Fertility is central to the study of human populations. For a project, you could consider the advantages and disadvantages of the measure of fertility that demographers use, such as crude birth rates or surrogate fertility, an indication of how many children a person may have. Alternatively, you could research the biological aspects that determine the fertility of a population, such as nutrition, public health and the economy.
Another process relevant to demography, mortality, or the death rate, affects the size of a population. Consider the various causes of death that can affect the mortality rate of a population, such as famine and infectious diseases. You could also study this in an historical context, by analyzing the impact of events, such as the Irish Potato Famine of the 1840s or the 1918 Flu Pandemic, on population. A project could also investigate differences in life expectancy by sex or race.
Migration can be internal, such as a change of permanent residence within a country, or international, where a person moves to a new country. Consider the migration differences between a developed country, such as the U.S., and a developing country, like Brazil, and the push and pull factors, including job opportunities that influence a person to migrate. You could also research the effects of migration, such as how an immigrant adjusts to a new community, and the cultural and linguistic diversity that migration brings to a town, city or country.
A sub-field of population studies, family demography is concerned with the composition and size of the household and the related processes of marriage and divorce. Study the historic changes of the family and its influence on population, for instance, the increase in pre-marital sex and social acceptance of cohabitation. You could also focus on unique family eras, such as the Great Depression where fertility and divorce rates dropped as people had less money. You could also compare and contrast the family demography of developing and developed countries.