the human sex ratio, both at birth and as a population matures, can vary significantly according to a large number of factors, such as paternal age, maternal age, plural birth, birth order, gestation weeks, race, parent's health history, parent's psychological stress
cold weather is an environmental stressor, and women subjected to colder weather abort frail male fetuses in greater proportion thereby lowering birth sex ratios. Cold weather stressors simultaneously extend male longevity thereby raise human sex ratio in its older age bracket.
maternal malnutrition generally appear to increase fetal death
Sex-selective abortion and infanticide are thought to significantly skew the naturally occurring ratio in some populations.China, where the introduction of ultrasound scans in the late 1980s has led to a birth sex ratio (females to males) of 0.883 because of single child policy of china these practices are based on a cultural preference for one sex, typically males, over the other. A son is often preferred as an "asset", since he can earn and support the family; a daughter is a "liability" since she will be married off to another family, and so will not contribute financially to her parents. The patriarchal structure of a society is the single most important factor skewing the sex ratio in favor of males, accentuated in some cultures by the burden of raising a dowry for a daughter's marriage
violence against women is causing gender imbalances in many developing countries.These authors report that more girls have been killed in the last 5\
0 years, just because they were girls
causes of the gender imbalance are warfar notably in the wake of WWI in western Europe, and WWII, particularly in the Soviet Union d large-scale migration, such as that by male labourers unable to bring