Sunday 07 February 2016


Why giving birth to sons could be bad for your health

Giving birth to sons is bad for a mother's health, according to a new study.

Giving birth to sons is more stressful for mothers and could shorten their livespans, a study suggests
Giving birth to sons is more stressful for mothers and could shorten their livespans, a study suggests Photo: ALAMY

Scientists suspected having big families shortens a parent's lifespan, because reproduction and childcare can take their toll on wellbeing.

And it was believed sons were more costly than daughters because they take up more energy, although previous results have been mixed.

So researchers used demographic data from pre-industrial Finland to show, irrespective of access to resources, mothers but not fathers with many sons suffered from reduced survival.

But this association reduced as mothers got older, reports the Royal Society journal Biology Letters.

Dr Samuli Helle, of the University of Turku, said: "Our results provide evidence that Finnish mothers traded long post-reproductive lifespan for giving birth to many sons."

Previous studies have suggested sons are especially costly for the mother because they are, on average, born heavier and place more physical stresses on the body.

They also raise levels of testosterone in their mothers', which can age the immune system, making it is less able to defend the body.

Sons need more care after birth. Daughters prolong mothers' lives by helping in tasks such as obtaining food and rearing younger siblings, at least in the traditional society studied, so daughters might aid long-term survival.

To minimise factors that vary across a modern population such as quality of healthcare and family income, Dr Helle and colleagues collected information on individuals born between the seventeenth and twentieth centuries.

This included post reproductive lifespan and offspring characteristics like number, sex and survival to the age of 15 for a total of 11,166 women and 6,360 men.

Dr Helle said: "Women's post reproductive survival declined with the number of sons they gave birth to. This association did not depend on their socio-economic status but was age dependent. That is, the survival costs of the number of sons born decreased linearly as women aged."

But the number of daughters born was not connected to women's lifespan and, in men, neither the number of sons nor daughters born were related to their survival, irrespective of their socioeconomic status.

Meanwhile, the number of sons and daughters surviving to adulthood were not related to either parent's survival.

Added Dr Helle: "Our results show producing sons shortened the post reproductive lifespan of Finnish women and this association detoriated as mothers aged.

"This supports our hypothesis that production of sons is more energetically costly than the production of daughters.

"The reduction of reproductive costs with age may be due to the potential beneficial social influences of surviving sons, as the number of adult sons was unrelated to post-reproductive lifespan in these women.

"No such associations were found in men, which is consistent with men investing less direct energetic resources in reproduction than women."

The researchers said that the harmful effect of producing many sons may be related to female hormones.

In mammals, accumulating evidence now suggests high concentrations of testosterone circulating in a mother's blood impairs her immune system.

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