Male-female ratio predicts age women have babies

13 February 2013

Poorer women will start having children younger if there aren’t enough men to go around, according to new research published in Biology Letters today.

Researchers found that richer women living in a wealthy area will respond to the problem of fewer men by delaying having children, potentially focusing on their education or career, and being prepared to travel further afield to find a mate.

The study from the University of Portsmouth is the first to show that, in England, the ratio of males to females in small urban geographical areas has a direct effect on the age women start having children.

Lead author Abby Chipman said the results suggest rich and poor women adopt different strategies if women outnumber men and adjust their reproductive timing based on the number of available men.

She said: “If there are more women than men, studies have shown that women have lower expectations of men. 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.

“The patterns we found suggest female-to-female competition is associated with poorer women adopting a ‘live fast, die young’ strategy.”

The study 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 factors used to determine neighbourhood deprivation include income, employment, health, education, housing and access to services.

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 ten per cent 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.

In England, about 105 boys are born to every 100 girls, but the distribution of males and females in any given area varies substantially.

Understanding ecological as well as economic factors may help us to understand and address social issues such as the UK’s high rate of teenage pregnancy.