This is a classic scholarly article in the academic study of Shi'i Islam, specifically Ithna 'Ashariyya (Twelver) Shi'ism by one of the great Islamicists, Etan Kohlberg, whose work on Shi'ism is standard in the fields of Islamic and Middle Eastern studies. In the spring of 2007, when I taught an undergraduate topics course in my department that I designed, "Shi'ism and Politics," this was one of the background articles that I assigned to students.
In this article, Prof. Kohlberg examines the transformation of "Imamiyya" or, generally-speaking, "Imam-based" Shi'ism to Twelver Shi'ism, which was marked by the beginnings of the belief in a twelfth Imam, Muhammad bin Hasan al-'Askari, reputed son of the eleventh Twelver Imam who is believed (by Twelvers) to have gone into occultation in Samarra, Iraq in the ninth century. A century later, once the last of his deputies died, Twelver Shi'is believe he went into al-ghayba al-kubra, "the greater occultation," in which he will remain until a time determined by God. Twelvers believe he will then return to usher in a period of absolute justice for the "true followers" of the family of the Prophet Muhammad, the Ahlul Bayt, as recognized by Twelver Shi'is. There are many Twelver ahadith in which various Imams say that the number of "true followers of the Ahlul Bayt" does not include all those who claim to be. In Twelver theology, the Imams possess supernatural powers and inherited special abilities to interpret the Qur'an (medieval Isma'ilis such as the Fatimids also believed their Imams had the ability to discern "inner" esoteric [Batin] meanings in the Qur'an). Twelvers believe that their Imams are second only to the Prophet Muhammad.
Whether or not the eleventh Imam, Hasan al-'Askari, had a son is debated between Sunnis and Twelver Shi'is, and it is not something accepted by all academic scholars of medieval Islamic history. Needless to say, practicing Twelver Shi'is would not agree with much of what Kohlberg argues in his scholarly, copiously-researched article. Indeed, Twelver Shi'is contend that a line of 12 Imams (religious and temporal successors to the Prophet) was ordained by God from the beginning and they point to ahadith (transmitted accounts of sayings and actions attributed to the Prophet, and in Shi'ism, also the transmitted accounts attributed to earlier Shi'i Imams) that include this number. The dating of these ahadith and their authenticity is disputed.
I was unable to upload the article to Scribd, as I normally do, so it is available for download via MediaFire, a free file-sharing web site, HERE.