which i got from two days with some of the worlds leading scientist researching Autism and genetics is that the autism suspect genes are showing up associated with many other mental and neurological conditions. Alziemers, epilepsy, sli, mental retardation. Dorothy, you are on the right track. While it is very difficult to really prove conclusively-most researchers have the same opinion as you-multiple genes and mono genetic variants are what sets the stage for these conditions at least.

    Much needs to be done to move forward with this consensus. There remains elusive pieces to this puzzle. The enviroinment-most notably viruses-along with the genes already on the suspect list are the emerging pieces.