Intellectual disability (here defined as IQ570) is common in
autism. Historically, the prevalence of intellectual disability in
autism is estimated at 70%,1but recent studies encompassing all
autism-spectrum conditions, including Asperger syndrome and
pervasive developmental disorder – not otherwise specified,
suggest that the prevalence of intellectual disability in autism-
spectrum conditions may be considerably lower.2,3It has been
suggested that the association between autism and intellectual
disability may be inflated because of clinical ascertainment bias.4
If this hypothesis holds true it has implications for studying
the causes of autism-spectrum conditions. A strong genetic link
between autism-spectrum conditions and intellectual disability
would argue for a search for genes influencing both traits. A
limited association would argue for separate genetic influences
on each trait. A possible ascertainment bias for intellectual
disability in autism-spectrum conditions limits the investigation
of this association in clinical samples. Instruments that assess
autistic traits on a quantitative scale5,6enable studying the
relationship in population samples. This study reports on the
association between autistic traits, IQ and academic achievement
in the extreme 5% scorers of a large community-based twin
sample. The genetic informative design allowed for exploration
of the genetic and environmental origin of the association.
Development Study (TEDS), a child twin sample representative
of the general population in the UK.7,8The sample characteristics
of TEDS are described elsewhere.9Zygosity in same-sex twins
was determined using polymorphic DNA markers (75% of
participants) or a parent-report questionnaire that has a reported
accuracy of 95%.10Participants’ IQ and academic achievement
were part ofthe longitudinal TwinsEarly
were assessed at ages 7 and 9. Measures of autistic traits were
collected when the twins were nearly 8 (parent report) and 9 (tea-
cher report) years.
Exclusion criteria were as follows: no first contact data
available (153 families); extreme pregnancy or perinatal difficulties
(165 families); unclear twin zygosity (300 families); not having
English as the spoken first language (146 families); specific
medical syndrome (not including suspected autism-spectrum
conditions) such as Down syndrome (225 families). After
exclusions data were available for 8104 twin pairs, of which
1340 were monozygotic male pairs, 1325 dizygotic males, 1496
monozygotic females, 1354 dizygotic females and 2589 dizygotic
twin pairs of opposite sex. At age 9, only twins born between
January 1994 and August 1995 were contacted, resulting in a
smaller sample size. Data on IQ, academic achievement and/or
autistic traits were available for 7965 7/8-year-old twin pairs and
for 3687 pairs at age 9.
The Childhood Autism Spectrum Test (CAST)5is a 31-item
questionnaire asking about behaviours associated with autism-
spectrum conditions. A CAST score of 515 is the cut-off for
identifying children at risk for autism-spectrum conditions. Items
can be divided into three subscales,11based on the DSM–IV
criteria12for autism: social impairments (12 items); communication
impairments (12 items); and restricted repetitive behaviours and
interests (7 items). The CAST shows good test–retest reliability
(r=0.83)13and satisfactory internal consistency (a=0.73 in
TEDS) for the full CAST and moderate a-values for the subscales
(social impairments: a=0.57; communication impairments:
a=0.66; restricted repetitive behaviours and interests: a=0.50).
Parents rated the child’s autistic traits at age 8. If the families gave
consent, teachers were asked to complete an abbreviated version of
the CAST (20 items)8when the twins were 9 years.
Association between extreme autistic traits
and intellectual disability: insights from a general
population twin study
R. A. Hoekstra, F. Happe ´, S. Baron-Cohen and A. Ronald
Autism is associated with intellectual disability. The strength
and origin of this association is unclear.
To investigate the association between extreme autistic traits
and intellectual disability in children from a community-based
sample and to examine whether the association can be
explained by genetic factors.
Children scoring in the extreme 5% on measures of
autistic traits, IQ and academic achievement
were selected from 7965 7/8-year-old and 3687 9-year-old
twin pairs. Phenotypic associations between extreme autistic
traits and intellectual disability were compared with
associations among the full-range scores. Genetic
correlations were estimated using bivariate DeFries–Fulker
Extreme autistic traits were modestly related to intellectual
disability; this association was driven by communication
problems characteristic of autism. Although this association
was largely explained by genetic factors, the genetic
correlation between autistic traits and intellectual disability
was only modest.
Extreme autistic traits are substantially genetically
independent of intellectual disability.
Declaration of interest
The British Journal of Psychiatry (2009)
195, 531–536. doi: 10.1192/bjp.bp.108.060889
Hoekstra et al
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H. Steven Moffic
Much scientific consensus has developed that global warming is a major threat to the well-being of our planet and ourselves. This
danger includes mental health. Violence, trauma and anxiety are all projected to increase. Psychology has also contributed to the
genesis and delayed responsiveness to global warming, given the use of denial, narcissism, and fear of change on the part of
politicians and citizens. Given the importance of psychiatry for this social problem, psychiatrists should be at the forefront of ‘going
green’ in terms of advocacy, modelling and solutions. We are not yet, but our ethical duty requires more.
The British Journal of Psychiatry (2009)
195, 536. doi: 10.1192/bjp.195.6.536