Autism Spectrum Disorder Development Based on Genetics, Not Environment

Internal Medicine World Report, March 2015,

Genetic factors seem to be the primary influence on the development of autism spectrum disorder, according to a study conducted on twins in the United Kingdom.

Genetics influenced the risk of autism spectrum disorder development (ASD) more than environmental factors, according to a study published in JAMA Psychiatry. The study was believed to be unique in its design, because it included low risk twins and pairs diagnosed with ASD, which captured the range of liability.

Researchers from King’s College London examined participants in the Twins Early Development Study, which included twin pairs born in England and Wales from 1994 through 1996 in order to establish the relative contributions of genetic and environmental factors in relation to ASD development. The researchers additionally sought to examine the genetic and environmental relationship between traits and diagnostic constructs of ASD.

The sets of twins were screened from the Childhood Autism Spectrum Test (approximately 6,000 pairs), the Development and Well Being Assessment (359 pairs), the Autism Diagnostic Observation Schedule (203 pairs), the Autism Diagnostic Interview Revised (ADI R) (205 pairs), and a best estimate diagnosis (207 pairs). Twins with high subclinical levels of autism traits, low risk twins, and those diagnosed with ASD were included in the study.

Identical twins’ risk for developing ASD was significantly higher than fraternal twins’ risk, which the researchers estimate the heritability of 56 to 95%. There was very minimal evidence that could be attributed to environmental influences in the development of ASD, though the genetic traits were substantial.

“We conclude that liability to ASD and a more broadly defined high level autism trait phenotype in United Kingdom twins 8 years or older derives from substantial genetic and moderate non shared environmental influences,” the authors added. “Genetic influences on diagnosed ASD are shared with those on autistic traits in the general population.”

The researchers warned that the results warranted some caution in their interpretation, because of the (ADI R) assessment completed by the same parent for both twins. The ADI R assessment was the only developmental history measure that showed significant environmental effects for the risk of developing autism. The authors wrote that the estimated influence of shared environment may have been inflated or biased from the rater.

“Our findings are broadly in line with those of recent twin and family studies and point toward strong genetic effects in ASD and no strong influence from shared environmental factors,” the authors concluded, though wrote that the findings contradict with other studies, too. “The strengths of the present study add validity to these conclusions because previous research has often lacked the rigor and systematic approach to the sample selection used.”