This paper is a review, not a new study. The authors looked back over research on how attention-deficit/hyperactivity disorder (ADHD) and early-onset conduct disorder tend to occur together, focusing on the pathway from ADHD to conduct disorder in boys.
They report that about one-third of boys with severe ADHD in the preschool years go on to develop early-onset conduct disorder. They also note that most boys with early-onset conduct disorder meet the criteria for ADHD as well.
The authors argue that both conditions grow from a shared underlying trait they call trait impulsivity. They describe this trait as strongly inherited, with genetic effects approaching and sometimes above 0.80, meaning genes explain a large share of it. Drawing on several large twin studies of children and adults, they say a single common factor explains much of why ADHD, oppositional defiant disorder, and conduct disorder show up together. Whether this impulsivity turns into conduct disorder, they add, depends on environmental risk.
They also cite genome-wide genetic studies suggesting that children with both ADHD and conduct disorder carry no extra genetic burden compared with children who have ADHD alone.
This is a selective theoretical review. It leaves out the purely inattentive form of ADHD and adolescent-onset conduct disorder, and the authors note that the genetic findings they cite sometimes conflict.
Source: Beauchaine T, Hinshaw S, Pang K. Comorbidity of attention‐deficit/hyperactivity disorder and early‐onset conduct disorder: Biological, environmental, and developmental mechanisms.. Clinical Psychology: Science and Practice (2010). doi:10.1111/j.1468-2850.2010.01224.x