What in the brain causes add/adhd?!

Fabulous question, and one that we still don't know the whole answer to.

Several neurotransmitters (the brain's chemical messengers) are involved with our ability to maintain and shift focus, prioritize, resist distractions, delay gratification, and so on. These abilities are disrupted in people with ADHD, so it is likely that the neurotransmitter systems responsible are "out of whack" in some way.

Some specific neurotransmitters that have been implicated in ADHD include dopamine (also involved with addiction), norepinephrine, and glutamate. Current ADHD drugs act primarily on these neurotransmitter systems.

Brain imaging studies have also found structural differences (like differences in average size of certain parts of the brain, or how quickly parts of the brain develop) in the brains of people with ADHD compared to those without ADHD. One of the early landmark brain-imaging studies, by Alan Zametkin and his colleagues, showed differences in the way the brains of those with and without ADHD metabolized glucose.

Some of these studies can be hard to interpret, though. Most of the studies have been done on hyperactive boys, so they may not accurately represent the brains of girls and women, adult men, people who primarily have problems with attention but aren't hyperactive, and so on..

Complicating things is that there may be several different causes of ADHD, accounting for some of the differences seen in ADHD subtypes and the patterns of comorbidity (coexisting disorders, like depression, anxiety, learning disabilities, etc.). And factors such as sex, age, and environment modify the way the chemical and structural pathways develop and work.

In sum: our knowledge in this area is still very much evolving. We have access to new brain-imaging techniques, genetic information, and larger and longer epidemiologic studies that can all help shed light on the causes of ADHD. It's a very exciting time for anyone with an interest in the way ADHD brains (and, really, all brains!) function.

I'm a scientist with an interest in ADHD. I've read many books and scientific papers on ADHD, including large portions of Steven Pliszka's book, "Neuroscience for the Mental Health Clinician" and Russell Barkley's book, "ADHD: What the Science Says". For people who aren't scientists, I'd recommend Barkley's book "Taking Charge of Adult ADHD".

It's a lack of the chemicals dopamine and serotonin. Someone without add/adhd is producing the correct levels of those chemicals.