Natural ratio[ edit ] World map of birth sex ratios, In a study around , the natural sex ratio at birth was estimated to be close to 1. This assumption has been questioned by some scholars. This has been explained by sex differences in genetic and biological makeup, with boys being biologically weaker and more susceptible to diseases and premature death.
It has been proposed that these environmental factors also explain sex differences in mortality. For example, in the United States, as of , an adult non-elderly male is 3 to 6 times more likely to become a victim of a homicide and 2. For example, the male to female ratio falls from 1. In the aggregated results of 56 Demographic and Health Surveys  in African countries, the ratio is 1. Some scholars argue that strong socioeconomic factors such as the dowry system in India and the one child policy of China are responsible for prenatal sex-selection.
In a widely cited article,  Amartya Sen supported such views. Other researchers argue that an unbalanced sex ratio should not be automatically held as evidence of prenatal sex-selection; Michel Garenne reports that many African nations have, over decades, witnessed birth sex ratios below , that is more girls are born than boys.
For mothers having babies after the first, this ratio consistently decreased with each additional baby from 1. The age of the mother affected the ratio: Fisher's principle Fisher's principle is an explanation of why the sex ratio of most species is approximately 1: Outlined by Ronald Fisher in his book, it is an argument in terms of parental expenditure.
Essentially he argues that the 1: Natural factors[ edit ] The natural factors that affect the human sex ratio are an active area of scientific research. Over articles have been published in various journals. Two of the often cited reviews of scientific studies on human sex ratio are by W. A few of these studies extend to over years of yearly human sex ratio data for some countries.
These studies suggest that the human sex ratio, both at birth and as a population matures, can vary significantly according to a large number of factors, such as paternal age, maternal age, plural birth, birth order, gestation weeks, race, parent's health history, and parent's psychological stress.
Remarkably, the trends in human sex ratio are not consistent across countries at a given time, or over time for a given country. In economically developed countries, as well as developing countries, these scientific studies have found that the human sex ratio at birth has historically varied between 0. In a scientific paper published in ,  James states that conventional assumptions have been: James cautions that available scientific evidence stands against the above assumptions and conclusions.
He reports that there is an excess of males at birth in almost all human populations, and the natural sex ratio at birth is usually between 1. However the ratio may deviate significantly from this range for natural reasons.
A scientific paper published by Jacobsen reported the sex ratio for , children born in Denmark between — The secondary sex ratio decreased with increased number of children per plural birth and with paternal age, whereas no significant independent effect was observed for maternal age, birth order, or other natural factors.
A research paper published by Branum et al. This study also found that the sex ratios at birth in the United States, between —, were lower in both black and Hispanic ethnic groups when compared with white ethnic group.
The relationship between natural factors and human sex ratio at birth, and with aging, remains an active area of scientific research. Environmental factors[ edit ] Effects of climate change[ edit ] Various scientists have examined the question whether human birth sex ratios have historically been affected by environmental stressors such as climate change and global warming.
Cold weather stressors simultaneously extend male longevity thereby raise human sex ratio in its older age bracket. They find an increased excess of male births during periods of the exogenous stress World War II and during warm years. In the warmest period over the years, the birth sex ratio peaked at about 1. Also, higher incidence of Hepatitis B virus in populations is believed to increase the male to female sex ratio, while some unexplained environmental health hazards are thought to have the opposite effect.
For example, Oster et al. They found no effect on birth sex ratio from Hepatitis B presence in either the mothers or fathers. These chemicals are believed to have accumulated in the tissues of fish and animals that make up the bulk of these populations' diets.
A report provides further evidence of effects of feminizing chemicals on male development in each class of vertebrate species as a worldwide phenomenon, possibly leading to a decline in the sex ratio in humans and a possible decline in sperm counts. Social status of the mother, known to be a factor in influencing the sex ratio of certain animals such as swine,  but apparently not in humans.
Latitude , with countries near the equator producing more females than near the poles. They analyzed the sex ratio of births from the files of Statistics Finland and all live births in Finland from to They found an increase in the proportion of males from to ; this was followed by a decrease and interrupted by peaks in births of males during and after World War I and World War II. None of the natural factors such as paternal age, maternal age, age difference of parents, birth order could explain the time trends.
The scientists found that the peak ratio of male proportion precedes the period of industrialization or the introduction of pesticides or hormonal drugs, rendering a causal association between environmental chemicals and human sex ratio at birth unlikely. Moreover, these scientists claim that the trends they found in Finland are similar to those observed in other countries with worse pollution and much greater pesticide use. Some studies have found that certain kinds of environmental pollution , in particular dioxins leads to higher rates of female births.
High birth sex ratios, some claim, may be caused in part by social factors. Reported sex ratios at birth, outside the typical range of 1. Another hypothesis has been inspired by the recent and persistent high birth sex ratios observed in Georgia and Armenia —both predominantly Orthodox Christian societies—and Azerbaijan , a predominantly Muslim society.
Since their independence from Soviet Union, the birth sex ratio in these Caucasus countries has risen sharply to between 1. They also consider the hypothesis that sons are preferred in these countries of the Caucasus, the spread of scans and there being a practice of sex-selective abortion; however, the scientists admit that they do not have definitive proof that sex-selective abortion is actually happening or that there are no natural reasons for the persistently high birth sex ratios.
However, during this period, there were also shifts in demographics that influence the sex ratio. Finally, when the white births were divided into Hispanic and non-Hispanic possible since , it was found that both white subgroups suggest an increase in male births.
For example, Ruder has studied 1. However, they report a significant effect of paternal age. Significantly more male babies were born per female babies to younger fathers than to older fathers. These studies suggest that social factors such as early marriage and quickly fertile couples may play a role in raising birth sex ratios in certain societies.
Some researchers have in part attributed the high male to female sex ratios reported in mainland China in the last 25 years to the underreporting of the births of female children after the implementation of the one-child policy , though alternative explanations are now generally more widely accepted, including above all the use of ultrasound technology and sex-selective abortion of female fetuses and, probably to a more limited degree, neglect or in some cases infanticide of females.
In the case of China, because of deficiencies in the vital statistics registration system, studies of sex ratios at birth have relied either on special fertility surveys, whose accuracy depends on full reporting of births and survival of both male and female infants, or on the national population census from which both birth rates and death rates are calculated from the household's reporting of births and deaths in the 18 months preceding the census. He compared the sex ratio in East and West Germany for the years to , with genetically similar populations.
The population stressors theory predicts that the East German sex ratio should have been lower in when East Germany's economy collapsed than expected from its previous years. Furthermore, the theory suggests that East German birth sex ratios should generally be lower than the observed sex ratio in West Germany for the same years, over time.
According to Catalano's study, the birth sex ratio data from East Germany and West Germany over 45 years support the hypothesis. The sex ratio in East Germany was also at its lowest in According to Catalano study, assuming women in East Germany did not opt to abort male more than female, the best hypothesis is that a collapsing economy lowers the human birth sex ratio, while a booming economy raises the birth sex ratio.
Catalano notes that these trends may be related to the observed trend of elevated incidences of very low birth weight from maternal stress, during certain macroeconomic circumstances. Neither gravidity nor parity seem to affect the male-to-female ratio. For example, James suggested  that Ein-Mor results are based on some demographic variables and a small data set, a broader study of variables and larger population set suggests human sex ratio shows substantial variation for various reasons and different trend effects of length of gestation than those reported by Ein-Mor.
In another study, James has offered the hypothesis that human sex ratios, and mammalian sex ratios in general, are causally related to the hormone levels of both parents at the time of conception. Gender imbalance in Bahrain caused by policies that restrict female spouses and children of immigrant workers Gender imbalance[ edit ] Gender imbalance is a disparity between males and females in a population. As stated above, males usually exceed females at birth but subsequently experience different mortality rates due to many possible causes such as differential natural death rates, war casualties, and deliberate gender control.
According to Nicholas Kristof and Sheryl WuDunn, two Pulitzer Prize-winning reporters for the New York Times, violence against women is causing gender imbalances in many developing countries. The first is a rapid decline in fertility, either because of preference for smaller families or to comply with their nation's population control measures.
Second, there is pressure for women to give birth to sons, often because of cultural preferences for male heirs. Third, families have widespread access to technology to selectively abort female fetuses. The authors describe similar gender discrimination and gendercide in Congo, Kenya, Pakistan, Iraq, Bahrain, Thailand and many other developing countries. Some of the factors suggested as causes of the gender imbalance are warfare excess of females, notably in the wake of WWI in western Europe, and WWII , particularly in the Soviet Union ; sex-selective abortion and infanticide excess of males, notably in China as a result of the one-child policy , or in India ; and large-scale migration, such as that by male labourers unable to bring their families with them as in Qatar and other Gulf countries .
Gender imbalance may result in the threat of social unrest, especially in the case of an excess of low-status young males unable to find spouses,  and being recruited into the service of militaristic political factions. Economic factors such as male-majority industries and activities like the petrochemical , agriculture , engineering , military , and technology also have created a male gender imbalance in some areas dependent on one of these industries.
Conversely, the entertainment , banking , tourism , fashion , and service industries may have resulted in a female-majority gender imbalance in some areas dependent on them. This increase in out-of-wedlock births was attributed to a change in the marriage market caused by the decline in the sex ratio. Qatar has the highest male ratio, with 2. For the group aged below 15, Sierra Leone has the highest female ratio with 0. The value for the entire world population is 1. Countries on the Arabian peninsula tend to have a 'natural' ratio of about 1.
This effect may be caused by emigration and higher male mortality as result of higher Soviet era deaths; it may also be related to the enormous by western standards rate of alcoholism in the former Soviet states. In the evolutionary biology of sexual reproduction , the operational sex ratio OSR , is the ratio of sexually competing males that are ready to mate to sexually competing females that are ready to mate,    or alternatively the local ratio of fertilizable females to sexually active males at any given time.
On occasion, regions with a high male-low female sex ratio, like Alaska, have shown a correlation with a higher rate of reported rape. High ratios of males make it easier for women to marry, but harder for men. These men will remain single and will be unable to have families, in societies where marriage is regarded as virtually universal and social status and acceptance depend, in large part, on being married and creating a new family. Analyses of how sex ratio imbalances affect personal consumption and intra-household distribution were pioneered by Gary Becker , Shoshana Grossbard-Shechtman ,   and Marcia Guttentag and Paul Secord.
It has been shown that variation in sex ratio over time is inversely related to married women's labor supply in the U. When there is a shortage of women in the marriage market, the women can "marry up", inevitably leaving the least desirable men with no marriage prospects. In many communities today, there are growing numbers of young men who come from lower classes who are marginalized because of lack of family prospects and the fact that they have little outlet for sexual energy.
There is evidence that this situation will lead to increased levels of antisocial behavior and violence and will ultimately present a threat to the stability and security of society. Human sex at birth was also analyzed and used as an example by Jacob Bernoulli Ars Conjectandi , where an unequal sex ratio is a natural example of a Bernoulli trial with uneven odds.
Willem 's Gravesande also studied it.