Some social
behaviors associated with autism are heritable, according to a new study. But
the extent to which genes and the environment influence these behaviors changes
as a child grows, the results show.
The
researchers examined toddlers’ so-called reciprocal
social behavior, or the ability to engage in interactions such as
sharing toys or conversing. Reciprocity skills vary in the general population,
and prominent difficulties in this area can be a sign of autism. To estimate
the heritability of these behaviors, the team measured them in a group of twins
— including identical twins, who share nearly all of their DNA, and fraternal
twins, who share about half.
Identical
twins tend to have more similar scores on measures of their social behaviors
than fraternal twins do, the researchers found, suggesting a strong genetic
component. But the degree of heritability changed as the twins developed,
suggesting that the influence of genetics and the environment on behavior is
not stable, says lead investigator Natasha Marrus, assistant professor of psychiatry at
Washington University in St. Louis, Missouri.
“Development
doesn’t look static when we watch it, and I don’t think the biology underneath
it is static either,” she says.
Because
differences in social reciprocity are observable before the age at which autism
is usually diagnosed, understanding how it develops could help researchers
chart the condition’s early trajectory, Marrus says.
The paper is
an important first step toward creating measures of behavioral dimensions that
appear in early development in autistic children, says Peter
Mundy, professor of education at the University of California, Davis,
who was not involved in the work. And it solidifies the idea that genetics
influences these behaviors.
“That’s
unassailable,” Mundy says. “It’s a good contribution.”
Social factors:
The
researchers analyzed data from the parents of 1,563 18-month-olds. The
parents watched a video of a typically developing
19-month-old interacting with an adult, rated how their child compared with the
girl in the video on 13 items, and answered 31 additional questions about their
child’s social behaviors, such as whether the child understands verbal requests
or tries to play with other people. They repeated both tests when the children
were 24 months old.
An analysis
revealed that the children’s scores on some survey items tended to be
correlated. A child who responds to her name, for example, is also likely to be
interested in what others are doing, the researchers found. Based on these
correlations, they identified five ‘dimensions’ of reciprocal social behavior:
social motivation; functional communication; restricted interests and repetitive behaviors; social avoidance; and social
orienting.
The team
repeated the tests in a group of 134 identical twin pairs and 205 fraternal
pairs, also at 18 and 24 months of age, and measured how similarly the twins
scored on each of the five dimensions. They did additional modeling to
differentiate the effects of genetics and shared and unshared environmental
factors.
At both ages
and for all five dimensions, identical twins had stronger similarities than
fraternal twins, indicating that reciprocal behaviors are strongly heritable.
But the
heritability of each dimension changed as the children grew. The influence of
genetics on social motivation and functional communication waned from 18 to 24
months. By contrast, its influence on social orienting nearly doubled between
screenings; it also increased, although to a lesser extent, for social
avoidance and repetitive behaviors.
The results
suggest that the interplay between genetics and environmental factors changes
over time, Marrus says, with the environment playing a stronger role at some
points than at others.
“Just because
something is heritable, it doesn’t mean that exact same level of genetic
influence operates over the entire course of your life,” Marrus says.
New
measures:
The apparent
change in genetic influence may actually reflect decreasing errors in the
measures used, Mundy says. A child’s language abilities, for example, would
play a key part in the parent’s assessment of the child’s social skills, and
language development is more varied in 18-month-olds than in 24-month-olds.
“I interpret
this as consolidation of true score over time,” Mundy says. “I would think it’s
not reflecting a major biological change.”
Dividing the broad
category of ‘social behavior’ into specific dimensions could ultimately help
researchers assess which interventions are effective, when and for whom,
says Giacomo Vivanti, associate professor in the Early
Detection and Intervention program at the Drexel Autism Institute in
Philadelphia, Pennsylvania, who was not involved in the work.
“The
boundaries between these constructs are really fuzzy,” Vivanti says. “We need a
more fine-grained understanding of, ‘What are we talking about when we talk
about social behavior?’
The findings
could help clinicians identify behaviors that may indicate autism in children
too young to be diagnosed, Marrus says. A child with difficulties in several
categories of reciprocal social behavior, for example, may have more underlying
genetics linked to autism and be more likely to have the condition than a child
with difficulties in only one category.
“Being able to
understand how to weigh all of those dimensions within an individual could be
very important, especially early in life, to figuring out what is this child’s
level of risk,” Marrus says.
That approach could also help researchers tease apart the heritable dimensions of other autism traits, such as motor skills development, Marrus says. Marrus and others have shown that eye-contact patterns, another indicator of social behavior, are heritable.
This article was published on Spectrum, the leading site for autism research news.
Lee A. Wilkinson, PhD, is author of the award-winning books, A Best Practice Guide to Assessment and Intervention for Autism and Asperger in Schools and Overcoming Anxiety and Depression on the Autism Spectrum: A Self-Help Guide Using CBT. He is also editor of a text in the APA School Psychology Book Series, Autism Spectrum Disorder in Children and Adolescents: Evidence-Based Assessment and Intervention in Schools. His latest book is A Best Practice Guide to Assessment and Intervention for Autism Spectrum Disorder in Schools (2nd Edition)