Classical demography refers to the study of human demography in the Classical period. It often focuses on the absolute number of people who were alive in civilizations around the Mediterranean Sea between the Bronze Age and the fall of the Western Roman Empire, but in recent decades historians have been more interested in trying to analyse demographic processes such as the birth and death rates or the sex ratio of ancient populations. The period was characterized by an explosion in population with the rise of the Greek and Roman civilizations followed by a steep decline caused by economic and social disruption, migrations, and a return to primarily subsistence agriculture. Demographic questions play an important role in determining the size and structure of the economy of Ancient Greece and the Roman economy.