Conduct Disorder (CD) is the descriptive diagnosis for troubled, bad, acting-out teenagers who violate the law and often use alcohol or drugs. It is on a continuum between Oppositional Defiant Disorder most commonly diagnosed in preadolescent children and Anti-Social Personality Disorder (ASPD), the label attached to most adults who regularly engage in criminal behavior.
So the researchers set out to see if they could distinguish ADHD from CD by giving both groups Ritalin with the belief that only those with ADHD would respond (this is called diagnosis via pharmacological dissection). To their surprise, the researchers could not round up enough adolescents who only had CD without any ADHD. In other words, virtually all the teens who met criteria for CD also met criteria for ADHD. So they wound up giving Ritalin to the CD comorbid with ADHD group and lo and behold some of their CD symptoms improved (makes sense - Ritalin decreases impulsive acts). Thus CD becomes another indication for using Ritalin in children.
The diagnostic ambiguity/utility question was lost or dropped in the article. Is there any value is saying ADHD is comorbid with CD when all CD children are also ADHD? How do these different labels guide different treatment? Also there are important legal implications. ADHD, which is presented as a neurological disability, increasingly is invoked as a legal defense or mitigating factor in sentencing with the notion that the acts perpetrated were not entirely under the person's control due to this neurological condition. Since criminal activity essentially defines CD and Anti-Social Personality Disorder, then potentially all offenders would be viewed as having ADHD (which is likely given the broad scope of the ADHD diagnosis). Thus, if all CD and ASPD are also ADHD, then invoking the ADHD diagnosis as a defense could be rendered meaningless if we are to hold people responsible for their behavior. Russell Barkley raises these very questions in ADHD and the Nature of Self Control as I do in the last chapter of Running on Ritalin.