ShadowGenius is correct. America was formed by the voluntary migration of people with certain characteristics such as boldness, risk taking, confidence, adventurism. Other, much older societies that have formed over the millennium creating a Gaussian distributions of personality types form bold risk takers at one side to timid, complacent followers at the other side.
Early America has been formed as a classified or “sieved” society. Early settlers in the “New World” had to take a very perilous, several month journey on rickety, crowed ships with a good chance on not making it across. Once arrived, there was no guarantee of even survival. America was formed from that risk taking side of the societal Gaussian distribution.
On the Discovery Channel there was a documentary on how a 10,000 year old “cave man” was found frozen in the Alps. They tested his DNA and did a sampling of the DNA of the people living in the town at the base of that mountain. They found that many in the town were related to that caveman. Imagine what timidity and lack of curiosity a line of people must have to never have moved away from their home for over 10,000 years.
The modern world with its advances was not advanced by these timid people too scared to move away from the base of their mountain. The first American settlements on the East coast developed their own Gaussian Distributions over several hundred years and formed their own risk adverse society. Luckily, there was the West where the new risk takers would move to seek fame and fortune. This kept happening until we ran out of western areas to move to.
That is why America is divided in two today. The Gaussian risk adverse personalities now match the risk takers and we are split. In short, America was exceptional because only the exceptional people of the world took the chance to get here, and that continues to this day with people risking all to come here to succeed.