This demographic balance could occur when the birth rate and death rate are equal. But be careful, because this is not always the case. ZPG also accounts for immigration (when a person migrates to a country) and emigration (when a person leaves their country to settle somewhere new). Therefore, a country that has reached zero population growth has a population where births plus immigration is equal to deaths plus emigration over the course of a year. When considering population growth in the entire world, of course, immigration and emigration do not apply and ZPG would be attained simply when the global birth and death rates are equal.