638 CHAPTER 14
focus on a narrow interest is different from a normal “passion” for a hobby or activ-
ity in that people with Asperger’s have signifi cant diffi culty putting aside their interest
to attend to necessary tasks, such as eating and going to school.Understanding Autism Spectrum Disorders
Let’s examine the factors that give rise to the autism spectrum disorders (autism and
Asperger’s). Autism is the pervasive developmental disorder that has been the most
widely investigated (and, as noted in Table 14.5, the prevalence rate is increasing),
whereas the factors that give rise to Asperger’s disorder have only recently been the
focus of study. Therefore, most of the discussion that follows pertains to autism,
unless otherwise noted.
Autism appears to be rooted primarily in neurological factors, which interact
with psychological and social factors. The symptoms themselves involve a range of
psychological and social factors.Neurological Factors
Like schizophrenia, autism is marked by signifi cant abnormalities in brain structure
and function. Genetics also appears to play a role in this disorder.
Brain Systems Children who have autism have an unusually large head circumfer-
ence, which is probably due to an above-average increase in white and gray matter
during infancy (Hardan et al., 2006). However, adults with autism do not have
larger than average heads, so the early accelerated growth is followed by slower
growth during childhood (Herbert, 2005).
The connections and communication among brain areas also appear abnor-
mal in autism (Minshew & Williams, 2007). Brain areas in the same region ap-
pear to communicate excessively, while there is too little communication among
distant areas (Courchesne & Pierce 2005), in particular, between the frontal lobes
and other brain areas (Murias et al., 2006). In addition, parts of the frontal lobe are
less active among those with autism than among control group participants, which
is consistent with the deficits in executive function that have been documented
in autism (Silk et al., 2006). Asperger’s is associated with a variety of problems
with motor coordination, which suggests abnormalities in the central nervous sys-
tem; these coordination problems continue into adulthood (Tani et al., 2006) and
are also seen in people with high-functioning autism (Jansiewicz et al., 2006).
Genetics How might these abnormalities arise? Genes appear to play a role. Re-
searchers have long observed that autism tends to run in families; 8% of siblings
of affected children will also have the disorder (compared to at most 0.2% of the
general population; Muhle, Trentacoste, & Rapin, 2004). Even stronger evidence
comes from twin studies: Monozygotic twin-pairs are up to nine times more likely
to have the disorder than are dizygotic twin pairs (Bailey et al., 1995; Folstein &
Rutter, 1977; Le Couteur et al., 1996; Steffenburg et al., 1989).
However, researchers have not located a single gene that always gives rise to
autism (Weiss et al., 2008). Instead, most forms of autism probably arise from
interactions among genes—perhaps 15 or more of them (Santangelo & Tsatsanis,
2005). Research suggests that one possible cause of the genetic mutations asso-
ciated with some cases of autism is the father’s age at conception (Croen et al.,
2007): Fathers who were 40 or older were almost six times more likely to have
children with autism than fathers younger than 30 (Reichenberg et al., 2006).
Older mothers were also more likely to have children with an autism spectrum
disorder (Croen et al., 2007).
Certain stimuli may trigger autism in genetically vulnerable children (Waldman
et al., 2008). Although researchers had earlier suggested that the cause of autism
might be thimerisol, an ingredient in a widely used vaccine for measles-mumps-
rubella, no studies have so far been able to document a causal link between this
vaccine and the disorder (Muhle, Trentacoste, & Rapin, 2004; Thompson et al.,
2007). In fact, autism rates continue to increase even among children who received
vaccines without thimerisol (Schechter & Grether, 2008), which indicates that fac-
tors other than thimerisol are at work. Another possibility was suggested by the