A reluctance to make eye contact seems to be connected with having a
small amygdala both in people with autism and in their healthy siblings. Thus
the link is probably inherited.
One of the major behavioral abnormalities displayed by individuals with
autism—a reluctance to interact with other people—appears to be
related to that almond-shaped danger-detector deep in the brain, the
The linking of autism's aloofness to the amygdala was made by a team of
scientists at the University of Wisconsin-Madison's Waisman Laboratory for
Brain Imaging and Behavior. The lead investigator was Brendon Nacewicz, and
the senior investigator was Richard Davidson, Ph.D.
“To our knowledge, we report the first relationship between amygdala
structure and... social impairment in autism,” the researchers wrote in
their article, which appeared in the December 2006 Archives of General
The researchers studied 54 subjects— 28 males aged 8 to 25 with
autism or an autism-spectrum disorder and 26 age-matched males with no known
psychiatric disorders. Subjects were evaluated for a type of social behavior
that individuals with autism were already known to perform poorly—making
eye contact with other people. In this case, subjects were evaluated for their
willingness to make contact with the eyes of photographed human faces. As
expected, the autistic group performed worse on average than the control
The researchers also measured the size of the amygdalae in the brains of
the autism and control groups. They found that the amygdalae were
significantly larger in the control group than in the group with autism.
After that, the scientists looked for a potential link between eye-contact
test performance and the size of the amygdalae within each group.
Amygdala size was not significantly linked with the control group's
willingness to make eye contact, but it was significantly linked with the
autism group's. In fact, the smaller an autistic subject's amygdalae, the less
time he or she spent making eye contact.
Thus, the amygdalae in individuals with autism seem to be abnormally small
compared with the amygdalae in mentally healthy persons, and their abnormally
small amygdalae in turn appear to be linked with their difficulty in making
What the connection between a smaller-than-normal amygdala and difficulty
in making eye contact means, though, is not clear.
For example, might an abnormally small amygdala slow an autistic person's
ease in making eye contact? Or might difficulty in making eye contact shrink
the amygdala? The researchers tend to favor the latter explanation. What they
think may be happening is that autistic children's fear of socializing affects
the amygdala, so that it at first expands abnormally, but then shrinks
abnormally. Indeed, a study of 3- and 4-year-old boys with autism found that
their amygdalae were abnormally large for their age, whereas in this inquiry,
individuals with autism who were older than age 13 had abnormally small
amygdalae for their age.
In any event, the link between a small amygdala and trouble in making eye
contact in individuals with autism may well be inherited, another study
conducted by this group at the Waisman Laboratory and in press with
Biological Psychiatry suggests. The lead investigator in this inquiry
was Kim Dalton, Ph.D., a research scientist at Waisman.
Relatives of individuals with autism often show milder indications of the
social deficits that individuals with autism possess—say, reluctance to
make eye contact with other people—suggesting that such deficits are
inherited. So with this in mind, Dalton and her colleagues tested not only 12
subjects with autism or autism spectrum disorder, but 10 of their nonautistic
siblings for their willingness to make eye contact with eyes on photographed
human faces. The scientists then compared the results for both groups with
those from 12 healthy control subjects. Both the autistic subjects and their
siblings were found to spend significantly less time than the control subjects
in making eye contact.
The researchers then measured the size of the amygdalae in all three
subject groups and compared results for the three groups, taking age
differences into consideration. The sibling group showed a significant
reduction in amygdala size compared with that of the control group—a
reduction that was nearly identical to that of the autism group.
Taken together, “these results provide the first evidence linking
objective measures of social impairment and amygdala structure...in
autism,” Davidson concluded in an accompanying press release by the
National Institute of Mental Health. “Finding many of the same
differences, albeit more moderate, in well siblings helps to confirm that
autism is likely the most severe expression of a broad spectrum of genetically
The two studies were funded by Studies to Advance Autism Research and
Treatment, National Alliance for Research on Schizophrenia and Depression,
National Alliance for Autism Research, and National Institutes of Health.
An abstract of “Amygdala Volume and Nonverbal Social
Impairment in Adolescent and Adult Males With Autism” is posted
at<http://archpsyc.ama-assn.org/cgi/content/abstract/63/12/1417>.An abstract of “Gaze-Fixation, Brain Activation, and Amygdala
Volume in Unaffected Siblings of Individuals With Autism” is posted