ADD does tend to run in families. In the last two decades, scientists have made progress in understanding the genetics of ADD. The pioneering works of David Comings, Florence Levy and others have demonstrated a genetic component to this disorder. Specific gene sites implicated in ADD include the dopamine transporter gene on chromosome 5, and the D4 receptor gene on chromosome 11. Child psychiatrist Florence Levy from Australia found that 81% of identical twins (who share identical genetic material) had ADD, while fraternal twins (who have sibling genetic material) share ADD only 29% of the time.
Based on both my clinical experience and the medical literature, it is safe to conclude that a very high percentage of ADD is passed down genetically. In my experience, if one parent has ADD, then 60% of the offspring will have it as well. If both parents have ADD, then 85 to 90% of the children will have it. Anyone who does this work over time has no doubt it is a genetic family disorder and that contributes to the high incidence of family dysfunction in ADD households. There are often multiple challenging people in an ADD family, not just one challenging child.