Sex Ratios in Population Pyramids
Understand why male and female bars differ by age, how sex ratios are calculated, and what births, mortality, and migration can reveal.
The paired sides of a population pyramid make sex differences visible at every age. Males conventionally appear on the left and females on the right. Equal-length bars indicate similar numbers or shares; asymmetry points to differences worth investigating.
How the sex ratio is expressed
A common definition is:
Sex ratio = male population ÷ female population × 100
A ratio of 102 means 102 males for every 100 females. A ratio of 85 means 85 males for every 100 females.
Always check whether a source reports males per 100 females or females per 100 males. The label matters more than the word “ratio.” This site uses males per 100 females throughout, including in the sex ratio ranking.
Why more boys are usually born
The sex ratio at birth is typically above 100 males per 100 females. That gives the youngest male bar a modest initial advantage in many pyramids. The exact ratio varies and can be affected by biological, statistical, and social factors.
A large departure from expected birth patterns requires careful source and context checking. A pyramid alone cannot establish a cause.
China in 2000 is a well-documented case. The UN estimates show about 116 boys per 100 girls at ages 0–4, far above the usual range. Demographic research has linked this to son preference combined with prenatal sex selection; the chart shows the imbalance but not those causes.
Why the balance changes with age
Male mortality is higher than female mortality in many populations and across many ages. As cohorts grow older, the initial male surplus often narrows, crosses parity, and becomes a female surplus. At the oldest ages, the female bar can be much wider.
Russia shows an unusually large gap. In the 2026 projection, there are about 93 men per 100 women at ages 20–64, but only about 52 per 100 at ages 65 and older. High adult male mortality in recent decades is widely documented.
This pattern depends on mortality history. Conflict, occupational risk, epidemics, smoking, healthcare access, and other factors can affect male and female survival differently.
Migration can dominate working ages
Migration is often selective by age and sex. Labour migration dominated by men can produce unusually wide male bars among young and middle-aged adults. Other migration streams may be female-dominated or involve families with a more balanced distribution.
The signature can be striking in small destination countries, where migrants make up a large share of residents. It can also leave corresponding gaps in origin populations.
Qatar is the clearest example in the dataset. In the 2026 projection, there are about 313 males per 100 females at ages 20–64, and about 244 per 100 for the population as a whole. The children’s bars, by contrast, are almost balanced.
Cohort marks and historical events
A sex imbalance in one age group may travel upward as the cohort ages. Comparing the same population across years helps distinguish a persistent cohort effect from a temporary flow or an estimation revision.
Look for:
- the ages where the two sides begin to diverge;
- whether the difference appears in one cohort or across many;
- whether it moves upward over time;
- whether migration and mortality sources support an explanation.
Russia’s pyramid contains a well-known example. People born during the Second World War, in the early and mid-1940s, form a small cohort. In Russia’s 2000 pyramid it appears as a notch at ages 55–59; by 2026 the same cohort has moved to ages 80–84, which is again narrower than its neighbors in the chart above.
Counts and percentages
Within a country profile, our bar tooltips report each male or female age group’s share of the total population. The table supplies exact displayed counts. The headline sex ratio uses the total male and female population for the selected year.
Comparing shapes across countries is easiest with percentages. Assessing the number of people affected requires counts.
Avoid overinterpreting two categories
International demographic series commonly publish age structure in male and female categories because that is how the source data and models are organized. This does not represent the full diversity of sex characteristics or gender identities. It is a limitation of the available comparable dataset and the conventional chart.
Apply these checks in the country comparison tool or follow the complete population-pyramid reading guide.
Sources
Frequently asked questions
What is a population sex ratio?
It is commonly expressed as the number of males per 100 females. The ratio can be calculated for the total population or a specific age group.
Why are there more women at older ages?
Women generally have lower mortality and longer life expectancy, so female numbers often exceed male numbers in older cohorts.
Can migration change a population pyramid’s sex balance?
Yes. Labour or refugee migration concentrated by sex and age can create large differences in adult cohorts.