What we now know about ADHD is that most people who have ADHD were born with an ADHD brain. The heritability of ADHD has been studied in many ways, including family studies, adoption studies, and twin studies. There is consistency of results among studies done in various parts of the world, showing that ADHD is not unique to the US.
In a study of twins in Australia, 82 percent of the time both twins in a set of identical twins had ADHD, whereas both non-identical twins had ADHD only 38 percent of the time. Research shows that one-third to one-half of parents with ADHD will have a child with ADHD. If a child has ADHD, there is a 30-40 percent chance that his or her sibling will also have ADHD.
It appears that other factors may contribute to ADHD’s prevalence or severity, so there is research being done to see if there might be a connection between ADHD and low birth weight, prematurity, head injuries, maternal smoking or drinking, exposure to pesticides or other toxins, and other possible factors.
There is a lot of information online about ADHD and genetics. Here is one place to start looking.