Poorer women who face a shortage of men will have children younger, while richer women will delay motherhood and focus on their careers, a new study suggests.
Lead author Abby Chipman of the study, which was conducted by the University of Portsmouth, said the results show that the ratio of males to females in small urban geographical areas of England had a direct effect on the age women start having children.
"The patterns we found suggest female-to-female competition is associated with poorer women adopting a 'live fast, die young' strategy," she said.
If women outnumber men, poorer women are more likely to have children younger
"We found 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."
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"If there are more women than men, studies have shown that women have lower expectations of men," she adds.
Ms Chipman said that in poor neighbourhoods there was less geographical movement, partly because fewer people own cars and higher levels of unemployment.
She added that the results helped confirm that people's experiences of their local neighbourhoods are an important factor related to when they were most likely to give birth.
The research was funded by a bursary from the department of psychology and was published in the journal Biology Letters.
It compared birth rates with data on neighbourhood deprivation for more than 2,500 urban neighbourhoods, each with about 8,000 residents, using data from the Office of National Statistics.
It examined women aged 15 to 50 and compared birth rates, deprivation and the male-to-female sex ratio in each area.
The results showed that every step change in the male-to-female ratio had an effect on the local birth rate.
For example, for every 10% shift in the sex ratio towards an over-supply of women, 7.5 more babies will be born to women in the 25-29 year age group in an average urban neighbourhood.
Ms Chipman said that scientists had known for some time that women were more likely to give birth when young if they were faced with tough living conditions, and that the sex ratio can influence behaviour with the scarcer sex being most in demand.
She gave the example that when men outnumber women, women become more selective resulting in poor men being less likely to marry.