It's possible that the traits that in some combination lead to ADHD are adaptive on their own, or in some people, or in lower doses (like having 1 copy of a gen

It's possible that the traits that in some combination lead to ADHD are adaptive on their own, or in some people, or in lower doses (like having 1 copy of a gene instead of 2). If that's the case, your non-ADHD relatives might be more fit, and the burden of some family members having ADHD isn't enough to outweigh the benefits to the rest of the family. Kind of like having one copy of the sickle cell anemia gene gives you resistance to malaria, but two makes you likely to die from sickle-cell disease.