Angelica Ronald and Rosa Hoekstra have written a nice review of recent twin studies in autism spectrum and autism traits. The review is online prior to print in the American Journal of Medical Genetics Neuropsychiatric Genetics.
They note in the review that twin studies have evolved from a perspective of looking at narrowly defined autism, to the broader category of autism spectrum to an even broader phenotype, the dimensional character of autistic traits in the general population.
I don't know much about autism traits in the general population but this twin studies review highlighted some current understanding.
- Relatives of individuals with autism spectrum disorder show elevated autistic traits
- Autistic traits in the general population show a smooth distribution through the normal range to the clinical extreme--this suggests a biological continuum rather than a normal or disease clinical status
- Understanding the cause of variation of autistic traits in the general population is likely to aid in the understanding of autism
- The heritability of autistic traits in the population is very heritable (genetic) ranging from 40 to 60% in young twins but increasing to 60 to 90% in adolescence.
Another insightful section of the manuscript dealt with the degree of genetic and environmental overlap between different autistic symptoms. Autism spectrum is made up of a triad of symptoms: social impairment, communication impairment and restrictive behaviors and interests. Twin studies suggest these different symptom clusters may be "fractionable", meaning they may have some independence from each of the other clusters. The authors note more genetic and twin studies should focus on one symptom cluster of the triad, rather than only studying those who have all three components.
The authors note that twin studies have helped explain the relationship between autism and intellectual disability and low IQ. There appears to be some common genetic contribution to autism and intellectual disability. However, since some individuals with autism and ASD have relatively normal intelligence, the picture is complex. Studying ASD in those with normal intelligence may provide further insight into specific autism genetic influences.
What needs to be done in the future?
Need for twin studies to tease out genetic and environmental factors that explain the common co-occurence of autism, ASD and autism traits with conduct disorder, sleep disorders, antisocial behavior and depression.
More attention needs to focus on genetic and environmental effects on developmental change in ASD and autistic traits in children (only two current studies published) and in adults (zero studies published).
More research on age-appropriate measures of autism traits--measures appropriate for one developmental age may not be appropriate for other ages.
More studies are necessary to look at cognitive phenotypes in autism and autism traits .
Photo of St. Louis Cardinal Daniel Descalso Courtesy of Yates Photography
Ronald, A., & Hoekstra, R. (2011). Autism spectrum disorders and autistic traits: A decade of new twin studies American Journal of Medical Genetics Part B: Neuropsychiatric Genetics, 156 (3), 255-274 DOI: 10.1002/ajmg.b.31159
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