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Women who outnumber men in poor communities are likelier to have babies at a younger age as competition drives them to lower their expectations of the opposite sex, a study said on Wednesday.
The findings are a useful tool for understanding unwanted teen pregnancies, its authors believe.
Researchers at Britain's University of Portsmouth compared birth rates, gender ratios and the socio-economic background in more than 2,500 English wards, or urban neighbourhoods, each with about 8,000 residents.
In poor neighbourhoods where men were the minority, women had children earlier, the investigators found.
But in more affluent neighbourhoods where men were similarly outnumbered, women focussed on education and career, later becoming the group with the higher birth rate in their 30s.
Study co-author Abby Chipman said that in poorer neighbourhoods the increase in the birth rate was mainly among 15-to-24-year olds, while in more affluent areas it was among those aged 34 to 39.
"If there are more women than men, studies have shown that women have lower expectations of men," she said.
"We found that poor women are more likely to rush to start their 'reproductive careers', while rich women are more likely to delay having children.
"We speculate that instead they begin to accumulate resources and education that will be of benefit to their future offspring."
Previous studies have shown that when men outnumber women, the female sex becomes more selective and poorer men are less likely to get married.
This is a situation that worries policymakers in India, China and other countries where there is a looming deficit of young women as a result of gender selection in pregnancy in favour of males.
The new research, published in the Royal Society journal Biology Letters, contends that when the pendulum swings in the other direction and women compete for men, pregnancies often result.
Amongst urban women aged 25-29, every 10 percent shift in the sex ratio towards an over-supply of women resulted in the birth of at least seven more babies in their ward, it found.
Scientists have long associated early pregnancies with poverty. But the new work shows how, at the micro-scale of the neighbourhood, the sex ratio can sharpen this tendency, the authors said.
"An oversupply of females within a local economically deprived environment leads young women to adjust their strategies for reproductive success," it said.