Front Matter

10 Introduction to Autism Spectrum Disorders

been conclusively linked to ASD as the single leading causative factor. In fact,

over 1,000 genes, or thousands of single nucleotide based mutations (single

nucleotide polymorphisms, SNPs), have been suggested as possible causal

factors of ASD. It should be noted that in the example we gave earlier of the

sickle cell gene, it is an SNP, where a single nucleotide A is replaced with T.

Therefore, looking for thousands of SNPs that might cause ASD is illogical.

Moreover, several epigenetic influences have been identified as potential co‐

factors in ASD’s complex etiology. Numerous environmental factors can

potentially induce genetic mutations [41–56]. We will cover this topic in

detail in Chapter 5.

It has been hypothesized that exposure to higher levels of male hormones

during the fetal or perinatal periods of development could heighten the risk

of autism [56–59]. The extreme male brain (EMB) theory of ASD argues that

exposure to fetal testosterone could cause gender differences in various

autistic traits. A causative link between the organizational impacts of fetal

testosterone on cerebral development and on ASD development is often

suggested using a 2D:4D digit [60] ratio (Figure 1.5), a presumed biomarker.

Low ration High ration

55

4 3 4

3

2

11

2

Ring 

nger

(4D)

Index 

nger

(2D)

RIGHT HAND

2D ÷ 4D

Figure 1.5 A loose association of abnormal 2D:4D ratio with autism. Source: Adapted from

https://i1.wp.com/www.handresearch.com/news/pictures/finger‐digit‐ratio‐stock‐traders‐

income.jpg.