There is no definitive answer as to what causes ADD and ADHD, as of 2015. However, a combination of factors, including genetics, brain chemistry and chemical exposure, all play a role, according to WebMD.
Several differences appear in the brain of a person diagnosed with ADHD. Neurotransmitters, or the signals between areas of the brain, and nerve pathways differ. Additionally, the chemical dopamine is lower with those with ADHD. Dopamine is linked to attention, learning and movement.
The brains of those with ADHD also show delays in maturation in the areas concerning attention and planning, according to the National Institute of Mental Health. Additionally, brain damage in those areas can cause ADHD symptoms in those previously asymptomatic.
Studies show that pregnant women who smoke or drink during pregnancy are more likely to have children with ADHD. Babies born prematurely or who have low birth rate are also at greater risk, says WebMD. Fetuses exposed to environmental toxins, such as PCBs, are at a higher risk as well.
Heredity is also a theorized factor. Children with one parent who have ADHD are 50 percent likely to have ADHD themselves. The chance is 30 percent if a sibling has the disorder, reports WebMD. Although the theory that sugar and television cause ADHD, there is no evidence to suggest this.