Sex Ratio

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In most human populations, male and female subpopulations are territorially integrated, but the sex ratio–the ratio of males to females–varies from place to place, especially among small localized populations. When reference is made to the short-lived ratios of men and women in workplaces, institutions, and organizations, the term gender balance is more appropriate and is increasingly preferred.

Measures, Accuracy, and Sub-Population Ratios

The sex ratio is usually expressed as a masculinity ratio–the number of males per 100 females. It may also be given as a masculinity proportion (i.e., percentage of males) or as a percentage excess or deficit of males. Sometimes the ratio is given in transposed form, as the number of females per 100 males (e.g., in India), and sometimes per 1,000 rather than per 100. The United Nations has unsuccessfully attempted to standardize the usage.

Published international data on sex ratios are neither very common nor very accurate, as census enumerations of the two sexes can vary in their reliability. One sex is often less completely enumerated than the other, particularly males in the West (especially among illegal immigrants and those opposed to authority) and females in numerous less developed countries (e.g., Afghanistan, Bangladesh, India, and Pakistan, where son and male preference predominates and females are habitually disadvantaged in numerous ways).

Sex ratios are calculated for many sub-populations (e.g., age, ethnic, and educational groups) as well as for different events, such as conceptions (known as primary sex ratios), births (secondary sex ratios) and deaths, and migration. Population sex ratios, sometimes termed tertiary sex ratios, are determined by sex-differentials in fertility, manifested in the preponderance of male births; sex-differentials in mortality, especially the normally greater longevity of females; and sex-differentials in mobility. The numerical significance of these three factors upon sex ratios of populations varies over time and space.

Sex Ratios at Birth

Sex ratios at birth have been, historically, the least important numerical influence on sex ratios of large populations in the past, being remarkably consistent within the range of 104–108 male births per 100 females. This biological disparity seems to be related particularly to hormonal levels at conception. The male surplus at conception is believed to be high but it is reduced by excess male mortality before birth, especially when health conditions are poor; sex ratios at birth rise somewhat when health conditions improve. First-order births tend to have slightly higher sex ratios than later-order ones, as seen in the sex balance of births in post-war baby booms. However, in many Asian cultures, the persistence of son preference in combination with access to techniques of fetal sex determination has resulted in a surge of female abortions in the late-twentieth century, substantially raising sex ratios at birth (recorded sex-ratios are also raised by under-enumeration). In a number of Asian countries, these ratios have risen to levels that are highly anomalous: 110 in South Korea, 111 in India, and 117 in China. In these countries, levels for later-order births are much higher. Beyond the practice of sex-selective abortion, the possible spread into large Asian and Muslim populations of the latest techniques of pre-implantation sex selection, sometimes known as gender choice, may further distort sex ratios at birth with major social and psychological implications. A growing excess of males is already affecting the marriage market in parts of China in the early twenty-first century.

Sex Ratios at Death

As all humans are mortal, the preponderance of male births implies higher numbers of deaths among males than among females. In age-specific terms, the differences between the sexes are, however, also pronounced: typically, male death rates are higher at every age. Thus the male numerical advantage at birth is eroded as a cohort ages, as can be observed in age-specific population sex ratios. When mortality is high, this effect is strong; hence females achieve a numerical equality with males at a relatively low age, such as around age 20. With low mortality, despite the female advantage in death rates, most males and females survive to middle age. Thus it takes longer in a cohort before the number of females equals the number of males, and, in a population, numerical equality of females with males occurs at a relatively high age. For example, in the United Kingdom in 2000, the number of males exceeded the number of females at every age up to roughly age 50. In the age group from 45 to 49, the sex ratio was 100.2; in the age group 50 to 54, it was 99.1. Since the sex ratio at birth is about 105–not greatly different from equality–the figures just cited indicate that in low mortality populations male and female numbers are broadly balanced in a large lower segment of the age pyramid. With mortality further lowered, the upper limit of that age segment is expected to rise further. Projections suggest, for example, that in the United Kingdom by the middle of the twenty-first century, the population sex ratio even at the age range from 60 to 64 will still exceed 100, reflecting the enduring influence of the sex ratio at birth on the sex ratio of the population up to the threshold of old age. But sex-differential mortality has a major effect on population sex ratios once survival rates start falling rapidly, as they do beyond age 60. Survival rates fall for both sexes but do so more steeply for males. In the United Kingdom, for example, expectation of life at birth in 2000 was approximately 80 years for females and 75 years for males. But the population sex ratios among the elderly (reflecting in part past differences in survivorship) show large male deficits: the sex ratio is 70.1 at ages 75 to 79 and 45.2 among those 80 years and older.

Several exceptions to this broad depiction of the pattern of population sex ratios and the mortality factors influencing them should be noted. One was signaled above, with reference to the anomalous sex ratios at birth owing to higher mortality among female fetuses found in some countries in Asia. Should such elevated sex ratios at birth persist, they would have a major effect on the balance of the sexes over time in a broadening segment of the age pyramid, spreading from lower to higher ages.

A second qualification has to do with anomalous female mortality relative to male mortality. Cultural factors operating to the disadvantage of females in a number of countries, notably in Pakistan, India, and Bangladesh, have tended to counteract the biological female advantage in survivorship, causing either excess female over male mortality or greatly reducing the natural female mortality advantage over broad age groups, especially among children and in younger adult ages. In such circumstances, population sex ratios can exceed the sex ratio at birth well up to the upper ranges of the age pyramid. But even in such populations, among the oldest of the old the number of females typically exceeds the number of males. As in recent decades, female mortality improvement has been faster than male mortality improvement even in these countries. This anomalous pattern of population sex ratios is expected to be gradually attenuating in future years.

Thirdly, in some populations, especially in Eastern Europe, while overall mortality is relatively moderate, the female mortality advantage over males is found to be exceptionally high. For example, in the Russian Federation in 2000, the expectation of life at birth for the entire population was 66 years but the gap between female and male life expectancy was 12 years: 60 years for males and 72 years for females. Such wide disparity in mortality strongly affects population sex ratios. In most East European countries, the sex ratios for those aged 60 and over were below 60, and overall population sex ratios were below 90. In Russia, for example, in 2000, the corresponding ratios were 53 and 88. However, exceptional war-time mortality, even after 55 years of peace, has contributed to this sharp imbalance.

Sex Ratios of Migrants

Sex ratios of migrants tend to vary much more than ratios at either birth or death. Some migrations have been male-dominated, as for example in the early colonization streams from Europe to the New World and in the major gold rushes; some have been female-dominated, including the widespread migrations of domestic servants to Latin American cities and much rural-urban migration in the West; and other migrations have been more or less balanced, especially when they have been forced by political or environmental conditions. Transient and temporary circulatory movements tend to be even more gender-selective. Gender selectivity tends to vary with the evolution of migration streams, men being more preponderant at earlier stages, with the proportion of women increasing over time. In the last quarter of the twentieth century, sex ratios of migrants changed rapidly in many countries, as women's greater autonomy and opportunities were reflected in their increased mobility. However, migration has less consistent effects on the overall sex ratios of populations than either births or deaths, as it tends to be spasmodic, linear, and localized in impact. In addition, its effects on the sex ratios of populations tend to decrease with increasing size of a real unit–at its limits, the sex ratio of the total world population is unaffected by migration.

Patterns of Sex Ratios

Toward the end of the twentieth century, the estimated sex ratio of the total world population was 102, having risen from 100 in 1950, reflecting improvements in mortality. The overall sex ratio in more developed countries was 94, well below that of the less developed countries, which was 103–the difference being mainly a result of the older age distribution in the former region. Sub-continental variations in sex ratio are greater than global variations over time, and range from 90 in Eastern Europe to 106 in South Central Asia, largely for the cultural and health reasons already cited.

Variations are much greater at the country level, mostly because migration plays a larger role in the population change of smaller countries. While the majority of countries have sex ratios of 96 to 103, a number of East European countries, including Russia, have ratios below 90 and several very populous Asian countries (e.g., Bangladesh, China, India, and Pakistan) have ratios of 105 or more. The main causes of low sex ratios of countries are recent conflicts and wars and high levels of male mortality (e.g., Belarus, Russia, Ukraine), as well as the emigration of men (e.g., Barbados, Lesotho, and Portugal). The main causes of high sex ratios are the immigration of men (e.g., oil-rich states of the Gulf, Libya, Brunei), the emigration of women (e.g., Ireland in the past, the Philippines, and many Pacific islands), and unusually high female mortality (e.g., Bangladesh, India, Nepal, and Pakistan).

Large countries like India, China, and the United States exhibit marked regional variations in sex ratios reflecting cultural and social differences. Owing largely to the influence of migration, there are also considerable rural-urban and local variations in sex ratios whose range generally increases inversely with population size. Thus, small mining towns have high sex ratios while retirement towns have low ones. Since populations are constantly changing and redistributing themselves, the patterns of sex ratios within countries are never stable.

See also: Gender Preference for Children; Induced Abortion; One-Child Policy; Partner Choice; Sex Selection; Spontaneous Abortion; Women's Status and Demographic Behavior.


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John I. Clarke