The variation of populations over time, also known as their population dynamics, depends on biological and environmental processes that determine population changes. A population's growth rate is strongly influenced by the proportions of individuals of particular ages. With knowledge of this age structure, population growth can be more-accurately predicted. Age structure data allow the rate of growth (or decline) to be associated with a population's level of economic development. For example, the population of a country with rapid growth has a triangle-shaped age structure with a greater proportion of younger individuals who are at or close to reproductive age . This pattern typically occurs where fewer people live to old age because of sub-optimal living standards, such as occurs in underdeveloped countries.