<P> Infant mortality is higher in boys than girls in most parts of the world . 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> <P> In the United States, the sex ratios at birth over the period 1970--2002 were 1.05 for the white non-Hispanic population, 1.04 for Mexican Americans, 1.03 for African Americans and Indians, and 1.07 for mothers of Chinese or Filipino ethnicity . Among Western European countries c. 2001, the ratios ranged from 1.04 in Belgium to 1.07 in Switzerland, Italy, Ireland and Portugal . In the aggregated results of 56 Demographic and Health Surveys in African countries, the ratio is 1.03, albeit with considerable country - to - country variation . </P> <P> Even in the absence of sex selection practices, a range of "normal" sex ratios at birth of between 103 and 108 boys per 100 girls has been observed in different economically developed countries, and among different ethnic and racial groups within a given country . </P> <P> In an extensive study, carried out around 2005, of sex ratio at birth in the United States from 1940 over 62 years, statistical evidence suggested the following: </P>

How many of each gender in the world