.. In Demography, an age pyramid is a graphic display which depicts a population with the lowest age category at the bottom, females on the left, males on the right (some demographers place them on the opposite sides), and each successively higher age category above those. . Different countries may use different age ranges for each category .. It is called a pyramid because, for most populations, there are more in each category near the bottom, and fewer in each category above the previous category. . Slightly more males are born than females, so there is a slight skew, with the bottom right side slightly bigger than the bottom left.  Infant mortality tends to be slightly higher for females, so the bottom two or three categories on the left might be slightly lower.  Males around late teens and early twenties tend to take more risks, become recruited to military duty, and drive recklessly, so there is a small indentation at those ages. . Women tend to live longer than men, so the upper level categories tend to be larger on the left side . In low income or least developed countries, birth rates tend to be high, and life expectancy tends to be low, so the pyramids tend to be short and wide. . In the Nordic countries, the birth rates tend to be low, and life expectancy high, so age pyramids tend to be tall and thin.  Countries like USA, Canada and the rest of Western Europe are approaching those of the Nordic countries, so the pyramids tend to be moving towards the Nordic pattern. . The dependency ratio is a fraction with the number above the line including persons of ages, young and old, who are likely to not be working, with the bottom number being the total population.  Some wealthy countries tend to have a high dependency ratio because the number of older people, compared to the total population, tends to be high.  In least developed countries, the ratio is high for very different reasons: the number of children, relative to the population as a whole tends to be high.  AIDS is having a noticable effect in some countries, taking a bite out of the sexually active ages. . For many established rural communities, people of the middle or working ages may leave to seek jobs elsewhere, and this takes a bite out of the pyramid in the middle categories.  This may be less on the left side than the right side if more men migrate than women.  If they leave their children to be raised by the grandparents, then the bottom categories will be wider, and this contributes to the dependency ratio of the sending community. . nb: Some day, when I have enough time, I will draw some illustrations for this page. PB .

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