1) Demographist 2) Population scientist 3) Sociologist 1) Demographist 
[n] - a scientist who studies the growth and density of populations and their vital statistics
demographer 1. Someone who studies human populations, including their size, growth, density, and distribution, and statistics regarding birth, marriage, disease, and death. 2. One who specializes in human statistical data.
demographist noun a scientist who studies the growth and density of populations and their vital statistics
a scientist who studies the growth and density of populations and their vital statistics