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Autism and Environmental Factors, First Edition. Omar Bagasra and Cherilyn Heggen.
© 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.
7
A Who’s Who of pesticides is therefore of concern to us all. If we are going
to live so intimately with these chemicals eating and drinking them, taking
them into the very marrow of our bones – we had better know something
about their nature and their power.
Rachel Carson, 1962, Silent Spring
Until recently, autism spectrum disorder (ASD) was considered essentially a
heritable mental disorder, a notion based on surprisingly sparse data from
small clinical studies. Population based studies of the heritability of other neu
ropsychiatric disorders and comorbidities among them have also been limited.
Now, rapidly accumulating evidence published in peer‐reviewed scientific
journals has suggested that up to 40–59% of the variance observed in ASD
might be due to environmental factors [1–7]. In this chapter, we will examine
this dichotomy –environment versus heritability/genetics theories. The herit
ability theory arose primarily from misinterpreted concordance in maternal
twin pairs, which we have described in previous chapters. Since the genes in
maternal twin pairs are identical, it was assumed that autism is inherited or
genetic in origin; however, glaring dissimilarities in the maternal twins were
overlooked [1–7]. Of note, when Leo Kanner first described the syndrome he
assumed that it was a heritable illness. However, he noted that there was a
common thread found in how each autistic child was raised, prompting him to
suspect the potential influence of the environment. The concept of environ
mental contributions in ASD is well regarded in some cases (i.e., exposure to
valporic acid) and thoroughly debunked in other cases (Bettelheim’s “refrigera
tor mother” theory, see Chapter 2). In contrast to single nucleotide mutational
disorders such as Parkinson’s, Huntington disease, phenylketonuria, caustic
fibrosis, neurofibromatosis, muscular dystrophy, and sickle cell anemia, or
syndromes that are defined by a single microdeletion or several recurrent
7 Autism and Exposure to Environmental Chemicals
