I do believe that Americans have A.D.H.D. more than other nations. It is my belief that due to ever changing "factors" for diagnosing a patient with A.D.H.D., especially with the recent changes to the psychiatric "Bible," the DSM, there is only a list of criteria for diagnosing a patient with any particular disorder. Due to no specific and objective requirements for such a diagnosis, a diagnosis of A.D.H.D. is purely subjective. Additionally, A.D.H.D. is more commonly diagnosed in an early stage, such as childhood, which such a diagnosis leads to an increase in profit for pharmaceutical companies. For example, if a child misbehaves occasionally, it is seen as "abnormal," and could lead to a diagnosis of A.D.H.D. when the actual cause is something else.
Many people with ADHD are being diagnosed and don't have it. Many people can control their attention issues, but choose not to. It's easier to blame a disease than take the proper actions to correct the behavior. Many people suffer with attention problems because we have a constant need to be entertained. This is not ADHD. This is conditioning by our environment and we can condition ourselves to act otherwise. It's amazing how many people claim they have ADHD and say they can control it using mind over matter.
Although A.D.H.D. is somewhat commonly diagnosed in the United States (about one in 20 children), a lot of the research surrounding A.D.H.D has come out of the United States. In order to fully determine whether or not other countries have a similar ratio, it needs to be studied more in other nations. It's very likely that it exists in various races and cultures, but goes underdiagnosed in them.
No, Americans do not have ADHD more than other nations, because psychologists and educators in the United States simply like to diagnose ADHD more than other countries do. ADHD is a fad in America. Educators often get extra funding to help deal with it. There are no statistics that it is actually more prevalent in America.
Americans are diagnosed with ADHD about the same as the rest of the developed world. And even more so in the urban areas where there are so many distractions. The same goes for much of western Europe and Canada, and even the more civilized cities in South America, like Buenos Aires and Rio.