ADHD has been researched extensively in the United States and a number of countries throughout the world. Sophisticated brain-imaging technologies and genetic research in recent years, particularly since the mid-2000s, has dramatically increased our knowledge of the probable causes of ADHD.
We now know there are multiple causes, although we certainly do not know all of them. Based on hundreds of well-designed and controlled scientific studies, the evidence clearly suggests that ADHD is the result of brain differences: abnormalities in size, maturation, and levels of activity in the regions of the brain involved in executive functions and self-regulation.
Heredity is the main known cause of ADHD, accounting for most cases. In other cases, there are problems and factors that occur prenatally, during birth, or in childhood that might interfere with a child's brain development and be contributing causes of ADHD.