What do we know about the cause(s) of autistic spectrum disorder? Discuss.
The DSM-5 defines autistic spectrum disorder (ASD) as a range of conditions classified as neurodevelopmental disorders (American Psychiatric Association, 2013). ASD encompasses autistic disorder, Asperger’s disorder, childhood disintegrative disorder and pervasive developmental disorder-not otherwise specified; suggesting that these previously separate disorders are a single heterogeneous condition that varies in symptom severity. This variance in symptom severity tends to be in two key domains: deficits in social communication and social interaction as well as in restricted repetitive behaviours and interests. With the increasing number of individuals diagnosed with ASD, 1 in 68 children as reported by the Centre for Disease Control and Prevention (2014), there is great interest in understanding the etiology of the disorder. This essay shall discuss what is currently known about the causes of ASD, highlighting areas where further research is needed.
Results of epidemiological studies suggest genetic factors play an important role in the etiology of ASD (Geschwind, 2013). Although it is not common for parents to have more than one child with autism, the recurrence risk estimate of having a second child with autism increases to 8.6%, and up to 35% for families where two or more children are affected (Ritvo et al., 1989). Evidence from twin studies also supports autism as largely genetically determined. For example, Smalley et al. (1988) reported concordance rates for ASD of 64% in monozygotic (MZ) twins and 9% in dizygotic (DZ) twins. Moreover, results of studies comparing concordance rates of MZ and DZ twins estimated the heritability of ASD as 92% (Bailey et al., 1995). The co-occurrence of autism with chromosomal and genetic disorders such as Fragile X syndrome (Hagerman, Hoem & Hagerman, 2010) provides further evidence to suggest that autism is largely genetic. Despite this, a specific gene attributable to ASD has yet to be identified. In his review, Betancur (2011) concluded that autism is not a manifestation of one single gene but rather a behavioural expression of multiple interacting genes. Furthermore, the identification of a ‘broader autistic phenotype’ suggests the presence of gene-environment interactions. The broader autistic phenotype refers to the subclinical autistic traits found in relatives of individuals with autism (Folstein & Rutter, 1977). These subclinical traits increase the concordance for autism in MZ twins to 93% and 15% for DZ twins. Researchers suggest that this phenotype is modified by environmental factors in individuals diagnosed with ASD.
With genetic studies failing to fully explain ASD, neurobiological explanations have also been posited. For example, there is evidence that abnormities in the frontal lobes, amygdala and cerebellum play a role (Penn, 2006). Studies have also found structural connectivity abnormalities in white matter in...