In anthropology and demographythe human sex ratio is the ratio of males to females Men above women a population. More data is available for humans than for any other species, and the human Men above women ratio is more studied than that of any other species, but interpreting these statistics can be difficult. Like most sexual species, the sex ratio in Men above women is approximately 1: Due to higher female fetal mortality,  the sex ratio at birth worldwide is commonly Men above women to be boys to girls although this value is subject to debate in the scientific community.
The sex ratio for the entire world population is males to females est. Gender imbalance may arise as a consequence of various factors including natural factors, exposure to pesticides and environmental contaminants,   war casualties, sex-selective abortionsinfanticides, aging, and deliberate gendercide. Human sex ratios, either at birth or in the population as a whole, are reported in any of four ways: If there aremales andfemales the ratio of males to females is 1.
Scientific literature often uses the proportion of males. This article uses the ratio of males to females, unless specified otherwise. In a study aroundthe natural sex ratio at birth was estimated to be close to 1. This assumption has been questioned by some scholars.
Infant mortality is higher in boys than girls in most parts of the world. 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 ofan adult non-elderly male is 3 to 6 times more likely to become a victim of a homicide and 2.
For example, Men above women male to female ratio falls from 1. In the United States, the sex ratios at birth over the period — were 1. In the aggregated results of 56 Demographic and Health Surveys  in African countries, the ratio is 1.
There is controversy whether sex ratios outside Men above women range are due Men above women sex-selection, as suggested by some scholars, or due to natural causes. 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 belowthat is more girls are born than boys.
In an extensive study, carried out aroundof sex ratio at birth in the United States from over 62 years, statistical evidence suggested the following: 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: 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 Men above women. 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 inMen above women 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 forchildren 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 Men above women 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. Various Men above women have Men above women 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. Causes of stress during gestation, such as maternal malnutrition  generally appear to increase fetal deaths particularly among males,  resulting in a lower boy to girl ratio at birth.
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.
The effects of gestational environment on human sex ratio are complicated and unclear, with numerous conflicting reports. For example, Oster et al. They found no effect on birth sex ratio from Hepatitis B presence in either the mothers or fathers.
A survey by the Arctic Monitoring and Assessment Program noted abnormally low sex ratios in Russian Arctic villages and Inuit villages in Greenland and Canadaand attributed this imbalance to high levels of endocrine disruptors in the blood of inhabitants, including PCBs and DDT. These chemicals are believed to have accumulated in the tissues of Men above women and animals that make up the bulk of these populations' diets.
A report provides further Men above women 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. Other scientific studies suggest that environmental effects on human sex ratio at birth are either limited or not properly understood.
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They analyzed the sex ratio of births Men above women the files of Statistics Men above women 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 Men above women 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 pollutionin particular dioxins leads to higher rates of female births. Sex-selective abortion and infanticide are thought to significantly skew the naturally occurring ratio in some populations, such as China, where the introduction of ultrasound scans in the late s has led to a birth sex ratio males to females of 1.
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 Azerbaijana 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 Men above women 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.
As an example of how the social composition of a human population may produce unusual changes in sex ratios, in a study in several counties of California where declining sex ratios had been observed, Smith and Von Behren observe "In the raw data, the male birth proportion is indeed declining. 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 sinceit was found that both white subgroups suggest an increase in male births. Several studies have examined human birth sex ratio data to determine if there is a natural relationship between the age of mother or father to the birth sex ratio. For example, Ruder has studied 1. However, they report a significant effect of paternal age. Men above women
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. Reported sex ratios at birth for some human populations may be influenced not only by cultural preferences and social practices that favor the birth or survival of one sex over the other but also by incomplete or inaccurate reporting or recording of the births or the survival Men above women infants.
When unusual sex ratios at birth or any other age are observed, it is important to consider misreporting, misrecording, or under registration of births or deaths as possible reasons. 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 policythough alternative explanations are now generally more widely accepted, including above all the Men above women of ultrasound technology and sex-selective abortion of female fetuses and, probably to a more limited degree, neglect or in some cases infanticide Men above women females.
In the case of China, because of deficiencies in the vital statistics registration system, studies of sex ratios at birth have Men above women 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. Catalano has examined the hypothesis that population stress induced by a declining economy reduces the human sex ratio.
He compared the sex ratio in East and West Germany for the years to Men above women, 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.
Men above women 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. A research group led by Ein-Mor reported that sex ratio does not seem to change significantly with either maternal or paternal age.
Neither gravidity nor parity seem to affect the male-to-female ratio. For example, James suggested  that Ein-Mor results are based on some Men above women 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 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 TimesMen above women against women is causing gender imbalances in many developing countries.
Commonly, countries with gender imbalances have three characteristics in common. 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 Men above women 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.