Genetic and environmental contribution to the overlap between ADHD and ASD dimensions in adults: a twin study

Introduction  Attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) frequently co-occur. One previous twin study in adults has found a stronger phenotypic and genetic overlap between traits of... [ view full abstract ]

Introduction 

Attention-deficit/hyperactivity disorder (ADHD) and autism spectrum disorder (ASD) frequently co-occur. One previous twin study in adults has found a stronger phenotypic and genetic overlap between traits of ADHD and traits of ASD related to repetitive and restrictive behaviours and interests (Polderman TJ, Hoekstra RA, Posthuma D, Larsson H. The co-occurrence of autistic and ADHD dimensions in adults: an etiological study in 17,770 twins. Transl Psychiatry. 2014).

To obtain more insight into the aetiology of the co-occurrence of these disorders in adulthood, we estimated the contribution of genetic and non-genetic effects to the association between ADHD traits and ASD traits in adults.

Methods

We use data from the Swedish Study of Young Adult Twins (YATSS) on 2122 twin pairs aged 20-28. Two dimensions of ADHD — inattention (IA) and hyperactivity/impulsivity (HI) — were assessed using the WHO Adult ADHD Self-Report Scale-V1.1. Two dimensions of ASD — repetitive and restrictive behaviours (RRB) and social interaction and communication (SIC) — were assessed using the Autism — Tics, AD/HD and other Comorbidities inventory. 

We used structural equation modelling to decompose co-variance between ADHD and ASD dimensions into genetic and environmental contribution and to estimate correlations between genetic and environmental effects acting on each ADHD and ASD dimension. The model was adjusted for age and sex.    

Results

At the phenotypic level, IA was associated with both dimensions of ASD (r=0.36; r=0.35), while HI was more strongly associated with RRB (r=0.40), than with SIC (r=0.24). Genetic and non-shared environmental effects accounted for a similar proportion of the phenotypic correlations. The highest genetic correlation was observed between HI and RRB (0.57), while the lowest was observed between HI and SIC (0.31).   

Conclusions

The phenotypic and etiologic overlap between ADHD and ASD seems to be, at least partially, dimension-specific in adults. Both genetic and environmental effects seem to be important for such overlap.