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The document discusses population pyramids and their shapes. It explains that a population pyramid studies the age and sex composition of a population and resembles a pyramid shape. The bars show different age groups with males on the left and females on the right. Expanding populations have a more triangular shape with high birth and death rates, while constant populations are bell shaped with equal birth and death rates. Declining populations have a narrow base and tapered top, indicating low birth and death rates common in developed countries. Examples of different pyramid shapes for Nigeria, Australia, and Japan are provided.