What were the privileges of the elite based upon?

It's not a simple answer and several points can be the answer. In general, the explanation for the social privilege is based upon social class, age, height, IQ, disability, ethnic or racial category, gender, gender identity, sexual orientation, and religion. However, all these ideas are studied outside Academia, because they don't have theoretical support.

Explanation:

During History, the elite social privileges were explained based on their birth, and the idea that God gave the power to a certain group of people to rule others. This idea gradually turns into different approaches during the 18th and 19th centuries when Science and Social Science began to attempt to explain society. From now on, social privileges were based upon race and intelligence. Today, these ideas are discredited by Academia, but several people still believe it.

a social contract.

"a social contract is an agreement between citizens and their leader where the citizens give up certain freedoms in exchange for protection. the constitution of the united states is an example of a social contract."

(1)representation based on state population (2)all states have equal representation apex