What Evidence Supports a Role for the Environment in Autism?

Several studies have now shown that a low birth weight is associated with an increased risk of developing autism. To understand if the correlation is genetic or environmental, researchers examined birth weight and autism prevalence in both fraternal and identical twins.

Fraternal twins develop from two separate eggs, making their DNA as similar as two, non-twin siblings. Identical twins, however, are from the same egg; their DNA is identical.

The increased risk of autism with a low birth weight was the same for fraternal and identical twins, indicating that the correlation between birth weight and autism may be—at least in part—environmentally based.

To learn more about the growing awareness of environmental factor contribution to autism, please explore the current section of Autism Reading Room.

For decades, science has focused on the genetic causes of autism. However the increasing prevalence cannot be wholly explained through genetic factors. The belief that autism is a genetic disorder originated with the findings of several twin studies that reported high autism (50–70%) concordance rates for identical twins and low (0–10%) concordance rates for fraternal twins.1,2,3 However, a recent study of a larger and more diverse cohort in California reported similar (50–70%) autism concordance rates for identical twins but much higher (35%) concordance rates for fraternal twins.4 Further analysis estimated that genetic factors were responsible for 38% of Autism Spectrum Disorder liability and environmental factors for 58%.4 These findings represent a paradigm shift, suggesting that environmental factors play a more prominent role than previously thought in the etiology of Autism and Autism Spectrum Disorders.


In this section we provide an overview of the research findings suggesting a role for the environment in the development of autism.  We also introduce a novel categorization system to summarize the strength of the scientific evidence supporting an association between a specific environmental factor and autism/Autism Spectrum Disorders. Each environmental factor is assigned a category based on the preponderance of evidence, the concordance of individual study findings, or the existence of a validated animal model of autism.



The Pyramid of Environmental Factors Associated with Autism. Each environmental factor is assigned a category based on the number of studies, agreement among studies, and the existence of an established animal model of autism.

  • Rare and Direct: Factors rarely encountered but directly linked to autism in numerous, peer reviewed publications and in established animal models.
  • Moderately Associated: More than 10 repeated, peer-reviewed publications citing a positive association and/or an established animal model of autism.
  • Weakly Associated: Fewer than 10 repeated, peer-reviewed publications citing a positive association OR the existence of numerous discordant studies.

  1. Bailey A, Le Couteur A, Gottesman I, Bolton P, Simonoff E, Yuzda E, Rutter M. (1995) Autism as a strongly genetic disorder: evidence from a British twin study. Psychol Med 25:63-77. PMID: 7792363.
  2. Folstein S, Rutter M (1977) Infantile autism: a genetic study of 21 twin pairs. J Child Psychol Psychiatry 18:297-321. PMID: 562353.
  3. Steffenburg S, Gillberg C, Hellgren L, Andersson L, Gillberg IC, Jakobsson G, Bohman M (1989) A twin study of autism in Denmark, Finland, Iceland, Norway and Sweden. J Child Psychol Psychiatry 30:405-416. PMID: 274559.
  4. Hallmayer J, Cleveland S, Torres A, Phillips J, Cohen B, Torigoe T, Miller J, Fedele A, Collins J, Smith K, Lotspeich L, Croen LA, Ozonoff S, Lajonchere C, Grether JK, Risch N (2011) Genetic heritability and shared environmental factors among twin pairs with autism. Arch Gen Psychiatry. PMID: 21727249.