a1 Centre for Research in Autism and Education, Department of Psychology and Human Development, Institute of Education, University of London, London, UK
a2 Department of Psychology, Institute of Psychiatry, King's College London, London, UK
a3 Analytic and Translational Genetics Unit, Massachusetts General Hospital, Boston, MA, USA; Department of Medicine, Harvard Medical School, Boston MA, USA
a4 King's College London, MRC Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry, De Crespigny Park, London, UK
a5 Genes Environment Lifespan Laboratory, Centre for Brain and Cognitive Development, Department of Psychological Sciences, Birkbeck College, University of London, London, UK
Background Autism spectrum disorder (ASD) and attention deficit hyperactivity disorder (ADHD), and associated subclinical traits, regularly co-occur with one another. However, the aetiology of their co-occurrence remains poorly understood. This paper provides the first genetically informative, longitudinal analysis of the interaction between traits of ASD and ADHD, and explores their genetic and environmental overlap.
Method Parents of approximately 5000 twin pairs completed questionnaires assessing traits of ASD and ADHD when twins were aged 8 and 12 years. Cross-lagged longitudinal modelling explored their developmental association, enabling a consideration of phenotypic-driven processes. Overlapping aetiological influences on traits at age 12 years were explored using bivariate twin modelling.
Results Traits of ADHD at age 8 years were more strongly predictive of traits of ASD at 12 years than traits of ASD at 8 years were of traits of ADHD at 12 years. Analysis of traits by subscales assessing specific symptom domains suggested that communication difficulties were most strongly associated with traits of ADHD. Bivariate modelling suggested moderate genetic overlap on traits in males (genetic correlation = 0.41), and a modest degree of overlap in females (genetic correlation = 0.23) at age 12 years.
Conclusions Traits of ADHD at age 8 years significantly influence traits of ASD at age 12 years, after controlling for their initial relationship at age 8 years. In particular, early ADHD traits influenced later communication difficulties. These findings demonstrate the dynamic nature of co-occurring traits across development. In addition, these findings add to a growing body of literature suggesting that traits of ASD and ADHD may arise via similar aetiological processes.
(Received April 03 2012)
(Revised September 24 2012)
(Accepted October 01 2012)
(Online publication November 16 2012)
c1 Address for correspondence: M. J. Taylor, M.Sc., Centre for Research in Autism and Education, Department of Psychology and Human Development, Institute of Education (University of London), 25 Woburn Square, London WC1H 0AA, UK. (Email: M.Taylor@ioe.ac.uk).