Over 1000 articles have been published in various journals. The scientific studies are based on extensive birth and death records in Africa, the Americas, Asia, Australia, and Europe.Two of the often cited reviews of scientific studies on human sex ratio are by W. A few of these studies extend to over 100 years of yearly human sex ratio data for some countries.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.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 scientific paper published in 2008, James cautions that available scientific evidence stands against the above assumptions and conclusions.
Scientific literature often uses the proportion of males.
This article uses the ratio of males to females, unless specified otherwise.
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.
Consequently, the sex ratio tends to reduce as age increases and among the elderly, there is usually a greater proportion of females.