"My son will be the death of me yet."
That familiar lament by mothers everywhere may have a kernel of scientific truth, researchers say.
A study of family church records in earlier centuries found that having sons shortened the life span of Finnish mothers by about 34 weeks per son. Daughters nurtured to adulthood helped prolong mothers' lives.
Baby sons, researchers suggest in the journal Science, make a much greater physical demand on the mother's body than do typically smaller daughters, and this may actually lead to a shorter life for the mother.
"Boys are usually born much heavier than girls," said Samuli Helle, a researcher at the University of Turku in Finland and the study's co-author. "It seems that boys are much more demanding to produce than girls."
Helle said the study looked at a nomadic people called the Sami during the period 1640 to 1870. He said the toll of having sons may not apply today.
"Nowadays we have better medical care," Helle said. "Resources are not as likely to limit females' life span. There might be some effect, but it will not be as huge as in the Sami people."
Helle and his co-authors used the records because of their accuracy and because they gave a measure of mortality before advanced medical care.
The study concentrated on women who produced children and then went on to live past age 50.
"You can actually cancel the negative effect [on life span] of one boy by producing about three girls," said Helle. "The girls stayed in their natal group for quite a long time. All the children had a great influence on their parents' lives, but the girls had a more positive effect than the boys."
The Sami were a nomadic people who followed the migration of reindeer. They led a hard life, but they were very successful, said Helle. Infant mortality in the group was very low, so there was "no need to ... replace those who died at an early age," he said.