How has the idea of citizenship changed throughout history?

The idea of citizenship has expanded to include all people who fulfill certain criteria, and in principle anyone can obtain a citizenship of many countries if they work towards it thought the process of naturalization.

In the past, citizenship was often limited: it included only men (as in ancient Greece) or only certain ethic groups (as in the case of Early US).

The best sociological research method would have to be using experiments although it has some disadvantages as well