Ideal age-sex pyramid are usually symmetrical but prevailing pyramids are symmetrical or uneven balanced in their age sex distributions. Asymmetry age-pyramids may be caused by demographic factors like; fertility, mortality, and migration non demographic factors like; war, marital behaviors, disasters, famine, population policies, epidemic medical facilities etc.
FERTILITY AND MORTALITY
Any changes in fertility and mortality affect population growth and thus causes changes in pyramid. Low birth rate and slow death rate lead to slow population growth and in the long run, to a high old age population and decreasing proportion of children thus illustrate the regressive pyramid with a widen apex and a narrow base.On the other hand, birth rate are high in developing countries same as death rate too showing the progressive pyramid with a broad base and a narrow apex.This type of pyramid of pyramid is represented by the Demographic Transition Model (D.T.M). This factor has a short term modifying effect on the shape of the population.