I found this on the NIH website:
What Causes ADHD?
One of the first questions a parent will have is "Why? What went wrong?" "Did I do something to cause this?" There is little compelling evidence
at this time that ADHD can arise purely from social factors or child-rearing methods. Most substantiated causes appear to fall in the realm of
neurobiology and genetics. This is not to say that environmental factors may not influence the severity of the disorder, and especially the degree of
impairment and suffering the child may experience, but that such factors do not seem to give rise to the condition by themselves.
The parents' focus should be on looking forward and finding the best possible way to help their child. Scientists are studying causes in an effort to
identify better ways to treat, and perhaps someday, to prevent ADHD. They are finding more and more evidence that ADHD does not stem from the home
environment, but from biological causes. Knowing this can remove a huge burden of guilt from parents who might blame themselves for their child's
behavior.
Over the last few decades, scientists have come up with possible theories about what causes ADHD. Some of these theories have led to dead ends, some
to exciting new avenues of investigation.
Environmental Agents.
Studies have shown a possible correlation between the use of cigarettes and alcohol during pregnancy and risk for ADHD in the offspring of that
pregnancy. As a precaution, it is best during pregnancy to refrain from both cigarette and alcohol use.
Another environmental agent that may be associated with a higher risk of ADHD is high levels of lead in the bodies of young preschool children. Since
lead is no longer allowed in paint and is usually found only in older buildings, exposure to toxic levels is not as prevalent as it once was. Children
who live in old buildings in which lead still exists in the plumbing or in lead paint that has been painted over may be at risk.
Whole article: NIH ADHD
There doesn't seem to be any mention of mercury poisoning, but they do mention lead being a possible factor.
To say that mercury may be an underlying cause to ADHD is interesting, but to dismiss any other idea as "denial" is absurd.


) It was just I wasn't doing
what my parents wanted me to be doing, so they wanted to blame it on a disorder and take me to numerous doctors to get me "fixed". 

