Note: This article is part of a series on the history of autism treatment.
Twin Studies and Autism
The choice to complete twin studies is instrumental in understanding Autism, Autistic Spectrum Disorder (ASD) and autistic traits. Four twin Studies were completed between 1970 and 1995. They demonstrated that Autism is highly heritable. In the last decade 30 more twin studies have been completed.
In the first studies (Folstein and Rutter, 1977; Ritvo et.al., 1985; Steffenburg et. al., 1989; Bailey et. al., 1995), identical twins were found to have a higher probability of both acquiring autism, as opposed to those that were fraternal. In identical twins where they shared all genes, a third (36%) of the twins studied developed autism. Fraternal twins who shared 50% of their genes showed a 0% probability of developing autism. In those cases, one had autism, the other did not. When the view was from a broader autism phenotype, the probability increased in both cases. In other words, considering autistic traits found in disorders such as PDD or autism spectrum disorder, the probability of being diagnosed with autism increased. The increased percentage was 92% for identical twins and 10% for fraternal twins.
A 2008 study indicated an increase in those numbers using the same broader scope of traits in fraternal twins. The identical twins numbers stayed in the same range.
In the first ASD twin study (Taniai et. al., 2008) where a behavior scale was used (CARS) with case vignettes in Japan, the heritability was 73% in males and 87% in females. There is some question though in using a western instrument with case studies from Japan. The second study was based on parent reports of symptoms of autism. The study concluded with similar numbers from the previous study. This was not as reliable because of the possible cultural discrepancy but it gave them a much larger sample size. A third study conducted on the heritability of autism was found to be at 80%, indicating that there is a strong occurrence of genetics related to the diagnosis of Autism.
Since the earlier studies have indicated a high heritability of Autism, there have been new studies that have indicated a high heritability for ASD, which presents an overall broader view of those with autistic traits.
Genetic and Environmental
There are several categories of genetic and environmental factors that need to be considered and how these impact twin study correlations. It is indicated that these factors could be working together but in many variants which makes interpretation extremely complex. Twin studies are useful in identifying the environmental factors and whether they are shared or non shared. Environmental factors are not always separate from genetic factors and this needs to be considered as well. Two concepts that are important to remember are gene-environment interaction and gene-environment correlation. These concepts can give us a view into our understanding of ASD and especially in relation to the twin studies.
For many of the twin correlations that were discovered, fraternal twins had less than half of the correlations than the identical twins. There is also an indication that a non-shared environment plays a causal role. Identical twins have similarities in genetics, but not necessarily in environment, in particular the non-shared environment. This could be due to a variety of factors, but they are mostly based on experiences in the environment not shared by each other. By this definition, the environment plays an important role in identifying the non-shared experiences and how they help determine autistic traits within the framework of twin studies. Through the studies, there is an overlap of genetic and environmental factors in reading autistic symptoms. The three core sets of autistic symptoms can be traced to genetic factors, however they are caused by a largely different set of genetic influences. This leads researchers to believe that more studies are needed to be conducted on causal explanations instead of researching autism as a whole. The genetic correlation may be present, but there are certain symptoms that are displayed in certain people with the condition that can lead to a diagnosis of Autism. These symptoms need to be studied further to be able to fully understand the components of Autism.
According to two separate studies attempting to make a correlation between Autism and intellectual disabilities (Hoekstra et. al. 2009, 2010; Nishiyama et. al. 2009), a genetic overlap was found to be present. It was also discovered that the presence of autistic traits indicated a propensity toward intellectual disabilities. However, the findings may have been inflated because of the inclusion of subjects with severe intellectual disabilities who only had mild autistic traits. The twin studies were completed to research the general population in regards to studying the correlation between Autism and intellectual disabilities. There was only a modest correlation between the two factors in this study, but it could have been limited by the few subjects that had severe disabilities. These studies give us some clues to move toward, but more studies are needed to come to more conclusive results.
Twin Studies have played an instrumental part in pointing researchers closer to some much needed answers in the discovery of autistic traits. Even though more studies need to be conducted in the research of Autism, these studies have allowed researchers to discover correlations that start to put some of the pieces of the puzzle together. Several of these conclusions include the correlation of the genetic risk factors being present in both ADHD and ASD, different causal pathways will be associated with different autistic symptoms and the genetic causes of Autism are largely distinct from the genetic causes of intellectual disabilities. These findings give researchers cause to be hopeful that more discoveries are on the horizon.