Parents often quip that their kids -- especially their rambunctious little boys -- are "going to be the death of me," and new research shows that they may be right: Having sons can shave an average of eight and a half months off of a mom's life.
(The affect on dads? None, apparently.)
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The study, by evolutionary ecologist Dr. Samuli Helle of the University of Turku in Finland and Dr. Virpi Lummaa of the University of Sheffield in the United Kingdom, was published this week in the journal Biology Letters. He and his team looked at the post-childbirth survival rates of 11,166 mothers and 6,360 fathers in pre-industrial Finland, using records kept by the Lutheran Church there.
"Irrespective of access to resources, mothers, but not fathers, with many sons suffered from reduced post-reproductive survival," they wrote in the study.
The subjects were born in eight different parts of Finland during the 17th to 20th centuries -- a time period when the mostly agricultural society did not have access to modern medical care or birth control. After crunching the numbers, the researchers found that a mother who bore six sons would live on average another 32.4 years after the youngest son's birth, while a mother who gave birth to girls would live approximately 33.1 years after her youngest daughter came along.
The shorter life expectancy was the same regardless of the mom's social or financial status, though Helle said that "societal and cultural reasons could also play a factor."
"Adult sons may be beneficial for their parental well-being and thus survival in some countries, but girls may be beneficial in other countries," he told Livescience. In Finland during the 1700s and 1800s, a girl helped her mother by shouldering many of the household responsibilities, which could have increased the mother's life span slightly. Dads weren't affect, probably because reproduction takes far less of a toll on a man's body than it does on a woman's.
Still, Helle said in a statement: "The research shows the more sons you have the lower post-reproductive survival was. Biologically, there is a bigger cost associated with having a boy than a girl, so that is one explanation for the shorter lifespan." Male babies are usually bigger than female babies, which may have meant that they required more nutrients from the mother's body during gestation, researchers suggest.
But modern moms with boys shouldn't worry too much: The study is correlational, not causal (that is, it shows a link between having sons and dying earlier, but doesn't prove that one causes the other). Besides, Helle points out, life these days is very different from that experienced by the mothers in the study, the vast majority of whom were born before 1960. Twenty-first century families are smaller than they were in the 1700s, 1800s, and early 1900s, and better nutrition, medical care, and birth control options mean that women are far healthier now than they used to be.
"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 said.
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