Anil has noticed that infertility has been affecting many of his intergenerational friends and family. Based on this observation, Anil is writing his senior thesis on the effects that certain cities, water quality, and hormones might have on his friends. Anil will most likely approach his findings and paper using which theory?

When the population of people remain constant for a period of time with no significant change to their numbers that is referred to as zero population growth.

This doesn't mean that no one has died for that period but it actual means that there was not any net change .

No net change refers to the fact that for that period the number of children given birth to are equal to the number of the people who have died.

When the rate of death and birth has reached equilibrium that when zero population growth occurs .

This also is true if we assume that no people entered the country or left the country in that particular population.

The fact is that this is not practical hence for zero population growth to occur these two factors also have to be equal so the people living the country must be equal to the people entering the country.

Immigration =migration

Birth = death then we will have zero population growth .

People having less kids than usual (maybe due to infertility ) may also lead to steady population growth .