Limited Genetic Covariance Between Autistic Traits and Intelligence: Findings from a Longitudinal Community Based Twin Study

Background:

Intellectual disability is common in individuals with autism spectrum conditions (ASC), but the prevalence of intellectual impairment may be overestimated in ASC due to ascertainment bias (Skuse, 2007). Therefore, the strength of this association is unclear and its relevance to finding the underlying (genetic) causes of ASC remains elusive. Furthermore, little is known about the longitudinal relationship between autistic traits and intellectual abilities. Instruments that assess autistic traits on a quantitative scale enable the study of this association in community based samples, free of the possible effects of ascertainment bias.

Objectives:

1) To investigate the association between autistic traits and intelligence in a large general population twin sample, both in the full-range scores and in the most extreme scoring 5% of this population. 2) To assess this association longitudinally over multiple time points in childhood. 3) To examine the etiology of this association.

Methods:

Parental ratings of autistic traits, using the Childhood Autism Spectrum Test (CAST, Scott et al., 2002), were collected in a sample of 8,848 twin pairs when the children were 8, 9, and 12 years old. Data on intelligence were collected when the twins were 7, 9, and 12 years. Phenotypic and longitudinal group correlations in the children with high CAST scores or with low IQ were compared to the associations among the full-range scores. The genetic and environmental influences on the overlap between autistic traits IQ and on the stability of this relationship over time were estimated using structural equation modeling.

Results:

Autistic traits were only modestly negatively related to intellectual ability, both in the extreme scoring groups and in the sample as a whole (r = -.14 to -.27). This association was mainly due to autistic trait items assessing communication difficulties. Cross-age correlations between CAST and IQ scores were similar to within-age correlations, and changes in autistic traits did not predict a change in intelligence, indicating that the modest association between these traits was stable over time. Genetic model fitting showed that autistic traits and IQ were influenced by a common set of genes and a common set of environmental influences that continuously affect these traits throughout childhood. The association between autistic traits and IQ was explained by genetic effects and by environmental influences shared between the twins. Importantly, the genetic correlation between autistic traits and IQ was modest (r_g = -.27).

Conclusions:

The findings from this general population twin study suggest that whilst both autistic traits and IQ are stable traits across middle to late childhood, the association between these traits is consistently modest and driven by communication problems characteristic for autism. Individual differences in autistic traits are highly heritable and are substantially genetically independent of intellectual functioning. Future molecular genetic and neurobiological studies of ASC that seek to elucidate the pathways from genes to autism should include a focus on genetic influences that spare intellectual abilities.