Autism spectrum disorders (ASD) is typically diagnosed on the basis of behavioral symptoms, without reference to etiology. However, considerable research has been devoted to investigations of etiological factors. While no single cause has been identified, the available data suggest that autism results from different sets of causal factors—including genetic, neurobiological, and environmental—that manifest in characteristic behavioral symptoms.
It is largely agreed that ASD is the result of hereditable genetic differences and/or mutations, although not all children can be identified as having a genetic linkage or mutation that is obvious to family members. Findings in support of a genetic link include research results showing that ASD is more common in boys than girls—most likely due to genetic differences associated with the X chromosome (Chakrabarti & Fombonne, 2005)—and twin studies that show a 60% to 90% rate of concordance for identical twins compared with a 0% to 10% rate of concordance for fraternal twins (Bailey et al., 1995). In a study conducted by Ozonoff et al. (2011), almost 20% of infants with an older biological sibling with ASD also developed ASD; the risk for developing ASD was greater if there was more than one older affected sibling.
Given the current availability of rapid, precise gene-sequencing tools and the accessibility of large numbers of DNA samples, significant progress in identifying genetic factors associated with ASD has been made (Coe, Girirajan, & Eichler, 2012; lossifov et al., 2012; Neale et al., 2012; O'Roak et al., 2012; Sanders et al., 2012).
Abnormalities in the genetic code may result in abnormal mechanisms for brain development, leading in turn to structural and functional brain abnormalities, cognitive and neurobiological abnormalities, and symptomatic behaviors (Williams, 2012).
Neurobiological differences associated with a diagnosis of ASD include
- problems with genetic code development involving multiple brain regions, including frontal and anterior temporal lobes, caudate, and cerebellum (Abraham & Geschwind, 2010);
- structural and functional abnormalities of the brain, including
- increased gray matter in the frontal and temporal lobes (Carper & Courchesne, 2005; Hazlett, Poe, Gerig, Smith, & Piven, 2006; Palmen et al., 2005),
- decreased white matter compared with gray matter by adolescence (Volkmar, Lord, Bailey, Schultz, & Klin, 2004),
- anatomical and functional differences in the cerebellum and in the limbic system (Volkmar et al., 2004);
- differences in the brain's response to the environment, including
- decreased neural sensitivity to dynamic gaze shifts in infancy (Elsabbagh et al., 2012);
- preference for nonsocial versus social processing and hemispheric asymmetries in event-related potentials (ERPs; McCleery, Askchoomoff, Dobkins, & Carver, 2009);
- disruptions in normative patterns of social neurodevelopment that contribute to a diminished attention to social stimuli (Jones, Carr, & Klin, 2008).
Given the complexity of autism risk, researchers have begun to investigate how pre- and post-natal environmental factors (e.g., dietary factors, exposure to drugs and environmental toxicants) might interact with genetic susceptibility to ASD. A number of environmental exposures have been indentified for future study, including lead, polychlorinated biphenyls (PCBs), insecticides, automotive exhaust, hydrocarbons, and flame retardants (Landrigan, Lambertini, & Birnbaum, 2012; Shelton, Hertz-Picciotto, & Pessah, 2012). However, no specific environmental triggers have been identified at this time.
Research focused on the environmental risks involved with the development of ASD is quite complicated, as researchers must include how the environmental factors interact with individual genetic information.