Refers to the proportion of males to females in a given population, usually expressed as the number of males per 100 females. Three types of ratios are distinguished: primary sex ratio (assessed immediately after fertilization), secondary sex ratio (assessed at birth or hatching), and tertiary sex ratio (assessed at maturity, and giving a ratio in which there are a larger number of females due the fact that they tend to live longer than men)). For humans, the secondary sex ratio is on average 105:100, but varies from country to country, especially in those where fatal feticide and infanticide are still practised. In theory, this ratio should be 1:1, as the number of X and Y spermatozoa from a male are equal. One explanation as to why this is not the case is offered by the maternal dominance hypothesis. According to this hypothesis, derived mainly from non-human primate studies, the more dominant a female the more likely she is to have male offspring. That the hypothesis is applicable to humans received some support from a meta-analysis carried out on six studies published between 1969 and 1991, which showed that dominant women had a significantly higher number of male offspring (see Grant, V.J. Maternal personality, evolution and the sex ratio: do mothers control the sex of the infant. London: Routledge, 1998). Allied to the hypothesis is a well-known and replicated finding that the effects of environmental stress can increase testosterone production in the adrenal cortex of women, where most of this hormone originates in women. An elevation in testosterone leads to increased manifestations of dominance in females, and thus to the birth of more males. Thus, during and after the two world wars, there were recorded increases in the number of male births in those countries directly affected by these conflicts, due not only to environmental stress but also to women taking over traditional male occupations at these times. Differences between countries in sex ratios may not be only due to environmental stress, sex-selective abortion, infanticide, and abandonment. In a controversial study published by Emily Oster in the Journal of Political Economy (2005, 113, 1163-1216), it was reported that mothers infected with hepatitis B virus were more likely to bear males (150:100) than uninfected women (first noted in Greece in 1982). She estimated that the prevalence of hepatitis infection accounts for 75% of the sex ratio imbalance in China, 20% to 50% of that in the Middle East and Egypt, but less than 20% in India, Pakistan, Bangladesh and Nepal.
See Adrenal cortex, Infanticide, Population (biology and ecology), Sex, Sex chromosomes, Spermatozoa, Testosterone