declining with age, due to a combination of mortality and an increase in the number of births over time. There are usually more females than males in the older age ranges since women have a greater life expectancy, for a variety of reasons including women partaking less in risky behaviors. The shape of the pyramid can also reveal the age-dependency ratio of a population. Populations with a high proportion of children and/or of elderly people have a higher dependency ratio. This ratio refers to how many old and young people are dependent on the working-age groups (often defined as ages 15–64). According to Weeks' Population: an Introduction to Concepts and Issues, population pyramids can be used to predict the future, known as a population forecast. Population momentum, when a population's birth rates continue to increase even after fertility rate has declined to replacement level, can even be predicted if a population has a low mortality rate since the population will continue to grow. This then brings up the term doubling time, which is used to predict when the population will double in size. Lastly, a population pyramid can even give insight into the economic status of a country from the age stratification since the distribution of supplies is not evenly distributed through a population. In the demographic transition model, the size and shape of population pyramids vary. In stage one of the demographic transition model, the pyramids have the most defined shape. They have the ideal big base and a skinny top. In stage two, the pyramid looks similar but starts to widen in the middle age groups. In stage three, the pyramids start to round out and look similar in shape to a tombstone. In stage four, there is a decrease in the younger age groups. This causes the base of the widened pyramid to narrow. Lastly, in stage five, the pyramid starts to take on the shape of a kite as the base continues to decrease. The shape of the population is dependent upon what the economy is like in the country. More developed countries can be found in stages three, four, and five, while the least developed countries have a population represented by the pyramids in stages one and two. Types Each country will have a different population pyramid. However, population pyramids can be categorised into three types: stationary, expansive, or constrictive. These types have been identified