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Cousin marriages are decreasing among Iranians. Since the Pahlavi era fewer Iranians have practised cousin marriages. There is a strong preference for marrying a first cousin, but no specific preference for the father's brother's daughter. For the quarter of women married after age 21 it was found that the incidence of consanguinity declined to 28%. Additionally, the proportion of cousin marriage among urban families stayed constant: it was only rural families that drove the increase. For all periods the proportion of cousin marriage among highly educated women was somewhat lower than among uneducated women. It is hypothesized that decreases in infant mortality during the period may have created a larger pool of eligible cousins to marry.