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Autism spectrum disorder (ASD) is typically diagnosed on the basis of behavioral symptoms, without reference to etiology. However, researchers have devoted considerable efforts to investigating etiological factors. Although no single cause has been identified, the available data suggest that autism results from different sets of causal factors, including genetic, neurobiological, and environmental factors.

Genetic Risk Factors

Researchers largely agree that ASD is the result of hereditable genetic differences and/or mutations. Findings that support a genetic link include research results showing the following:

  • ASD is more common in boys than in girls—it is most likely linked to genetic differences associated with the X chromosome (Chakrabarti & Fombonne, 2005).
  • The rate of concordance for identical twins is higher compared with the concordance rate in fraternal twins (Bailey et al., 1995; Colvert et al., 2015).
  • 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 (Ozonoff et al., 2011).

Given the current availability of rapid, precise gene-sequencing tools and the accessibility of large numbers of DNA samples, researchers have made some progress in identifying genetic factors associated with ASD (Coe et al., 2012; De Rubeis et el., 2014; lossifov et al., 2012; Neale et al., 2012; O'Roak et al., 2012; Sanders et al., 2012).

See Bourgeron (2016) for a summary of research on the genetics of autism.

Neurobiological Factors

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).

Structural and functional abnormalities in the developing brain include

  • increased gray matter in the frontal and temporal lobes (Carper & Courchesne, 2005; Hazlett et al., 2006; Palmen et al., 2005);
  • decreased white matter compared with gray matter by adolescence (Volkmar et al., 2004);
  • anatomical and functional differences in the cerebellum and in the limbic system (Volkmar et al., 2004); and
  • synaptic deficits that affect anatomical structures and neuronal circuitry (Ecker et al., 2013).

See also Ha et al. (2015) for a review of brain characteristics in ASD.

Differences in the brain's response to the environment may result in symptomatic behaviors that include

  • 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 et al., 2009); and
  • disruptions in normative patterns of social neurodevelopment that contribute to diminished attention to social stimuli (Jones et al., 2008).

Pinpointing the pathological nature of ASD and understanding the relationship between genetic mutations and neurobiological outcomes is complicated by several factors, including the

  • genetic heterogeneity underlying ASD,
  • biological pleiotropy of risk genes (single genes affecting multiple traits),
  • challenges differentiating primary from secondary effects in developmental syndromes, and
  • dynamic nature of brain development (Willsey & State, 2015).

Environmental Factors

Researchers have begun to investigate how pre- and postnatal environmental factors (e.g., dietary factors, exposure to drugs and to environmental toxicants) might interact with genetic susceptibility to ASD.

Researchers have identified a number of environmental exposures for future study, including lead, polychlorinated biphenyls (PCBs), insecticides, automotive exhaust, hydrocarbons, and flame retardants (Landrigan et al., 2012; Schmidt et al., 2014; Shelton et al., 2012). However, no specific environmental triggers have been identified at this time.

For more information on the causes of ASD, see IACC (2017).

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