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We explore the possible reasons for this below. From comparison of mortality rates in infant boys and girls, it becomes clear that boys are at higher risk of complications in the first few days of life: preterm births, asphyxia, birth defects and heart anomalies. But why? First of all, boys are more likely to be born prematurely : the share of boys born before full-term pregnancy is higher than for girls. Boys tend to have a higher birthweight than girls — which can increase the risk of waiting to term to deliver — meaning that more boys are induced before the end of the pregnancy term.
The fact that preterm births are more common for boys contributes to this. Although boys are, on average, heavier than girls at birth, they are less physiologically mature at birth. This means they are at higher risk of having delayed physiological function such as lung function and adverse neurological outcomes. The reason for this difference has been an important question for decades — the answer is still not clear. But there are some leading hypotheses: surfactant production for lung function has been observed earlier in female fetuses, leading to improved airway flow in the lungs; estrogen has been shown to affect lung development positively in females; males, on average, have a higher birthweight meaning they may trade-off increased size for functional development; and the uterus may be less hospitable to male fetuses — the introduction of a Y chromosome in females can create and immunoreactive response to the central nervous system.
Boys are also at higher risk of infectious diseases such as syphilis, malaria , respiratory infections, tetanus and diarrheal diseases. This is more generally true for a broad range of infections, spanning person-to-person, vector-borne, blood-borne, and food and water borne diseases.
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But why are boys more susceptible to infection? Overall, boys have weaker immune systems. There are two key hypotheses for why. The Y-chromosome in boys increases their vulnerability. Biologically, males and females are differentiated by chromosomes: females have two X chromosomes XX and males one X and one Y chromosome XY. Having two X chromosomes means that the newborn has a stronger immune system because X chromosomes contain a larger number of immune-related genes.
This makes males more vulnerable to many infectious diseases.
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They are also more susceptible to specific genetic diseases where the defective genes are carried on either the X or Y chromosomes; this is because boys have only one X chromosome so a single recessive gene on that X chromosome results in the disease. But the stronger immune response of females comes with a cost. Sex hormones may be another key reason for weaker immune systems in males.
Males have much higher amounts of testosterone which seem to inhibit two major parts of the immune system — B and T-lymphocytes. Overall, male hormones weaken the immune system relative to females.
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This is not restricted to childhood: the female advantage carries into adulthood. More specifically, differences in maturity, sex chromosomes, and hormones. In circumstances where both sexes are treated equally, we would therefore expect infant and child mortality rates to be slightly higher for boys. The sex ratio — the number of males relative to females — at birth and in childhood are male-biased in almost every country. In the chart below we see the sex ratio — measured as the number of males per females — at different ages through adolescence and adulthood.
In the global average for adolescents and young adults we see the result of both the male-bias in birth ratios and the large impact of populous countries such as China and India with very skewed sex ratios: At age 15 and 20, males outnumber females by to But as we move through adulthood we see that this ratio is lower and lower. For year-olds the ratio is close to 1-to-1; for year-olds there are only 89 males per females; and in the very oldest age bracket year-olds there are only 25 men per women.
You will see that for some countries this decline in the sex ratio with age is even more extreme: in Russia, for example, by age 50 there are only 88 males per females; by 70 years old there are almost twice as many women as men. In every country in the world women tend to live longer than men. On average, women live longer than men — this is true for every country in the world.
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This fact plays an important role in how the sex ratio changes with age through adulthood. Today, and at several points historically, the sex ratio at birth in some countries is too skewed to be explained by biological differences alone. In a recent study Chao et. Most of these countries are in Asia.
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Is there a biological or environmental difference, or is it the result of discrimination? The reason for this skew in sex ratio has been previously challenged. One of the leading hypotheses was put forward by economist Emily Oster. The authors concluded that hepatitis B rates could not explain the skewed sex ratio in China. Other studies — such as that by Lin and Luoh in Taiwan — have also found minimal to no effect of hepatitis B on the sex ratio.
After the hepatitis B hypothesis was debunked, no clear evidence of a biological factor in such skewed sex ratios has emerged. But the natural variability of the sex ratio is too small to explain the high ratios in some countries.
source link The evidence for sex-selective abortion and discrimination against girls is now strong across several countries. Not only does the increase in sex ratios coincide with the availability of prenatal sex determination technologies, there is also clear evidence from studies investigating the use and promotion of such methods. In India, for example, prenatal diagnosis PD became available in the s, shortly after legalization of medical abortion in Results from some of the earliest studies on abortions following the availability of prenatal sex determination are striking.
Results from another six hospitals in the city found 7, of the 8, aborted fetuses in were girls. The evidence that highly skewed sex ratios at birth have been largely the result of gender discrimination and selective abortions has been well-established across several countries. We discuss the reasons for this discrimination here. There are some additional hypotheses as to why the sex ratio at birth is skewed in some countries. Sex discrimination can occur prenatally in the form of sex-selective abortions, as we discuss here or postnatally when it can lead to the death of a child in the very worst cases.
The death of a child due to sex discrimination can be brought about in a deliberate killing of an infant infanticide or can be caused by neglect or poor and unequal treatment. Over time, prenatal discrimination has increased as both abortions and sex determination technologies have become more readily available. Nonetheless, postnatal discrimination still occurs and has a long history. Infanticide or infant homicide — the deliberate killing of newborns and infants — has a long history.
Rather than being an exception, then, it has been the rule. And humans are not alone.
From birds to rodents; fish to mammals; we find evidence of infanticide across the animal kingdom. There are some common misconceptions today surrounding the practice of infanticide. A number of researchers have studied the demographic, health and cultural profiles of prehistoric societies. In rare cases they can use indirect evidence of the fossil record; but many rely on modern hunter-gatherer societies today. Estimates for infanticide in prehistoric societies are very high. They did this using census and interviews gathered over a seven year period.
Infant mortality rates in the past were very high — most studies suggest around a quarter of newborns did not survive the first year of life. They also found large sex differences: infanticide rates were four times higher for girls than boys. Other studies of modern hunter-gatherer societies analysed the sex ratio of infants to estimate the prevalence of infanticide.
Very skewed sex ratio of infants is suggestive of select infanticide. In studying 86 hunter-gatherer bands across North America, South America, Africa, Asia and Australia, researchers found high levels of female infanticide across 77 of them. The practice of infanticide was not just common in prehistoric societies, but was also very common in many — but not all — ancient cultures.
There is of course significant evidence of female-selective infanticide throughout history: we see that in the sex ratio of many hunter-gatherer societies above through to skewed ratios in Medieval England.
Even today, cases on infanticide still exist, despite being outlawed in most countries. This remains reported across countries with a strong son preference: India and China are the most documented examples. Infanticide is the most direct case of postnatal sex selection. More often overlooked is the excess mortality which results from neglect and unequal treatment of girls. In almost every country young boys are more likely to die in childhood than girls — as we explore here , there are several biological reasons for is. But this is not true in a few counties — India is one notable example today.
There, girls die more often than boys. When we compare infant under one year old and child under-5s mortality rates between boys and girls in India we see that the difference is bigger for the older age group. While infant mortality rates are approximately the same, the child mortality rate for girls is higher.
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In the chart here we see mortality rates for boys on the y-axis and girls on the x-axis for various causes in India. This data is shown for children aged years old in Here we see that for many, death rates are significantly higher for girls. Some of these — hepatitis, measles or tuberculosis, for example — we expect to be higher for girls. But not for infections, respiratory and diarrheal diseases. Note also the much higher mortality rates for nutritional deficiencies and protein-energy malnutrition for girls.
Poorer health outcomes for girls across some countries — often in Asia and not restricted to India — has been well-documented. Social preference for a boy has resulted in unequal treatment of young girls in a number of ways. Power proportionately increases as the sample size for study increases. Accordingly, an investigator can control the study power by adjusting the sample size and vice versa. A clinical study will be expressed in terms of an estimate of effect, appropriate confidence interval, and P value.
The confidence interval indicates the likely range of values for the true effect in the population while the P value determines the how likely that the observed effect in the sample is due to chance. A related quantity is the statistical power; this is the probability of identifying an exact difference between 2 groups in the study samples when one genuinely exists in the populations from which the samples were drawn. The calculation of an appropriate sample size relies on choice of certain factors and in some instances on crude estimates.
There are 3 factors that should be considered in calculation of appropriate sample size- summarized in Table 1. The each of these factors influences the sample size independently, but it is important to combine all these factors in order to arrive at an appropriate sample size.