This is something that surprisingly escaped me in the past year. Apparently two guys wrote abook about how Americans (a self selected group of "crazy" people who actually packed up and left their homeland) are likely to exhibit hypomania - sort of like ADHD, but different. They wrote a book called the Hypomanic Edge.
I found out about this from the New York Times Magazine issue that had the "Year's best ideas".
Here is the link
...and the first paragraph of the short article by Emily Bazelon:
For centuries, scholars have tried to explain the American character: is it the product of the frontier experience, or of the heritage of dissenting Protestantism, or of the absence of feudalism? This year, two professors of psychiatry each published books attributing American exceptionalism to a new and hitherto unsuspected source: American DNA. They argue that the United States is full of energetic risk-takers because it's full of immigrants, who as a group may carry a genetic marker that expresses itself as restless curiosity, exuberance and competitive self-promotion - a combination known as hypomania.
link to Boston Globe article
Anyway this caught my attention because of the link to some previous work done on the association between some variants at the DRD4 locus associated with novelty seeking behavior and linked to migration behavior in humans (Human Nature paper by Chen). There was also a paper about this in PNAS a few years back (I don't have the author of paper with me) and a paper in the same issue by Henry Harpending and Greg Cochran called "In our genes" where they talk about the worldwide population differences in this allele and some behavioral associations with "novelty seeking behavior", migration, and cad vs. dad societies.
This book, "Hypomanic Edge" might be an interesting read.