New Research
Investigating the Association Between Autistic-Like and Internalizing Traits in a Community-Based Twin Sample

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Abstract

Objectives

Recent research has suggested that children with autistic spectrum disorders often experience comorbid symptoms of anxiety and depression. However, despite this overlap, no quantitative genetic studies have addressed the phenotypic overlap and the etiologic association between internalizing and autistic-like traits within the general population. This study aimed to investigate the phenotypic and etiologic relation between internalizing and autistic-like traits using a community-based twin sample.

Method

We investigated the co-occurrence of these traits in a population-based sample of 3,233 twin pairs aged 8 to 9 years, using both parent- and teacher-report questionnaires. Bivariate structural equation modeling techniques were used to determine the extent to which internalizing and autistic-like traits shared common genetic and environmental influences.

Results

Our results showed that there was a modest phenotypic correlation (r = 0.26-0.29) between autistic-like and internalizing traits. The traits were both substantially heritable but were largely independent with regard to their genetic influences (rG = 0.12-0.19). Shared environmental influences were modest but were largely common to both traits. Similar results were found using both parent- and teacher-reported data.

Conclusions

Internalizing and autistic-like traits showed moderate phenotypic overlap within the general population. This association was explained in small part by shared genetic factors, but the results suggested that most genetic influences were specific to either internalizing traits or autistic traits. Given these findings, we discuss the potential mechanisms that may underlie the relation between these traits. J. Am. Acad. Child Adolesc. Psychiatry, 2009;48(6):618-627.

Section snippets

Autistic-Like Traits and other Behavioral Problems

A twin design can be used to disentangle the degree to which the covariation between two traits is attributable to shared genetic or environmental influences. Three existing studies have investigated the etiologic association between autistic-like traits and other behavior problems, including withdrawn behaviors such as shyness,22 ADHD,1 and aggression and delinquency.22a In general, these studies suggested significant covariation between autistic-like traits and other behavioral problems and a

Autistic-Like Traits and Internalizing Behaviors

Despite high rates of co-occurrence between ASD and internalizing disorders, little is known about the etiologic explanation. Furthermore, no studies to date have analyzed the phenotypic or the etiologic association between internalizing and autistic-like traits within the reference range. We predicted that there would be substantial phenotypic overlap between these traits. With regard to the etiologic association, we predicted that there would be some genetic and environmental influences that

Participants

Data came from the Twins Early Development Study (TEDS), a population-representative sample of twins born in the United Kingdom between the years 1994-1996.14, 23 The study included data collected at age 8 and 9 years. Children were excluded if there were reports of extreme prenatal or perinatal difficulties or severe medical disorders. Children with diagnosed ASD were also excluded from the data collection, along with their cotwin. As such, our analysis focused on typically developing

Results

The descriptive statistics for the internalizing and autistic-like trait measures are presented in Table 1. The results show that, for the CAST, the male subjects scored higher than the female subjects (parent-reported data: F1,6264 = 265.52, p < .01; teacher-reported data: F1,2769 = 118.98, p < .01). Zygosity had a significant effect on the mean scores for parent-reported CAST, with DZ twins scoring significantly higher than MZ twins (F2,6264 = 5.31, p < .01). There were no significant

Discussion

The current study examined the phenotypic and etiologic association between autistic-like traits and internalizing traits within a population-based sample of 8- to 9-year-old twins, using parent- and teacher-reported data. To the best of our knowledge, it represents the first investigation of the phenotypic overlap between these traits in the general population and the first quantitative genetic investigation of their etiologic overlap in either a clinic- or population-based sample.

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    Twins Early Development Study is funded by a program grant from the U.K. Medical Research Council (G500079).

    The authors thank Prof. Robert Plomin and his team for the use of data from the Twins Early Development Study and all of the families who participated.

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