Which class of American society attributes membership to birth rather than to merit

Hard to ascertain as it is partially true for many classes

Explanation:

To be rich is easier if you are born rich but that does not mean that there are no people getting rich on their merit or from other classes. Similarly, many people go poor from a situation in the middle class or the working class.

Being born into a class makes one susceptible to being in the class throughout their class be it middle class or working class too as much as the upper class of the society.