No-one knows for certain what causes ADHD, and it is possible that a number of factors may contribute to its development.
ADHD tends to run in families so the leading theory is that ADHD is an inherited neurodevelopmental disorder. ADHD is thought to be as heritable as height.
Other theories about the cause of ADHD include oxygen deprivation around the time of birth, premature birth or low birth weight, brain injury, or exposure to chemical toxins in early childhood. There is no evidence that food intolerances cause ADHD although it is possible that in some children high levels of sugar intake, food colourings and preservatives may worsen the symptoms. There is no evidence for claims by some chiropractors that “neonatal retained reflexes” cause ADHD.
In line with the theory that ADHD involves neural development rather than just learned behaviour, children with ADHD tend to have a higher rate of other neurological difficulties, such as bed wetting that persists beyond the usual developmental age, clumsiness and poor co-ordination, difficulty knowing left from right, poor handwriting, learning disorders, and difficulty telling the time on a clock-face.
There is no evidence that persistent childhood abuse causes ADHD, although such childhood trauma may result in symptoms that look a lot like ADHD (e.g., dissociation can cause children to tune out and not pay attention to their surroundings, they may be restless and agitated as a consequence of constant threat).