one child policy
created in 1997
under the age of 20 there are 30 million more boys than girls

key words
demographic transition model= a mesure of population change of birth rate and death rate, natural birth rate = the increase of birth to death rate.

Population growth for a particular country is determined by four factors, births (B), deaths (D), immigration rate (I), and emigration rate (E):
growth rate of population = B-D+(I-E)