ADD is due to several factors. It is unlikely that one etiology leads to all cases. Family genetic factors have been implicated. Heritability is estimated to be between 0.55 and 0.92. No gene has been described or found that has been replicated in several studies.
ADD does occur in known biological syndromes, such as the fragile X and fetal alcohol syndrome.
If six or more out of the nine symptoms in both domains are present for at least six months to a degree that is maladaptive and inconsistent with developmental level, the combined subtype is diagnosed. This is the most common subtype in clinical populations, but may not be the most common subtype in the general population. Most of our knowledge base comes from studies of elementary school-aged boys. There are fewer data available on girls, preschoolers, adolescents and adults. The inattention and hyperactive-impulsive symptoms are present, but are manifested in more developmentally appropriate ways in preschoolers, adolescents and adults.