For decades, researchers have disagreed and debated about the biological causes of autism. It is a spectrum of widely varied disorders that are typically diagnosed based on behavioral criteria, not physical etiology. This symptomatic approach has hampered a more direct assessment of the spectrum of autism, but that may be changing. Advances in several areas, such as gene sequencing, neurophysiology, and the role of the gut-brain axis come together in new ways that enhance discoveries formerly confined to respective disciplines.

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Neurophysiology and the Gut

While there are few solid causal links between any autism spectrum disorder (ASD) and the gut’s microbial mechanisms that regulate immunity via nerve cell channels and a system of neurons throughout the gastrointestinal tract, there are defined links that bear further examination. First, comorbidity of digestive disorders and identifiable difference in fecal flora occur in most patients with ASD, irrespective of the severity or manifestation of the disorder.

While it is as yet unclear that any abnormality in the gut flora or morphology is responsible for ASD, what is manifest is the difference in neurophysiology. As reported by the American Speech-Language-Hearing Association (ASLHA), those who are diagnosed with an ASD typically exhibit characteristic differences in brain formation. These include increased gray matter in the frontal and temporal lobes, decreased proportion of white matter, and both functional and anatomical differences in the limbic system and the cerebellum.

These structural abnormalities are accompanied by developmental differences that have physical foundations. Young children show a pronounced preference for asocial processing, which occurs in a different portion of the brain. The orthodox model of development indicates children should gravitate to tasks and pastimes that utilize social processing methods, and encourage them to make eye contact, seek physical interaction, and develop social and spatial reasoning skills. The young child with an ASD will choose different skill development, which alters the normative patterns in the neural net that are required for attention to social details and interaction in later life.

Genetics or Environmental Stimuli

In the first decade of the 21st century, many researchers engaged in twin studies. The working hypotheses included the possibility that ASD has a genetic factor, or are heritable. The actuality turns out to be a bit more complicated than even the intricate mysteries of the human genome. Twin studies, or designed observational experiments involving twins and siblings, seemed to indicate the high likelihood that autism was genetic and could be passed to offspring.

When examining the possible genetic origins of ASD, there are a few critical factors to consider. First, upwards of 70 percent of all diagnosed cases of ASD occur in males. Second, regarding exhaustive twin studies, instances of twin siblings both exhibiting signs of ASD run exceptionally high in identical twins, with far fewer cases of fraternal twins sharing a disorder, especially if they do not share a sex. In siblings from the same pairing, the incidence of ASD is dramatically lower, although still possible.

A meta-study published in a 2016 issue of J. Child Psychological Psychiatry appears to substantiate the genetic heritability of ASD. While some researchers still uphold the importance of shared environmental factors that may influence the expression of genes responsible for the occurrence of ASD, the metastudy casts doubt upon such influences as causal.

There is still much research to be done. Scientists continue to probe the mechanisms of heritability and the influence of shared environmental factors for the incidence of ASD. While it is still unknown precisely which genes are responsible, many researchers believe that the biological causes of autism are genetic, and influence the formation of critical areas of the brain, functional operation of the formative neural net, and the distribution of gray matter.