ADHD and Autism Overlap: Adult Twin Study

Clinical studies show high rates of ADHD in adults with autism or autism spectrum disorder (ASD). 

These studies have estimated the prevalence of ADHD in adults with ASD at between 28 and 44% of individuals. The prevalence of ADHD in the adult general population without ASD is estimated at 2.5%.

Given the overlap between these two developmental disorders, it makes sense to examine genetic factors that might contribute to co-occurrence.

TJC Polderman along with colleagues from the Netherlands, the UK and Sweden looked at this issue using a sample of adult twins. Twin studies provide a powerful tool for teasing out genetic from environmental influence by comparing identical (monozygotic) to fraternal (dizygotic) twin pairs.

All twins in the studies rated their ASD and ADHD symptoms from those used as criteria in the Fifth Revision of the Diagnostic and Statistical Manual (DSM-5). This produced scores in four key symptom domain scores:

• ASD social/communication deficits
• ASD repetitive and restrictive behavior interest
• ADHD inattention
• ADHD hyperactivity/impulsivity

Using the twin study design the authors were able to identify:

• A strong genetic contribution to the overlap of ASD repetitive scores with ASDH inattention in both men and women
• A strong genetic contribution to the overlap of ASD repetitive scores with ADHD hyperactivity/impulse control in both men and women
• ASD social/communication deficit showed only modest

The authors conclude their study is one of the first suggesting ASD repetitive/restriction symptoms contribute specifically to high ADHD symptom levels. One potential explanation would be a common genetic factor or factors that produce both repetitive/restrictive ASD symptoms along with inattention and hyperactivity.

The authors note their findings support giving attention to specific symptom domains in designing research in ADHD and ASD. Specific symptom domains may have specific genetic and environmental risk factors.

Readers with more interest in this topic can access the free full-text manuscript by clicking on the PMID link in the citation below.

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Polderman TJ, Hoekstra RA, Posthuma D, & Larsson H (2014). The co-occurrence of autistic and ADHD dimensions in adults: an etiological study in 17,770 twins. Translational psychiatry, 4 PMID: 25180574