Wealthier mothers tend to have more sons while hard-up women produce more daughters, a study claims. Scientists have unearthed a fascinating link between the financial status of a woman and the proportion of boys and girls she can expect to have.
The discovery adds to the increasing evidence that the sex of a baby isn't just a matter of chance but is influenced by lifestyle and environment.
According to evolutionary theory, when conditions are good, and babies are likely to be healthy, a mother's best chance of passing on her genes to another generation is to have boys.
Fit, healthy boys will see off rivals and can potentially father hundreds of children, ensuring the survival of the family line. But if a mother is unfit or malnourished, a baby boy is a poor investment.
A weak, sickly male is unlikely to beat off competition from other males and may not become a father or even survive.
In these circumstances it makes more evolutionary sense to have a girl who does not face competition to become pregnant to continue the family line.
To test the influence of a mother's wealth and fitness on the sex ratio, Dutch researchers used a database of more than 95,000 Rwandan mothers compiled in 2002.
They compared the proportion of boys each woman had, with her marital status - an indicator of her wealth and health.
Like many African countries, Rwandan men are allowed more than one wife. Within a polygamous marriage, there is usually a tight pecking order.
Higher ranking wives tend to have more influence, and income, than lower ranking ones. And the more wives a man has, the less food and money there is to share.
As expected, lower ranking wives produced more daughters on average than the higher ranking wives or the women in monogamous marriages, the researchers report in the journal Biology Letters.
The biggest difference was between third or lower-ranking wives who had 106 daughters for every 100 sons compared with those in monogamous marriages who had 99 daughters for every 100 sons.
Women have no control over whether they have boys or girls. But some studies suggest that unborn baby girls are tougher - and less likely to be miscarried if the mother is stressed or ill.
The influence of status on the sex of a baby is likely to be stronger in countries such as Rwanda, where 75 per cent of the population are below the poverty line and the average life expectancy is just 50, than in countries such as Britain.