In 1800, the population of the modern-day territory of Iran was approximately 6.3 million. This figure would see modest growth throughout the 19th century, as several wars and a mass famine in 1870-1871 (modern estimates put its death toll at around 1.5 million people) were largely balanced out by a surge in migration to Iran; this migration came as the Russian Empire expanded into the Caucuses, and caused a wave of refugees to flee southwards to avoid forced expulsion and ethnic cleansing in the North Caucasus region, particularly from 1864 onwards. As a result, the population of Iran reached ten million by the turn of the 20th century.

Twentieth century growth Iran’s population would begin to grow rapidly in the 20th century, as the discovery of oil in the country in 1908 led to an economic boom, and the socio-economic reforms implemented under Reza Shah would see a number of medical and healthcare advancements across the country. Although unpopular with religious fundamentalists, Reza Shah's reforms had long-term influence on the demographic development of Iran, even after his abdication in 1941. Following the Second World War, Iran became increasingly westernized and developed relatively strong relations with the U.S.; however, western influence, economic imbalances and the oppression of the Mohammed Reza Shah's regime became the driving forces behind the Iranian Revolution, which was one of the most significant moments in the history of the region.

Growth after the Revolution The 1979 Iranian Revolution saw the removal of the Shah and an end to Iran's so called westernization; the monarchy was replaced by an Islamic, theocratic regime led by the Supreme Leader, Ayatollah Ruhollah Khomeini. During Khomeini's decade in charge he oversaw Iran's transition into an Islamic Republic, which implemented radical political and cultural changes in the country, and this coincided with an increased population growth rate in the 1980s. This growth was promoted by the Iranian government, who encouraged a baby boom during the Iran–Iraq War between 1980 and 1988, as part of an effort to increase future Iranian military manpower. As a result of this strategy, the population of Iran would grow from approximately 38.6 million in 1980 to over 56 million just a decade later. Following the implementation of a UN-brokered ceasefire in 1988, population growth in Iran would slow, as economic sanctions and government implementation of family planning policies would lead to a drop in fertility. Population growth has continued steadily into the 21st century, however, and in 2020, Iran is estimated to have a population of 84 million.

In 1800, the region of present-day Iraq had an estimated population of just over one million people. The population of Iraq would grow slowly throughout the 19th century, reaching just over two million by the beginning of the 20th century. However, Iraq’s population would begin to rise rapidly in the 1920s, as modernization programs implemented by the British administration, aided by the discovery of oil in 1927, would see mortality rates fall and living standards rise for much of the country. As a result, Iraq would grow to have a population of just under seven million by the time of the Iraqi Republic's establishment in 1958. Population growth would continue to increase following the creation of the republic, doubling to fourteen million by the 1980s; according to these estimates, the Iran-Iraq War of the 1980s did little to change the trajectory of Iraq's population growth, nor did the Gulf War of 1990-1991, despite Iraq suffering up to half a million fatalities in these conflicts. Iraq's population growth did slow in the early 2000s however, with the U.S.-led invasion of Iraq in 2003. While the initial invasion would see relatively few casualties (compared to previous conflicts), the resulting economic turmoil and political instability, combined with the rise of the Iraqi insurgency and civil war in the region, would cause population growth to slow for several years. Population growth would recover starting in the 2010s, and by 2020, Iraq is estimated to have a population of just over forty million.

UK residents by broad country of birth and citizenship groups, broken down by UK country, local authority, unitary authority, metropolitan and London boroughs, and counties. Estimates from the Annual Population Survey.

Pairwise FST values between breed groups.