note of 11/16/2018

Population Ratio
Defined as the number of the males per 100 females in the population.
Developed countries have more females than males
the Demographic Transition
Why natural increase rises or falls
Historically no country has ever reverted bact to a previous stage
stage 1 : low Growth rate
Marked by very high birth and death rates
no long term natural increase
no country presently is in stage 1
stage 2
Marked by rapidly declining death rates and very high birth rate
Europe and North American entered stage 2, as a result of the ondustrial revolution(1750)
Africa asia and Latin America entered stage 2 around 1950,as a result of medical revolution-improve medical care
Stage 3: Moderate growth
Marked by rapid decline in birth rate and steady decline in death rate.
Natural increase is moderate
Gap between CBR and CDR is narrower in stage 3 countries than stage 2 countries.
Population grows, because CBR is still greater than CDR
Most Europe countries and North America countries transition to  stage 3 during 20 century.
stage 4:Low Growth
Marked by very low brith rate and death rate
no long term natural increase and possibly a decrease
Country reaches stage 4 when population gains by CBR are diminished by losses because of CDR
Coudition known as zero population Growth.