According to the census taken in 1956, the total population of Iran was 18,944,821; it has increased since then at the rate of 2.4 to 2.5 per cent a year. The centre of the country is largely empty and life has been driven either towards the exterior or towards the interior of the mountains, to the points where there is an adequate water-supply. There are extraordinary variations in density: in the neighbourhood of Tehran, 44 inhabitants per sq km; on the shores of the Caspian, from 20 to 30. According to the 1956 census, the urban population constitutes 30.1 per cent of the total population, and the rural population (including seminomads) 69.9 per cent. In Iran, the marriage rate is generally higher among women than among men. Consanguinity in marriage is on the decline, as demonstrated in part by the growing number of marriages between Iranians and foreigners.