Population size is affected by a number of factors. The size of the population obviously can increase when the birth rate rises, unless the death rate also increases accordingly. Likewise, the size of the population can decrease when the death rate rises, unless the birth rate also increases commensurate with the death rate. A rise in fertility can precipitate an accompanying rise in the birth rate, and a decrease in fertility can lower the birth rate; therefore, conditions that mitigate against fertility, such as advanced age, are a factor in the population size. Migration rates can affect population size in the same manner as birth and death rates can; people migrating to an area can increase the population just as new births do, and people migrating away from an area can decrease it just as deaths do. These factors can be made to balance each other to some extent; for example, replacement migration has been suggested as a remedy for declining population due to an aging population with lower birth rates (ôReplacement Migration: Is It A Solution To Declining And Ageing Populationö).
There are a number of factors that can affect these rates. War, for example, can reduce the birth rate because it removes men from the family home. Pestilence can increase the death rate and lower the birth rate. Pollution and radiation can increase the death rate by causing disease. Immigration can raise or lower any of the other factors depending on what age and gender of people leave the country (Bouvier). Homosexuality can lower the birth rate, since same-sex partners cannot reproduce. The age range for procreating individuals can affect the birth rate, as well; when people are having sex at an earlier age, more babies are born, and when a large segment of the population waits to have children, the population declines. Habits, such as smoking, overeating, and drug abuse can lower the population by precipitating early death (
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