Population geography, a subfield of geography, studies spatial variations in the distribution, composition, migration and growth of population. It focuses on where populations are found and how their size and composition are regulated by the demographic processes of fertility, mortality and migration. Disciplines other than geography that contribute to population geography include demography, sociology and economics. Its earliest form, as part of spatial science, was pioneered by scholars such as Trewartha, Zelinsky and W.A.V. Clark from the USA and J.B. Garnier and Pierre George from France. Over time it has widened its scope to cover themes such as gender, religion, age and disability, going far beyond the vital statistics of births, deaths and marriage or migration.