Dominant, aggressive women are more likely to have sons than daughters, scientists believe.
A study in New Zealand found a possible link between high testosterone levels in women and giving birth to boys.
The findings question the idea that the sex of babies is determined by chance.
A team led by Dr Valerie Grant of the Department of Medical and Health Sciences at the University of Auckland, found a link between high levels of the male hormone testosterone in cows‘ wombs and their likelihood to have a bull calf.
The link could explain patterns in human populations such as the phenomenon dubbed the “war time effect” – in which disproportionate numbers of boys are born at the end of periods of hardship such as wars.
Previous studies have found links between dominant behaviour in female animals and higher levels of testosterone. Stress is also believed to boost levels of the hormone.
The team extracted follicles from cow ovaries and tested for testosterone before fertilising the eggs. Those eggs which had been exposed to higher levels of testosterone were more likely to develop into male embryos.
“Results showed that follicular testosterone levels were significantly higher for subsequently male embryos,” the team wrote.
The findings suggest that sperm carrying “Y” chromosomes – present in male animals – are more likely to fertilise an egg if it has been exposed to testosterone.
While in cows high testosterone was linked to dominant behaviour, in humans it was associated with everything from lower divorce rates to right wing political views and spatial ability.