Sex ratio

Definition and types

The sex ratio expresses the number of males per 100 females in a population or population subgroup. There are three key variants: the sex ratio at birth (SRB) — measuring the biological baseline; the overall sex ratio — counting everyone alive; and the sex ratio by age group — which reveals how mortality differences compound over lifetimes.

The natural SRB is approximately 105–106 males per 100 females across human populations with no sex selection. This slight male excess at birth is offset by higher male mortality throughout life, so most populations approach parity (100:100) in the 20–50 age range, and shift toward female surplus at older ages. The UN WPP 2024 shows a global sex ratio of approximately 101.7 as of 2024, slightly male-skewed due to the outsized influence of China and India.

Son preference and "missing women"

In societies with strong son preference and access to sex-selective technology (ultrasound + abortion, or more recently preimplantation genetic diagnosis), SRBs can deviate sharply from the biological baseline. Nobel laureate Amartya Sen coined the term "missing women" in 1990 to describe the female deficit implied by comparing observed sex ratios to what would be expected without sex-selective practices and differential female mortality.

China's SRB peaked at approximately 121 males per 100 females in 2004 — far above the natural 105 — driven by the interaction of the one-child policy and male-heir preference. The result: an estimated 30–40 million excess males in China's current young-adult population, creating a structural marriage squeeze as men cannot find female partners. India's SRB, around 109–111 (above natural), is driven by similar dynamics in northwestern states (Haryana, Punjab, Rajasthan) while southern states (Kerala, Tamil Nadu) are near biological norms. A 2021 paper in The Lancet estimated 4.7 million female births were "missing" in China and India combined in 2017 alone.

There are signs of partial correction: China's SRB fell from 121 to around 111 between 2004 and 2023, reflecting both policy changes (three-child policy, gender-neutral propaganda) and economic modernisation. India's national SRB is also declining slowly, driven by rising female education and urbanisation.

Ageing and the female surplus at older ages

Men have higher age-specific mortality rates at nearly every age — cardiovascular disease, accidents, suicide, and cancer mortality are all higher for men globally. This biological and behavioural differential compounds over decades: by age 80, women typically outnumber men 2:1 in most high-income countries. Japan's population aged 90+ is approximately 75% female. The age structure of any ageing society therefore progressively skews female.

This has direct implications for social policy. Long-term care facilities and pension systems disproportionately serve women, who live longer but often have lower lifetime earnings and pension savings. The OECD has flagged the female pension gap as a structural risk: women receive on average 26% lower pension income than men across OECD countries, yet they survive longer and need it more.

For the wider ageing context, see the UN WPP 2024 ageing article and the median age glossary entry.

War, migration, and temporary distortions

Sex ratios can be temporarily distorted by events that affect men and women differently. Wars kill disproportionately more men (both military deaths and civilian male excess mortality), leaving post-war societies with female surpluses. After World War II, the Soviet Union had a sex ratio among 20–35 year olds of approximately 60 males per 100 females in 1950. Labour migration also distorts ratios: Gulf states like Qatar and UAE have overall sex ratios above 300 males per 100 females because of the overwhelming male-majority migrant worker population.

Frequently asked questions

What is a normal sex ratio at birth?

Biologically, the natural SRB is approximately 105–106 males per 100 females. This slight excess compensates for higher male infant and childhood mortality, so sex ratios approach 100 (parity) by reproductive age. Source: UN WPP 2024.

Why does China have a skewed sex ratio?

China's one-child policy interacted with a cultural preference for male heirs, leading to sex-selective abortions. The SRB peaked at ~121 in 2004. The result: an estimated 30–40 million "missing women" and a marriage squeeze. It has since fallen to ~111 (2023).

Why do older populations have more women than men?

Men have higher age-specific mortality rates at nearly every age. The survival advantage compounds over time: by age 80, women outnumber men roughly 2:1 in most countries. Japan's over-90 population is 75% female. See the mortality rate glossary entry.

Is the global sex ratio balanced?

Globally, approximately 101.7 males per 100 females as of 2024 — slightly male-skewed because the male birth-rate excess and the large elevated-SRB populations of China and India outweigh the female surplus in older age groups. Source: UN WPP 2024.