<P> Gender imbalance may arise as a consequence of various factors including natural factors, exposure to pesticides and environmental contaminants, war casualties, gender - selective abortions and infanticides, aging, and deliberate gendercide . </P> <P> Human sex ratios, either at birth or in the population as a whole, are reported in any of four ways: the ratio of males to females, the ratio of females to males, the proportion of males, or the proportion of females . If there are 108,000 males and 100,000 females the ratio of males to females is 1.080 and the proportion of males is 51.9% . Scientific literature often uses the proportion of males . This article uses the ratio of males to females, unless specified otherwise . </P> <P> In a study around 2002, the natural sex ratio at birth was estimated to be close to 1.06 males / female . Some scholars suggest that countries considered to have significant practices of prenatal sex - selection are those with birth sex ratios of 108 and above (selection against females) and 102 and below (selection against males). This assumption has been questioned by some scholars . </P> <P> 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 . Recent studies have found that numerous preconception or prenatal environmental factors affect the probability of a baby being conceived male or female . It has been proposed that these environmental factors also explain sex differences in mortality . In most populations, adult males tend to have higher death rates than adult females of the same age (even after allowing for causes specific to females such as death in childbirth), both due to natural causes such as heart attacks and strokes, which account for by far the majority of deaths, and also to violent causes, such as homicide and warfare, resulting in higher life expectancy of females . For example, in the United States, as of 2006, an adult non-elderly male is 3 to 6 times more likely to become a victim of a homicide and 2.5 to 3.5 times more likely to die in an accident than a female of the same age . Consequently, the sex ratio tends to reduce as age increases and among the elderly, there is usually a greater proportion of females . For example, the male to female ratio falls from 1.05 for the group aged 15 to 65 to 0.70 for the group over 65 in Germany, from 1.00 to 0.72 in the United States, from 1.06 to 0.91 in mainland China, and from 1.07 to 1.02 in India . </P>

Ratio of girl and boy in world 2017