Population growth is driven by two factors: net migration (people moving in minus people moving out) and natural increases (births minus deaths). Since 1960, most growth in Thurston County has been due to migration, and natural increase has remained relatively stable.
Note that in the above chart, from 1960-2016 there has been continuous Natural Increase, and only one year where out-migration was greater than in-migration (1969).
Population increase due to natural increase is fairly steady, but there are large year-to-year differences in net Migration. Net migration is strongly influenced by economic conditions. When unemployment is high, net migration tends to be low. People also move here for jobs in nearby counties. Around 29% of the military and civilian work force on Joint Base Lewis-McChord, just to the north in Pierce County, lives in Thurston County. The number of Thurston County residents commuting to jobs in Pierce and King counties continues to grow. As housing prices rise in King County, this trend will likely continue.
The natural increase in population is the number of births minus the number of deaths in a calendar year.
Net migration is equal to the number of people moving to Thurston County minus the number of people moving out of Thurston County.