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sex-ratio Conventionally defined as the number of males per 1,000 females in the population of a society. More boys than girls are born each year, but the excess number of males at birth is gradually reduced by the higher male mortality rate, to an age-point where the number of women begins to exceed the number of men. In most countries, the overall sex-ratio is below 1,000, with women outnumbering men. However, since the Second World War the sex-ratio has been rising in many Western industrial societies, so that the excess of females is now confined to older age-groups. In Britain, for example, the cross-over point rose from age 25 in 1901–10 to age 47 in 1930–2 and to age 57 years in 1980–2. Due to improvements in health care, and the (relative) absence of wars, men now outnumber women for most of their lives in many countries. Other factors affecting the sex-ratio are sex-selective migration patterns, and the female infanticide practised in countries where females are treated as socially inferior.

The sex-ratio is regarded as an important social indicator. It affects marriage rates, women's labour-market participation rates, and (it has been argued) sex roles.