Cause of ADHD: Genetic and Environmental Factors

Cause of ADHD: Genetic Factors

Research into the cause of ADHD has continued to indicate some substantial genetic contributions. This data supports the conclusion that there may be some familial transmission involved in the cause of ADHD. Heritability estimates range from 60% to 80%.

Molecular genetic studies into the cause of ADHD have identified a number of individual genes as potential candidate genes in ADHD. Some of the research has focused on the dopamine (DA) neurotransmitters including the DA D2, DA D4, and DA D5 receptor genes and the DA transporter gene. However, the strongest evidence into the cause of ADHD links ADHD with the 7-repeat allel of DRD4, which mediates an intracellular response to DA, and the 10- repeat allel of DAT1, which is linked with an elevated dopamine reuptake back into the neuron. Also, preliminary data have also linked the 10-repeat allel of DAT1 with poor response to methyphenidate(Ritalin), which acts primarily by inhibiting the dopamine transporter in the striatum.

Cause of ADHD: Environmental factors

The best research into the cause of ADHD comes from twin studies which have implicated environmental factors into the development of ADHD. Some of the studies have found that a moderate-to-significant proportion of susceptibility to ADHD may be accounted for by factors that are not shared by both twins. The nonshared factors have been found to have the greatest effect on children with conduct disorder, and are believed to be somewhat less important contributors to the cause of ADHD. Examples of nonshared factors may include poor school performance, difficult temperament-inflated self-esteem, impulsivity, low verbal intelligence, and biological events such as perinatal insult and head trauma.

While shared family, peer and neighborhood risk factors may play a significant role in the development of oppositional defiant disorder and conduct disorder, they may play a lesser role in the cause of ADHD. These adverse factors may include large amounts of siblings within the family, families that have experienced separation, single-parent households, child neglect, parental conflict and frequently poverty. Parental child-rearing practices, such as harsh physical discipline and poor supervision have also been implicated in oppositional defiant disorder and conduct disorder but are not believed to be strongly related to the cause of ADHD.