About the cited study:
The sample of 96 children is relatively small, but larger than some university studies I have seen on ADHD. I don’t see a normal here, what are they basing this against? What is the prevalence for B blood type though? A correlation is not a causation. It could also be a statistical anomaly. Do they differentiate severity of ADHD? How was the ADHD diagnosed in children? What happens in adults with ADHD?
I see that this blog discusses the study and blood type http://adhd-treatment-options.blogspot.com/2009/05/does-blood-type-affect-adhd.html . The website does look at the science, but has some questionable conclusions and treatment. It does best when discussing genes I suppose.REPORT ABUSE