Maya Shatzmiller states that the demographic behavior of medieval Islamic society varied in some significant aspects from other agricultural societies. Nomadic groups within places like the deserts of Egypt and Morocco maintained high birth rates compared to rural and urban populations, though periods of extremely high nomadic birth rates seem to have occurred in occasional "surges" rather than on a continuous basis. Individuals living in large cities had much lower birth rates, possibly due to the use of birth control methods and political or economic instability. This led to population declines in some regions. While several studies have shown that Islamic scholars enjoyed a life expectancy of 59–75 years between the eleventh and thirteenth centuries, the overall life expectancy of men in the same societies was lower. Factoring in infant mortality, Lawrence Conrad estimates the average lifespan in the early Islamic caliphate to be above 35 years for the general population, compared to around 40 years for the population of Classical Greece and 31 years for the population of thirteenth century England.