932-992 A. D., 310-370 A. H.
Muslims were initially a small minority in the lands they conquered. But over the course of centuries they came to be a large majority of the population in the Middle East and North Africa. Strikingly, it may be possible to quantify, at least roughly, the progress of conversion.
A major production of Islamic society, from the earliest days until recently, is the biographical dictionary. As befits a patrilineal society, many of these dictionaries provide a nasab, or genealogy, for their subjects, a list of ancestors similar to the begats in the Bible, as well as a nisba, an affiliation, most often a geographic affiliation. An individual might be listed as Muhammed son of Ahmad son of Rustam, affiliated with Nishapur (a city in Iran). Note that the first two names (Mohammed, Ahmad) are clearly Muslim, while the name of the grandfather (Rustam) is Persian. There is a good case to be made that Rustam was the first member of his family to convert to Islam, and that he initiated a sequence of Muslim names among his descendants. It’s possible to use this information, along with some reasonable demographic assumptions, to construct a graph showing the course of conversion to Islam among a large group of biographic subjects. Here’s what we get for Iran:
The points fall nicely along a logistic curve. A logistic curve is what we often see with the spread of an infection, where the y-axis shows the number of people infected. Logistic curves also commonly show up when we look at memes rather than germs, where the y-axis might show how many farmers have adopted a new strain of corn. In either case, the rate of growth of the “infection” is proportional to the product of the number already infected and the number not yet infected. So conversion to Islam in the medieval period may fit a simple model of cultural transmission. (Note however that this method does not tell us about people who never converted to Islam during this period, like the ancestors of present day Christian Copts in Egypt.)
- Conversion seems to proceed more quickly in Iran than in other areas of the Arab empire (Iraq, Syria, Egypt, Tunisia). It looks as if the Christians who were the majority of the population in the latter areas are more resistant to Islam than the Zoroastrians in Iran.
- Converts, especially in the early stages, often leave behind their native communities and settle with other Muslims. This helps drive urbanization, with new cities growing up around Muslim military cantonments. This contrasts sharply with the ruralization which follows the end of the Roman Empire in western Europe.
- The early period when converts make up less than half the population coincides with a period of anti-Islamic revolts. As the Islamic fraction grows, these revolts move from more central to more peripheral regions. They eventually cease altogether as Muslims attain a secure majority.
- In the early period, local Muslim rulers are too insecure to risk rebelling against the central authority of the Caliph. It is in the later period, with local Muslims securely in the majority, that regions assert their independence, and the ummah (community of the faithful) fragments.
- Rather than assimilate to the locally dominant version of Islam, later generations of converts often carve out cultural space for themselves by adopting variant versions. Much of the sectarian segmentation of the Islamic Middle East today, between different legal schools and sects, traces back to differential timing of conversion during the medieval period.