http://newideas.net/genetic-causes-adhd-cardiff-study
According to this article, a research team believes they have found a genetic link for ADHD. In a genetic study they did, 15% of the kids diagnosed with ADHD had significant differences in their DNA as opposed to 7% of the control group (kids without ADHD). The main problem with this data is that 85% of the kids diagnosed with ADHD do not have a clear difference in their DNA. This would show that the problem could be partially genetic, but there is a clear nurture side to it. The argument given by the article is that the current ADHD screening isn't as thorough as it could be and that many other disorders are being diagnosed as ADHD. While this could be the case, until there is further evidence to back it up, I agree that ADHD is probably caused more by environmental factors than genetic. The rate of kids diagnosed with ADHD without the DNA difference (85%) is just too high to ignore. Until it is clearly shown that many of the kids diagnosed with ADHD do not have the disease, the environment argument seems more scientifically sound than the genetic argument to me.
I've noticed a lot of news reports and articles that are about ADHD being overdiagnosed. I believe environment has a lot to do with the development of ADHD especially at young ages when the brain is developing. Since children spend most of their time either with their family at home or in school, I would be curious to know how much of a factor school size has to do with it. I'm not sure what part of the environment would lead kids to develop ADHD but maybe being in a large class where there is a high student teacher ratio may be part of the cause (especially in pre k-3rd grade). With less one on one time with the teacher and more time to be distracted by fellow classmates may cause some children to develop a lack of concentration, organization, and initiative to finish tasks.
Just a thought...