Having many sons may shorten women's lives after their last birth more than having daughters, according to new research.

The findings, which came from a group of Finnish women born mostly before industrialization, are correlational, so they can't prove sons actually cause shorter lives for their moms relative to daughters. And because the effect varies across the world, social factors, rather than intrinsic biological effects, may be to blame.

But the study, published today (Feb. 26) in the journal Biology Letters, suggests that cultural differences in how sons and daughters have historically been raised may affect women's life spans.

"Adult sons may be beneficial for their parental well-being and thus survival in some countries, but girls may be beneficial in other countries," study co-author Samuli Helle, an evolutionary ecologist at the University of Turku in Finland, wrote in an email.

Costly process

Pregnancy and breast-feeding both require extra nutrients and calories.

That extra energy requirement can take a toll on women who have more children and shorten their lives, even if they start out healthier than other women to begin with, said Grazyna Jasienska, a biological anthropologist at Jagiellonian University in Poland, who was not involved in the study. [8 Odd Ways a Woman's Body Changes During Pregnancy]

And because males are born slightly heavier than females on average, researchers have hypothesized that they may require more nutrients, making them more reproductively costly to rear.

But studies throughout the world have shown conflicting effects. In China, males seem to confer a longevity advantage, for instance.

To see how the effect played out in Finland, Helle and his colleagues tracked Lutheran church parish records for more than 11,000 women and their children from the last three centuries. The vast majority of the women were born before 1960.

The more sons a woman had, the more likely she was to live a shorter time after last giving birth. The effect held whether women were rich or poor.

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Cultural effect

The reason for this shortening probably doesn't have to do with the extra energetic costs associated with boys during pregnancy and infancy, Jasienska told LiveScience.

Instead, it probably reflects the prevailing social norms at the time.

"Girls in many traditional societies are, as we know, much more helpful to mothers than boys," Jasienska said. "They may help with child care, they may help with many tasks."

And because the study was looking at a mostly pre-industrial society, where food was scarcer and woman had no birth control, the effects may not persist today, Helle wrote.

"One could speculate that owing to modern medical care, smaller family size and more abundant resources, the biological costs of reproduction might not play that important role in modern societies anymore," Helle wrote.

But other social differences could still make sons and daughters affect women's longevity differently, he added.

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