WASHINGTON (AP) _ Having sons may have shortened the lives of mothers centuries ago, while daughters added years to the maternal life span, according to a study based on church and family records.
The study analyzed life, death and births among a group of Finnish nomads in the days before modern medicine. It found that the life span of mothers was reduced by about 34 weeks per son, but was extended by a daughter who grew to adulthood.
Baby sons, researchers suggest in the study appearing Friday in the journal Science, make a much greater physical demand on the mother's body than do the 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 conclusion is based on church family records kept in Finland for a nomadic people called the Sami during the period 1640 to 1870. He said the toll on mothers' lives of having sons may not apply in the era of modern medicine.
``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 the effects of natural mortality before advance medical care.
The study concentrated on women who produced children and then went on to live past age 50. The researchers found that women who gave birth to sons had a shorter life span than those who had only daughters. Typically, a mother's life was shortened by about 34 weeks per son.
Having daughters who were raised to adulthood diminished the effect, actually helping the mothers to live longer, Helle said.
``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.''
During the study period, Sami families typically had four children per generation. The children tended to stay with the family, creating an extended family group that spanned several generations.
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 produce compensating children to replace those who died at an early age,'' he said.
During the preindustrial era, the life span average about 62 years for both men and women, said Helle. This was far longer than in many cultures during that era. Today, the average life span of a Finn is about 81 years for women and about 74 years for men.