THE pyramid is a traditional way of visualising and explaining the age structure of a society. If you draw a chart with each age group represented by a bar, and each bar ranged one above the other—youngest at the bottom, oldest at the top, and with the sexes separated—that is the shape you get. The pyramid was characteristic of human populations since the day organised societies emerged. With lifespans short and mortality rates high, children were always the most numerous group, and old people the least. Now the shape of the global population is changing. Between 1970 and 2015 the dominating influence on the global population was the fertility rate, the number of children a woman would typically bear during her lifetime. It fell dramatically over the period, meaning that the world shifted from having larger to smaller families. The age groups start to become markedly smaller only about the age of 40, so the incline starts much further up the chart than with the pyramid. The shape looks more like the dome of the Capitol building in Washington, DC. Between 2015 and 2060 the biggest influence upon the population will be ageing. Small families are already becoming the norm, the fall in fertility is slowing down and now almost everyone is living longer than their parents—dramatically so in developing countries. So, by 2060, the dome will have come and gone and the shape of the population will look more like a column (or perhaps an old-fashioned beehive).
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