Among the Turkish-speaking population of Iran, Azerbaijanis are the least proficient in Persian. The Turkish composition of Tehran is estimated from about one-third to over half of the citie's population. This ratio, of around 6-7 million people, places Tehran in the first place among the densest Turkish habitation in Iran. Azerbaijanis alone comprised at least one-third of Iran's population in 1850. It is likely that this proportion has continued to increase as Iranian Azerbaijanis exhibit greater fertility in rural areas.