Sex Ratio and Age Structure Across Regions

A population pyramid shows age groups stacked up, split into male and female halves. In a balanced population, each bar is roughly even on both sides. Sex ratio puts a number on that balance: divide the number of men by the number of women, then multiply by 100, giving men per 100 women. A ratio of 100 means equal numbers. Above 100 means more men, below 100 means more women.

The ratio rarely skews evenly across all ages at once. It skews inside specific age bands, and the band tells you the cause. Labor migration pulls in young working-age men for jobs in mining, construction, or oil, so the 20-to-40 bar bulges male while children and the elderly stay balanced. War kills young men disproportionately, hollowing out that same age band from the male side instead. Policies like China's one-child policy combined with a cultural preference for sons skewed the ratio at birth, so the skew shows up at the bottom of the pyramid and rises with that cohort as it ages. Reading where the skew sits, not just that it exists, is the skill.

This connects directly to the demographic transition and to migration flows covered alongside it. A country early in the transition with high birth rates shows a wide young base; a sex ratio skew layered on top of that base, from gendered migration or policy, changes the shape further. Pyramids are the visual record, and sex ratio is the arithmetic check that explains why one side of the pyramid bulges.