Factors influencing the shape of a population pyramid

• A decline in the birth rate of a population leads to a pyramid with a narrower base
• A sharp decline in the number of births during crisis times, such as during wars or economic depression, results in a pyramid with a narrower middle section a few decades later
• During wars, with great loss of life among young men, there will be a clear indentation on the male side of the pyramid
• A pyramid with concave sides reflects a high death rate, while one with more convex sides reflects a low death rate
• In a pyramid after the Second World War, the ages groups of 35 and 44, the baby boom, occurred in developed countries. As the baby boomers grow older, the widened section moves upward in the pyramid and the demand for medical and geriatric care increases
• At present, the most dramatic changes occur in sub-Saharan Africa, where HIV/Aids-related deaths have a devastating effect on the numbers of sexually active men and women
• Changes in populations occur when large numbers of young immigrants are rapidly absorbed into a population or when large-scale emigration of young adults takes places.