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Catalano team finds that a 1 °C increase in annual temperature predicts one more male than expected for every 1,000 females born in a year. have studied 138 years worth of human birth sex ratio data, from 1865 to 2003.
They find an increased excess of male births during periods of the exogenous stress (World War II) and during warm years.
The secondary sex ratio decreased with increased number of children per plural birth and with paternal age, whereas no significant independent effect was observed for maternal age, birth order, or other natural factors. reports the sex ratio derived from data in United States birth records over a 25-year period (1981–2006).
More data are available for humans than for any other species, and the human sex ratio is more studied than that of any other species, but interpreting these statistics can be difficult.
Like most sexual species, the sex ratio in humans is approximately 1:1.
Due to higher female fetal mortality, war casualties, gender-selective abortions and infanticides, aging, and deliberate gendercide.
This study also found that the sex ratios at birth in the United States, between 1981–2006, were lower in both black and Hispanic ethnic groups when compared with white ethnic group.
The relationship between natural factors and human sex ratio at birth, and with aging, remains an active area of scientific research.
Essentially he argues that the 1:1 ratio is the evolutionarily stable strategy.