Age structure diagrams show how a population is distributed. It divides the population into pre-reproductive, reproductive and post-reproductive phases.  The shape of the diagram can show you if a country is growing rapidly, slowly, or negatively. It can also show is there is zero growth. The wider the base of the diagram the more individuals below the age of fifteen. The more individuals under fifteen the more individuals poised to reproduce increases. The left hand side is always the males.