A Twin Study of Attention-Deficit/Hyperactivity Disorder Dimensions Rated by the Strengths and Weaknesses of ADHD-Symptoms and Normal-Behavior (SWAN) Scale

Background When symptom rating scales are used in the general population, there is severe skewness, with many individuals having no symptoms. While this has major implications for genetic designs that require extremely discordant and concordant (EDAC) siblings, little is known of the genetics of scales which seek to differentiate within the “no ADHD symptom” group. Methods Parents of Australian twins completed two attention-deficit/hyperactivity disorder (ADHD) questionnaires, the Australian Twin Behaviour Rating Scale (ATBRS), based on conventional DSM-IV symptom scores, and the Strengths and Weaknesses of ADHD-Symptoms and Normal-Behavior (SWAN) scale, which includes above-average performance on attention and activity. The two scales were compared in two age groups of same-sex twins, 528 pairs aged 6 to 9 and 488 pairs aged 12 to 20. Results Parents reported higher levels of activity and attention in their twins when reporting using the SWAN scale than when using the ATBRS, and while the monozygotic (MZ) correlations were similar on both scales, the dizygotic (DZ) correlations were consistently higher on the SWAN. On DSM-IV based scales, parents exaggerated differences within those sibling pairs in the “with few ADHD symptoms” category. Conclusions The SWAN may provide a more realistic description of the ADHD phenotype for the selection of twin and sibling pairs for genetic analysis